HomeMy WebLinkAboutChalkyitsik Village Council Level Two Biomass Feasibility Study 12-20-2011-BIOChalkyitsik Village Council
Level Two Biomass Feasibility Study
Alaska Wood Energy Associates
BIOMASS HEATING FEASIBILITY Level 2 Study
Chalkyitsik Alaska Final Review Document
Alaska Wood Energy Associates - 2 - December 20, 2011
Table of Contents
SECTION 1 4
1.1 GOALS AND OBJECTIVES 4
1.2 PROJECT SCALE 5
1.3 RESOURCE ASSUMPTIONS 6
1.4 FINANCIAL METRICS 7
1.5 LEVEL 2 SUMMARY 8
SECTION 2 11
2.1 EXISTING FIELD CONDITIONS 11
2.2 PRELIMINARY SYSTEM INTEGRATION PLAN 13
2.3 SCHEMATIC DESIGN DATA 16
2.3.1 PIPING 16
2.3.2 BOILERS 18
2.3.3 FAN COIL UNIT 18
2.4 RESOURCE ASSESSMENT 19
2.5 HARVEST SYSTEMS 20
SECTION 3 21
3.1 GENERAL 21
3.2 RECOVERED HEAT 21
3.3 WOOD HEAT 22
3.4 SUPPLEMENTAL HEAT 24
3.5 DISTRIBUTION 25
3.6 INTEGRATION OF RECOVERED HEAT 26
3.7 DESIGN CONSIDERATIONS FOR THE USE OF STICK FIRED BOILERS 27
3.7.1 SIZING, BOILER CONTROL, AND UTILIZATION RATE. 27
3.7.2 END‐USER ISSUES. 29
3.7.3 MATERIAL HANDLING. 30
2.7.4 EMISSIONS CONTROLS/EFFICIENCY. 31
3.7.5 MAINTENANCE. 31
3.8 SITING ISSUES. 32
SECTION 4 33
4.1 LIMITS 33
4.2 METHODOLOGY 34
4.2.1 ENERGY SAVINGS. 34
4.2.2 RECOVERED HEAT. 37
4.2.3 COST ESTIMATES. 37
4.3 BUSINESS STRUCTURE 38
4.4 PROJECT PERMITS 39
APPENDIX A. WILDLAND FIRES IN THE VICINITY OF CHALKYITSIK. 40
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APPENDIX B. HARVEST EQUIPMENT EXAMPLES. 41
APPENDIX C. LOWER BOILER SITE. 43
APPENDIX D. UPPER BOILER SITE. 44
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SECTION 1
1.1 Goals and Objectives
The objective of this Level 2 Cost and Feasibility Study report is to document the
progress and findings at this stage of the project. The scope of the project is to provide
enough analysis to allow the village to determine if the project or projects included
should proceed to investment grade studies, design, and eventual implementation. The
Level 2 Study looks at both small district heating “mini” plants as well as individual
building-level biomass boiler applications.
A mini plant for the purposes of this report is simply one or more boilers in a single
building serving two or more buildings, by piping the heat from the plant to the buildings.
The term “District Heating Plant” (DH Plant) denotes a larger plant, using chip-fired
boilers, and generally serving five or more buildings. The location and size of the
buildings in Chalkyitsik are such that no DH plant is feasible here; there is not enough
economy of scale to support the high first costs associated with DH plants.
The study is complete based on current knowledge; the author of the study has been to
Chalkyitsik twice for the purpose of doing site work for this study. Should the project
progress to an investment grade audit, further site visits may be required. The study is
based on data from the village, from the site visits, from the State, and other professionals
with experience in bush Alaska. As additional information comes to light in the design
phase, this is expected to result in changes to the savings and/or cost projections.
Some of the examples herein are taken from a similar analysis performed for Fort Yukon,
which is currently 90 percent done with an investment grade study and more than 35
percent complete with design.
The work on this project is being done by Alaska Wood Energy Associates. This team
consists of professionals from a number of different companies, representing various skill
and knowledge sets. Greg Koontz of efour, PLLC, Seattle, WA, is performing the
feasibility modeling and will also provide design services specific to the boilers and any
DH plant.
Obviously, a primary concern for any biomass-fired plant is availability of the wood
resources. This issue is the responsibility of Bill Wall, PhD of Sustainability, Inc. The
means and methods of procuring the wood and processing it will be documented in detail
elsewhere; they are summarized briefly in Section 2.4 below. For that reason, this report
does not address biomass supply in the same level of detail as the heating systems.
The objective for the team and the village at this point is to use the study to choose a path
forward into final design and construction; or, failing that, to determine under what
conditions the projects would be moved forward. This final report documents the L2
feasibility study that has been completed.
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1.2 Project Scale
In order to be successful, a DH plant must achieve a certain economy of scale. The
capital costs involved are quite large, and so the savings to the village must be on the
same scale in order to make economic sense. One of the reasons why a DH plant must be
large to be feasible is that they are projected to run on wood chips, not stick-wood. This
is primarily a function of boiler capacity, and the need for manual labor to feed stick fired
boilers.
As shown below in several of the Figures, the largest stick-fired boiler being considered
has a firing rate of 925 kBTU/h (thousands of BTU per hour). The largest chip-fired
boiler AWEA recommends has a firing rate of 4,265 kBTU/h - over four and half times
larger. Chip-fired boilers are automatically fed; stick-fired boilers are manually fed. One
consideration is the manual labor required to fire five stick-fired boilers four times a day
each in cold weather.
At the same time, the equipment needed to harvest wood and chip it to feed a chip-fired
DH plant represents a significant capital cost. Experience has shown that unless a village
has about six to eight large buildings (minimum) in a fairly tight cluster, a chip-fired DH
plant is not economically feasible. It follows that if there is no DH Plant to use chips,
then it is not feasible to make chips at all – so in villages with no feasible opportunity for
a DH Plant, the study focuses solely on stick-fired boilers, as in Chalkyitsik.
One new building has been added since the preliminary report (the new clinic, which is
expected to be in place at the same time, or earlier than the boiler plant is built).
Chalkyitsik has at least seven larger buildings, and these have all been included in the
study. These buildings exist in two clusters, of three and four buildings; the two clusters
are separated by about one third of a mile – too far apart to economically connect with
piping. The associated first cost, heat loss and pumping energy would be very
detrimental to the economics of the project.
As a result, the study looks at each of the seven buildings separately, and then at three
“mini-plants”. The seven buildings are 1) the school, 2) teacher housing, 3) the upper
water treatment plant, 4) the washeteria/old clinic, 5) the tribal offices, 6) the store, and
7) the new clinic (planned, but not yet constructed).
The upper water treatment plant is included twice – once with recovered heat and once
without. This building currently gets almost 100 percent of its heat from heat recovered
off of the village engine generators.
In addition to the individual building analyses, three mini-plants are included: 1) School
+ teacher Housing + Upper Water Treatment, 2) Washeteria/Clinc + Tribal Offices + new
clinic, and 3) Washeteria/Clinc + Tribal Offices + Store + new clinic.
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It is assumed that the space that currently serves as a clinic will continue to be used, and
thus must be heated to the same temperatures as is the case now.
1.3 Resource Assumptions
In order to compare all the mini plants and individual boilers on an equal basis, some
base level assumptions about these costs had to made and used in the performance /
financial model. The assumptions shown in Figures 1.1 and 1.2 below are based on
current estimates of recent fuel costs in the villages, plus projections of the cost of
obtaining stick wood.
Figure 1.1. Base Level Oil / Electrical Assumptions
The unit cost of oil has been increased to $7.00/gal since the preliminary report
The table lists four oil prices; this model is used in a number of villages, so it needs to be
flexible. The “low cost” is used when some buildings in the village get a better price than
others (such as schools in some villages). The “high cost” is what everyone else pays.
The other two are the cost of oil to the DH plant (in case it is different still), and the cost
to the power plant (in case they also get a different price). In this case, only one value
was used, $7.00 per gallon.
Figure 1.2. Base Level Wood Assumptions (chip/pellet data grayed out)
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The unit cost of cordwood has been increased to $300/cord since the preliminary report
The primary resources of concern in this study are the various energy sources, current
and proposed. Because the systems involved are closed piping loops, water and sewer
use is almost zero on an annual basis. Aside from filling and/or flushing the system,
there is no water use and thus no sewer use.
The fuels of concern, then, are No. 1 oil and electrical energy (current costs), and stick
wood (proposed costs), as seen in Figures 1.1 and 1.2. As will be seen in the following
sections, even if this project is implemented, oil and electrical energy will still be
required in the village.
It was noted that the upper water treatment plant benefited from “free” recovered heat
from the village generators. In this study, that heat has been treated as free; that is, the
power plant does not charge for the use of this recovered heat. If this is not the case, the
charge for the recovered heat can be included in a revision to this study. This would
obviously negatively affect the economics of any boiler plant serving the treatment plant.
Finally, figure 1.3 shows the properties of the stick wood that were used in the model.
These tables show the mix of species expected to be harvested in the area and the
expected moisture contents; this input is used to calculate the composite stick-wood
properties.
Figure 1.3. Stick-wood Properties
1.4 Financial Metrics
There are any number of financial metrics that can be employed to evaluate a project.
Many of these require that the source and means of financing the project be known.
Many require knowing the expected interest rate that money could be borrowed at, and
even the rate of return the client would expect to achieve if they invested the capital
elsewhere (not in the project).
In the case of potential projects in Chalkyitsik, much of this information is not known at
this time. The exact funding mechanisms are not known. The in-kind participation of the
village, if any, is not defined, and therefore the value of it cannot yet be determined.
Finally, forward-looking interest rates are not very predictable at this point in time.
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Since this study is not an investment grade study assumptions about all of the relevant
financial variables do not seem justified. For all these reasons, this study has used a
single financial metric to evaluate each potential heating plant – both as a stand-alone
investment and as a way to compare different technologies and combinations of
buildings.
Net simple payback (NSP) as used herein is defined simply as the implementation cost of
the project divided by the value in dollars of the annual year-one energy savings. Year
one savings are specified; it is assumed that resource rates will change year to year (or
faster).
All financial summaries used in this study use NSP as the sole financial metric for
evaluating each option. There are a number of variables, which do not factor into the
NSP as defined herein; perhaps the two most relevant here are the labor cost and the
maintenance cost.
It is not known how the plants would be manned. Stick-fired boilers require up to 12
loads of wood per day, manually fed, at peak load. However, in such cold weather, the
operator(s) may choose to simply feed the boiler during the day, and let the oil-fired
boilers take over at night.
The levels of unknowns regarding the amount of labor that would actually be applied to
each plant makes estimating labor costs difficult at best. Similarly, judging the
maintenance costs of the boiler plants in the harsh climate of the interior of Alaska
presents an issue. The Garn boilers are very simple, with not much to break.
Nevertheless, they have moving parts to maintain, and possibly fail.
Labor and maintenance costs are annual, and thus deduct directly from the energy savings
(lengthening the NSP). The point being made above is simply that the range of possible
values for annual labor and maintenance costs is so wide that they should not be used to
make financial decisions as a part of this study. Instead, as the project is developed in
each village, the decisions on boiler technology and plant size should go hand-in-hand
with discussions of how the boilers will be operated and maintained so that the true cost
can be determined prior to making the investment.
1.5 Level 2 Summary
A mathematical model was constructed to model the performance of the various mini
plants and individual boilers in the village, and to compare their financial performance.
The model is discussed in more detail in Section 4, and a sample of the calculations
involved is shown in Appendix A. To date, it is not known for sure how this project
would be financed, so the financial model does not include the cost of money.
At this point, as noted above, the key financial metric is net simple payback (NSP) at
current costs. Figure 1.4 shows a summary of the results of the model using the Base
Level resource assumptions.
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Figure 1.4. Project Summary
Some of the paybacks are very long, and it can be seen that in all these cases, the boiler
proposed is the smallest available; the Garn WHS 1500. These long paybacks indicate
that the associated building is small, and does not consume much oil on an annual basis.
Even with the smallest available boiler, the cost of a biomass boiler installation is
significant. Unless there are substantial potential annual dollar savings in the form of oil
displaced, a given building may not be able to support a biomass boiler installation.
The primary reason we group buildings into mini plants it is to improve the economics of
the projects. In the table above, items 1 through 6 and item 8 are the seven individual
buildings, modeled with the assumption that the water treatment plant is not getting
recovered heat. Item 7 is the water treatment plant again, this time assuming it does get
recovered heat. In item 7, the payback gets very long – with the recovered heat in place,
the building uses almost no oil, so there are almost no dollar savings to offset the first
cost.
Items 9 and 10 are the upper village mini-plant (item 9 assumes no heat recovery, item 10
includes heat recovery). Item 11 is a lower village mini plant with just the washeteria,
new clinic and the tribal offices. Item 12 adds the store to the plant in item 11.
Normally, adding the store would likely lower the payback – it is relatively large, and not
far away. However, because the store does not have a boiler, hooking the building into
the mini plant is more expensive than it would be otherwise.
One conclusion that is drawn from the study to date is that the projects are extremely
sensitive to the cost of oil, and less so to the cost of wood and electrical energy. Another
is that economy of scale is important to make these projects feasible.
All of the mini-plants are financially feasible. The two that make the most financial
sense are in line 10 for the upper plant and line 12 for the lower plant. Each of these
plants will displace the most amount of fuel oil and will payback in six years or under for
the capital cots. The real issue is developing an for profit business structure that is
sustainable and can get, 191 cords for the lower plant and 180 cords for the upper plant,
of wood harvested, seasoned and fed into the boilers on an annual basis. These two
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plants will displace 23,200 gallons of fuel in the lower plant and 23,600 gallons in the
upper plant.
Since the preliminary version of this report was submitted, Garn, the recommended
stick-fired boiler manufacturer, has begun to develop a new product. Informally, it is
known as the “Garn in a box”. As the name implies, it is a Garn boiler that comes as a
complete package. Currently, a Garn boiler requires a primary pump, a heat exchanger,
and some piping and controls in order to operate, as well as separate electrical
connections for each piece of equipment (the additional required piping and equipment
beyond the heat exchanger is considered part of the distribution system, not part of the
“boiler plant”). In the new configuration, Garn will pre-pipe and pre-wire all the parts
and pieces that make up the “boiler plant”. In addition, the boiler and remainder of the
plant will be enclosed in a container that is meant to act not only as the shipping
container but the as the boiler “building” as well.
On site, the end-user need only construct a concrete slab, and bring the power and
distribution piping to the slab. Once the Garn is placed on the slab by a lift truck or
similar, the piping and power can be connected in a day or two at the most. They are also
meant to be able to sit side-by-side with no clearance between. The only site-erected
“building” required is an enclosed storage or lean-to for wood, and perhaps an enclosed
walkway from the storage to the “boiler room” doors (for multiple-boiler applications).
Currently, Garn is planning to offer this package only in the WHS 2000 size. However,
this could work very well for Chalkyitsik – that is the recommended boiler size for the
two most likely mini-plants, items 10 and 12; each consists of two WHS 2000s.
The units are scheduled to go into construction in 2012. The estimated cost of the units is
relatively high, compared to just buying the boiler. As more are made, the cost may
come down. In the preliminary report, the cost estimate assumed that all Garn WHS
2000s were bought separately (as were the other sizes), and piped and wired on site.
And, for now, this appears to be perhaps slightly less expensive. However, there are
advantages to having much of the work done off-site. For this final report, therefore, all
of the Garn WHS 2000s are assumed to be the pre-packaged models. The prices are
higher; but the net simple paybacks are still short. Pricing the models this way allows the
village the flexibility to purchase either variety of the boiler. By the time an investment
grade study / concept design is done, more will be known about the “Garn in a box”, and
this new information will help inform any future equipment decisions.
Section 3 below includes a discussion about Garn boilers, and the design considerations
that apply to stick-fired boilers. All of that information is valid regardless of how the
Garn boiler is configured or packaged.
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SECTION 2
2.1 Existing Field Conditions
At this time, there are seven buildings included in the study, in clusters of three and four
buildings each. Each of these seven buildings has been evaluated for the installation of a
dedicated biomass boiler, and each cluster of buildings has been evaluated for a “mini
district heating plant”, where one or two biomass boilers located in a single building
would serve two to four buildings. Heat from the boiler “plant” would be piped to each
building served Economy of scale is important in biomass boilers, so these mini-plants
generally have a better payback than the dedicated building boilers, as the summary in
Section 1 indicated.
The seven buildings included in the study at this time are: 1) the school, 2) teacher
housing, 3) the upper water treatment plant, 4) the washeteria/clinic, 5) the tribal offices,
6) the store, and 7) the new clinic.
All of these buildings are currently served by boilers (the new clinic will be as well),
except the store, which has a direct-vented Toyotomi, ductless forced air heater. An
important consideration is that connecting a new external boiler to an existing one in a
building is generally very simple (and thus relatively inexpensive). Even connecting to a
building with an oil furnace can be easily done, by adding a heating coil in the furnace
ductwork.
However, because the Toyotomi heater has no ductwork, converting the store to biomass
heat would require a different method. It would require adding a fan coil unit (FCU) to
the space. The coil in the FCU would be fed hot water by the biomass boiler, and the
Toyotomi would remain in place as back-up heat. The FCU could be a floor-mounted
vertical unit (would look like a furnace, but has only a fan and a coil), or a horizontal unit
that is ceiling mounted.
A potential complication in hooking up the buildings to a biomass-fired boiler or boilers
exists in the washeteria, and is explained in detail in Section 3.7.2. The complication
relates to the clothes dryers in the building and their specific requirements.
The table in Figure 2.1 below lists the existing heating equipment in the subject
buildings, with some comments on condition.
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Figure 2.1. Existing Village Boilers
The Teacher Housing is the only building on the list that has no back-up heat. The
mechanical room in which the boiler is housed is not large enough to add another boiler.
The one boiler appears to be in good condition, however, as are the remaining boilers on
the list with the slight exception of the school boiler system.
The school boiler system was in generally good shape, but it could use some “touch-ups”.
There is a lot of insulation missing, which not only represents a heat loss to the system, it
also results in a lot of surface rust on the pipes that are continually wet. This is shown in
Figure 2.2 below:
Figure 2.2. School Boiler Piping/Header
The upper village water treatment plant, which basically serves the school and school
housing, is adjacent to the village power plant. Like many villages, a system has been
installed to recover heat from the engines. In many villages, this system is turned off, or
does not work, but in Chalkyitsik, the system is operating and appears to be in good
shape.
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Normally, the heat from the engines would be released to the atmosphere by running the
cooling water through radiators. The heat recovery system is installed in series with, and
upstream of the radiators; it has the ability to extract any amount of heat from zero to 100
percent of the heat in the cooling system. A 3-way control valve controls the flow of the
cooling water through a flat plate heat exchanger. One the “other side” of the heat
exchanger is the water treatment plant heating water. The heat exchanger transfers heat
from one loop to the other, cooling the engine cooling water while heating the building
heat loop. If the heating loop does not extract all the engine heat, the remaining heat in
the engine-cooling loop is routed to the radiators for final cooling. Figure 2.3 below
shows the key elements of this system.
Figure 2.3. Flat Plate Heat Exchanger / 3 – Way Valve / Radiators
2.2 Preliminary System Integration Plan
Hot water will be piped from the external biomass boiler into the existing building
mechanical room. Once in the building mechanical room, the new hot water distribution
piping will be tied into the existing hot water supply and return lines that feed the existing
boilers. Typically, four 2-position, 2-way automatic isolation valves will be installed in
the piping, as shown in Figures 2.5 and 2.6 below. The position of these valves will
determine whether the heat comes from the building oil-fired boiler, the biomass boiler,
or both. The existing building pumps will continue to serve the building-heating load.
The valves that control the origin of the heat will be controlled by the existing building
controls, where they exist, or by a small-dedicated control panel if needed. If this proves
too costly for very small installations, the switchover can always be done with manual
valves, but this relies on an operator.
Figure 2.4 below shows a schematic drawing of a typical installation for two oil-fired
boilers. In this scenario, each boiler is sized for 100 percent of the load; the boilers are
manually alternated so that they get roughly equal run time. In all cases (figures 2.4, 2.5,
and 2.6), light solid lines indicate existing equipment and piping, dark solid lines depict
new equipment and piping, and light dashed lines show the water flow through the
system. For convenience, it is assumed in all cases that Oil Fired Boiler – 1 is the active
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boiler, and boiler 2 has been isolated using the associated manual isolation valve. HWS
is hot water supply to the building; HWR is hot water return from the building.
Figure 2.4. Typical Oil-fired Heating Plant
Figure 2.5 shows the initial configuration of the combined oil and biomass heat, with the
biomass boiler providing all the heat. Closed “auto” (automatic) valves are solid, open
valves are not “blacked in”.
Figure 2.5, Combined Oil and Biomass Heat, all heat from Biomass
In the event that the biomass heat cannot meet the building setpoint for any reason, the
two systems can operate in series. The lack of adequate heat output from the biomass
boiler could range from small (an extremely cold day) to total (a boiler failure or
someone forgot to fire the boiler in time), but the operation would remain the same – the
oil-fired boiler would simply add enough heat to maintain setpoint, whether this is 1
percent or 100 percent of Load. This is shown in Figure 2.6.
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Figure 2.6, Combined Oil and Biomass Heat, Boiler Heat in series with Biomass Heat
The scenario gets only slightly more complex if recovered heat is to be used as well. In
this case, the existing heat exchanger (HX) remains in place, and the piping on the “hot
side” remains as is (the engine cooling loop is rejecting heat to the heating loop, so the
engine cooling loop is the “hot side” of the HX, and the building heating loop is the “cold
side”). On the cold side, the piping is re-arranged as shown in Figure 2.7 below:
Figure 2.7 Combined Oil and Biomass Heat, Boiler Heat with Recovered Heat
The stick-fired boilers proposed for use on this project incorporate a hot water storage
tank (see 2.3 below, and Section 3 for more detail). The water in this tank is normally
kept between 120 and 200 deg F. The existing 3-way control valve (upper right of
Figure) would be used to modulate the flow of heat through the heat exchanger, and thus
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keep the boiler storage tank as close as possible to 200 deg F (its maximum charged
temperature).
However, as it gets colder outside and the building heating load increases, tank
temperature will begin to drop as more heat is extracted from the tank. When the
temperature drops below 120 deg F, the recovered heat can no longer maintain the
building heat. At that point, the boiler operator will have to begin to fire the biomass
boiler. The recovered heat will continue to be used, however.
The HX is in series with, and upstream of, the biomass boiler, which in turn is in series
with and upstream of the oil-fired boilers. This determines the order in which the heat
sources are used: recovered heat first, then biomass heat, and then only when both of
these sources are fully loaded will the oil boilers be used. A failure of either the HX or
biomass systems is fully backed up by the existing oil boiler(s).
The exception to the proposed connections above is the Store, where no boiler exists. In
this case, a new fan coil (see 2.3 below) would be installed to utilize the hot boiler water.
The easiest means of “interconnecting” the fan coil unit (FCU) with the Toyotomi heater
is to simply set the Toyotomi thermostat three or more degrees below that of the FCU. If
the FCU can maintain the higher setpoint, the Toyotomi will not come on. If for some
reason, the FCU does not maintain setpoint, the Toyotomi will automatically come on
when the space temperature drops down to the lower setpoint.
2.3 Schematic Design Data
The subsection above shows how a proposed biomass boiler would be interconnected
with the existing boilers in each building. Note that a single boiler (or two or three in
combination) can serve multiple buildings. The piping from the boilers simply branches
out to each building – all the connections would still look like those in 2.2 above. The
subsections below contain preliminary design information on equipment proposed for this
project. More detail about all these elements can be found in Section 3.
2.3.1 Piping
If the proposed biomass boiler installations and plants proposed herein had to rely on
traditional rigid “arctic” piping, the payback would be significantly extended. Instead,
the basis of design is a flexible, pre-insulated system that uses a plastic carrier pipe
(carrier pipe is the pipe that “carries” the water, or the inner pipe). The carrier piping is
constructed of cross-linked polyester, or PEX. The outer pipe (or casing), which protects
the insulation, is made of corrugated plastic to make it flexible.
The piping comes on rolls that are dozens or hundreds of feet long. It is much lighter
than steel or copper pipe, and thus cheaper to ship into the villages. Standard easy-to-
install fittings are used anywhere in the piping to connect end-to-end, tee, or elbow as
required. The piping can be obtained as a single pipe within a casing, or in the smaller
sizes, supply and return can be combined into a single common casing. Figure 2.8 shows
an example of a PEX system.
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Figure 2.8, PEX Piping, Two Carrier Pipes in a Single Casing
Figure 2.9 shows the sizes of PEX piping available from Rehau, the manufacturer that the
preliminary performance modeling is based on.
Figure 2.9 Preliminary Piping Schedule.
The insulation rating, or R value, of the piping depends on the depth of bury, and the soil
conditions. The performance model has all these factors built in. The standard
installation detail is quite simple. A shallow trench is dug, between three and five feet
deep. A minimal amount of bedding is put down, and the piping is unrolled into the
trench. The trench is back-filled to about 6 inches above the top of the casing pipe. Then
a 4’ x 8’ sheet of “blueboard” (rigid insulation) is placed lengthwise in the trench above
the piping to increase the R value of the total assembly (blueboard not shown in Figure
2.10). Then the rest of the trench is back-filled as shown in Figure 2.10; the final
assembly can support road traffic, if need be.
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Figure 2.10 Preliminary Piping Installation Detail.
Within the buildings, piping will be steel or copper, with standard fiberglass insulation.
2.3.2 Boilers
Garn manufactures the proposed boilers; Dectra Corporation, located in St Anthony,
Minnesota owns Garn . A number of Garn Boilers are already installed in Alaska. Some
Design Considerations specific to stick-fired boilers are presented in detail in Section 3.
Figure 2.11 is a schedule of capacity and physical characteristics of the Garn boilers.
Figure 2.11 Garn Boiler Characteristics.
The model 3200 is quite a bit larger than the other two models, in every way. It is
expensive to ship to Alaska, and therefore has not been widely used in bush Alaska.
However, it is almost always less expensive to install one boiler with X capacity than two
boilers with ½ X capacity each, so the model 3200 has been included in the study.
2.3.3 Fan Coil Unit
Fan coil units (FCUs) are exactly what they sound like. A single housing contains a fan,
controls, and a heating and/or cooling coil (or both). In this case, only a heating coil
would be selected. This unit would pull in inlet air from the room, blow it across a hot
water coil (fed by the biomass boiler), and blow the heated air into the room. This
discharge air can be ducted, if desired, although the existing Toyotomi unit has no
ductwork. The picture in Figure 2.12 below is from the Trane Company; however, there
are many manufacturers of FCUs. Final selection would be made during design. The
FCU in the picture has both a cooling and heating coil (note the four piping connections).
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Figure 2.12 Trane Fan Coil Unit.
2.4 Resource Assessment
A significant amount of the forests around Chalkyitsik have burned in the past 60 years
as part of a natural fire disturbed ecosystem (Appendix A). Burned areas typically
regenerate to hardwoods first and then as succession moves through time the stands are
taken over by spruce either white or black depending on the quality of the site and the
return fire interval. Fires create a mosaic pattern of forests in all successional stages.
There are burned and mature stands within five miles of Chalkyitsik. Since there have
been recent fires (last 10 years), there is a lot of standing dead that is being utilized as fire
wood and could also be used as fuel for a district heating system. Residents in
Chalkyitsik have a lot of experience in gathering firewood and one fire wood supplier
gets up to 50 cords of wood annually for sale into the village using hand felling with
chain saws and transporting with a snow machine and sled.
There are two DH scenarios selected in this report as most feasible. Since the goal of the
project is to displace as much fuel oil as is financially feasible and ecologically
sustainable, the largest system appears to be the preferred. The plant of two Garn boilers
with heat recovery from the generators will require approximately 180 cords of wood
annually on the upper site at the school. The plant of two Garn boilers in the lower site
will require 190 cords with no heat recovery available. We estimate ranges of dry
standing burnt spruce to be approximately 12-14 cords per acre in many stands. The
community is interested in using these stands first whenever possible. Approximately
26-30 acres annually will be needed to fully support the two largest DH system proposed.
This resource assessment is based on several trips to Chalkyitsik and field inspections of
stands in all conditions from burned to early successional hardwoods to mature stands.
No formal inventory has been developed nor has systematic forest stand cruises been
conducted. However, two different experienced foresters have been on the ground
observing the forest and all agree that the level of harvest for a DH heat system is fully
sustainable within 5-10 miles of the village. If the project moves forward a harvest plan
will need to be developed and followed. Training on the proper cutting methods for
regeneration will need to be held with contracted woodcutters.
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2.5 Harvest Systems
A full analysis of harvest systems is beyond the scope of this report. A harvest system
can be developed as part of the final design and will depend on the final size and whether
both boiler plants are installed. The following describes the general approach to a harvest
system.
The key to a successful biomass program includes the integration of getting enough
quality wood on an annual basis securely stored and then fed to the boilers. There are
three different general harvesting systems that could work in Chalkyitsik: A. Hand
cutting and loading with snow machine transport; however if both boiler sites are
installed a total of 327 cords will be required and snow machines are inefficient for this
level of harvesting; B. Hand cutting and mechanized loading and hauling; C. Mechanized
cutting, loading, and transport. Of course, there are multiple scenarios for equipment for
systems B and C.
Photos of different types of equipment that could be utilized are in Appendix B.
Deciding which system to utilize will be an important decision for the community to
make in the development of a final business model. It is the recommendation of this
report to develop and use system B described below. Although system A yields more
hand labor for potentially more people, this may not be the most effective way to assure
an adequate supply of wood annually and may not be the most cost effective. System C
could be too expensive in capital costs and maybe overkill for the amount of wood
required for the Chalkyitsik DH system.
Hand felling and limbing will take a significant amount of labor in itself for producing
327 cords annually. Adding a tractor with a loading arm and a trailer for hauling
(Appendix B) will make the harvesting-transport system both affordable and more
reliable. The issue with the tractor for winter hauling will be maintaining the trail to the
wood stock piled in the forest. The tractor will have attachments for snow removal, both
a plow and a blower. Although not a versatile a track vehicle could be used instead of the
tractor as the primary transport vehicle.
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SECTION 3
3.1 General
There are often two general configurations of boiler plants examined in studies such as
this; single building applications (or very small groups of adjacent buildings) and district
heating (DH) plants. Likewise, two wood burning technologies are usually included;
stick fired and chip fired. An individual building boiler could be either a chip or stick-
fired boiler, but only chip boilers are large enough (in terms of heat output) to power a
DH plant.
However, chip-fired systems are very expensive to install, and require a great deal of
support equipment (chippers, etc). Experience has shown that in order to get the required
economy of scale to make a DH plant financially practical, a minimum of about six to
eight or more buildings (depending on size) must be connected to the plant. Chalkyitsik,
as noted in Section 2, has seven fairly large buildings included in the study, however,
they are widely scattered into two clusters. The distance between the two clusters of
buildings is too great to pipe between (for first cost, heat loss and piping energy reasons),
so no DH plant has been proposed or evaluated for Chalkyitsik. In the absence of a chip-
fired DH plant, there is no economical way to make wood chips, so by process of
elimination, this study includes only stick-fired boilers.
In general, stick fired boiler systems are smaller, less automated, require less associated
equipment, and their lower heat output range makes them appropriate for smaller scale
installations (single building or small group applications), such as Chalkyitsik. They are
simple and robust; but their simplicity means that they are more labor-intensive to
operate than chip fired boilers.
Overall, the study considers three sources of heat: 1) heat recovered from power
generator engines, 2) heat from wood, and 3) heat from oil.
3.2 Recovered Heat
Heat recovered from an engine generator and used in a boiler system or DH Plant is
“free” in the sense that there is no marginal cost increase to reject that heat to a heating
loop compared to the rejecting it to the atmosphere. The heat comes primarily from the
cooling jacket of the engine, and must be carried away from the unit to prevent it
overheating. In the absence of a co-located building or heating plant, the heat is normally
carried to a radiator, which cools the jacket water by releasing the heat to the atmosphere
(a fan blows air over a radiator coil).
Engine generators producing prime power are an ideal source of heat for any heating
plant. They run continuously, and the quality (temperature) of the heat rejected is almost
identical to the heat required by the boiler or plant. Generally, recovered heat is the
lowest cost form of input energy to the heating system, thus it is normally selected first
and used to the fullest.
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However, in some villages the owner of the power plant chooses to charge for the
recovered heat; in such cases the “waste” heat is “free” for them to provide, but not free
to the end-user. In this model, the recovered heat has been assumed to be free to the end
user. This will need to be confirmed in further studies, and, in the event that a cost for
the recovered heat is to be charged to the user, this charge will be incorporated into the
financial and performance models. Sections 3.6 below and 2.2 above describe how the
recovered heat will be integrated into the proposed biomass fired systems.
3.3 Wood Heat
As noted above, recovered heat (when available) is considered the primary heat source,
because it is generally the least expensive heat source. Wood heat is the thus considered
the secondary heat source. As with recovered heat, wood heat will always be used to the
extent possible before using the tertiary heat source (oil, in this case). The villages own
significant amounts of this wood resource in the surrounding lands, and AWEA believes
it can be produced in a usable form (cord wood) at a price significantly below that of oil,
on a BTU basis.
Solid fuel boilers require more infrastructure than oil-fired boilers. They require space
for wood storage and processing. Stick fired boilers require space to cut to length and
split wood to meet the boiler specifications.
Given the remoteness of the Yukon Flats, any equipment installed must be reliable and
well tested. It is also desirable that any boilers used be standard units, “off the shelf” so
to speak. The use of proprietary or customized equipment increases the chance that if
equipment failure occurs, it will be expensive and/or time consuming to get it fixed.
For stick-fired boilers, the team proposes to use Garn boilers. A number of Garn Boilers
are already installed in Alaska. Figure 3.1 is a picture of a Garn boiler.
Figure 3.1 Garn Boiler
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Note that aside from cordwood, a Garn boiler can also burn clean construction waste, slab
wood, and densified wood products (briquettes, etc). However, neither construction
waste nor slab wood was considered to be a reliable resource at the sites considered.
Densified wood products are not available in the interior; in fact, one of the primary
reasons to consider a stick-fired boiler was minimize the processing required for the fuel.
As Figure 3.1 shows, the Garn boiler consists of a burn chamber (the chamber hatch-style
door can be seen in 3.1) surrounded by a hot water storage tank. Because of the water
tank, the Garn boilers are much larger than chip-fired boilers for a given output. It also
means that Garn boilers have two output ratings: 1) burn rate, and 2) storage capacity.
The burn rate is the rate at which heat is released when the chamber is loaded with stick
wood per directions and fired. The storage capacity is how much heat the tank can hold.
In order for the storage capacity to have any meaning, the maximum and minimum
storage temperatures must be specified. For the Garn, the tank is fully charged when the
tank water is 200 deg F. It is depleted when the tank temperature is 120 deg F.
Heating a building is a continuous process; heat is continually withdrawn from the tank
by a pump and heat is transferred to a building. Heating the boiler tank is a batch
process. A discrete amount of wood is burned in each “batch”. No more fuel is added
until the previous burn is complete.
Fuel is not added continuously, thus the burns are a batch process. The storage tank
allows the Garn to bridge the gap between the batch process of burning a load of wood
and the continuous process of heating a building. Section 3.7 below offers a detailed
description of the implications of using stick-fired boilers.
Figure 3.2 below is reproduced from Section 2, and shows the characteristics of the Garn
boilers included in the study.
Figure 3.2. Garn Boiler Characteristics
The material handling associated with stick-fired boilers is much simpler than that
associated with chip fired boilers. In essence, the wood should be cut to length, and
should fall within an acceptable range of diameters. Larger diameter lengths may have to
be split. Ash must be removed from the burn chamber manually, and tube cleaning is
manual as well.
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The boilers must be fed manually regardless of how the wood is processed, but the actual
processing is a trade-off between simplicity and availability of equipment and more
manual labor versus more expensive, specialized equipment with correspondingly less
manual labor. If chainsaws and splitters are the primary material handling equipment,
then a great deal of manual labor is required to process the wood and feed the boilers.
The benefit is that this equipment is cheap, easy to fix (and quick to ship, if new is
needed), and abundant in the villages. If specialized harvesters and/or cutters are used,
much of the manual labor is removed. However, this equipment is expensive, cannot
easily be replaced and there is likely only one of each per village – so a failure means the
operation is down until it is repaired. Each village in which a Garn is installed will need
to consider the associated material handling carefully in cooperation with AWEA to
determine the best solution for the village.
3.4 Supplemental Heat
There are conditions under which biomass alone or the combination of recovered heat
and wood heat may not be able to meet the heating load. It is possible to supply enough
boiler capacity to nearly eliminate the need for oil (except in the case of biomass boiler
failure, which is always possible). However, it is not always the best answer financially.
In theory, one could pay someone to stand next to a Garn boiler and feed it continuously,
which would maximize its output. In reality, this is not practical; the study assumed a
maximum of four boiler “burns” a day. A burn is not necessarily a single loading and
firing of the Garn; in this study a burn means burning enough wood to raise the storage
tank temperature from 120 deg F (min) to 200 deg F (max) – in practice this might take
two or more successive loads of wood in a row on very cold days. Remember that heat is
being extracted from the tank even as the heat from the wood is charging it.
If the number of burns per day is limited, then it is possible (in fact common) that the
expected peak load of a building on the coldest days is slightly greater than the capacity
of a given boiler, or combination of boilers. In such a case, one could always either use
the next larger boiler, or add another to cover the peak loads. However, the economics of
this are generally bad – installing a very expensive boiler to run ten or so days a year
makes no financial sense. It is easier and less expensive to simply use the building’s
existing oil boilers on such days to supplement the biomass heat.
In the results summaries there is a value labeled “fraction of oil displaced”. A value of
1.000 means the biomass boiler has the potential to provide 100 percent of the building
heat, with no need for supplemental oil heat (in reality, there is maintenance downtime,
so coverage will probably never be 100 percent on an annual basis). In these cases, the
peak load of the building matched up well with the capacity of one or more of the stick-
fired boilers. Often, however, values such as 0.900 (90 percent) appear. In this case, the
peak load is predicted to exceed the capacity of the boilers for a very small number of
hours each year – and thus supplemental heat from the building oil boilers will be
required.
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3.5 Distribution
The heat generated by the external biomass boilers must be distributed to the various end-
users. Unless the biomass boiler is very, very close to the building (in which case the
piping could be above-ground), this is done by pumping hot water through buried
distribution pipes to each building. Traditionally, the piping used in this part of Alaska is
a rigid system of pre-insulated piping. A carrier pipe carries the fluid; this is standard
steel piping. Rigid foam insulation surrounds the carrier, and insulation is protected by
an outer spiral-wound metal jacket (or casing). See Figure 3.3 below.
Figure 3.3 Traditional “arctic pipe”
This system provides superior heat loss characteristics (i.e. very low losses), but it is
expensive, and installation must be very well planned. It is expensive primarily because
the whole piping system is rigid. It must therefore be installed below the permafrost or
frost heave will snap the pipe – in many areas of interior Alaska this means 18 to 20 feet
deep. The required trenching is expensive, requires large equipment, and takes time.
Installation must be well planned out because the pipe is difficult to modify in the field.
Cutting a piece to length means cutting through all three layers; it is difficult to get a
clean cut and subsequent clean connection to the next piece. For that reason, the system
is typically laid out in great detail in the plans, and each piece and each fitting made for a
specific spot in the system. Thus any mistakes in fabrication or any damage to a pipe in
the field can take a long time to repair.
As noted in Section 2, therefore, AWEA proposes to use a flexible plastic piping system,
manufactured by Rehau, which uses PEX carrier pipes. Figure 3.4 is reproduced from
Section 2.3:
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Figure 3.4 PEX piping
Because the system is flexible, it can be installed in the active layer of the soil – in the
Plant proposed for Fort Yukon, the proposed depth is 48 to 60 inches deep. Further
investigation will be needed to determine the appropriate depth in Chalkyitsik.
Connections are simple, so the layout does not need to be planned in great detail.
Because it comes in rolls, hundreds of feet of piping can be laid out in very little time.
Trenches are shallow and simpler to construct, generally using equipment that may
already exist in the villages.
The most significant negative aspect of the PEX system as opposed to the traditional
system is that the insulation is not as effective. Piping losses are greater with the PEX
system, and piping losses can have a significant effect on ongoing operating costs. In
applications with free or low cost recovered heat, these operational savings can be
leveraged on an ongoing basis to counteract the increased piping losses, allowing
Chalkyitsik to realize the first cost savings associated with the PEX system without the
additional piping losses excessively damaging the project financials.
This would not be the first installation of the PEX system in rural Alaska; thousands of
feet of this type of piping were recently installed in McGrath in less than one week.
3.6 Integration of Recovered Heat
Diagrammatically, the integration of recovered heat into a DH Plant is shown in Section
2.2. The concept is very simple. Water in the cooling loop from the operating engine(s)
producing power in the power plant is routed first to the biomass boiler plant.
In the Plant, the hot cooling water flows through a heat exchanger. A three-way control
valve on the cooling loop side of the exchanger controls how much heat is rejected to the
boiler loop. If the valve is wide open, all the flow goes through the heat exchanger.
A temperature sensor in the boiler storage tank is used to modulate the heat exchanger 3-
way valve, extracting as much heat as possible from the engine-cooling loop. If this is
not enough heat to meet the load, additional wood or oil heat will be added to primary
loop, as required. If the load is less than the available recovered heat, the three-way
valve will modulate, as required, to maintain the heating loop at setpoint.
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On the generator cooling loop side, the water leaving the Plant, having flowed either
through the heat exchanger or through the valve bypass, will return to the power plant
cooler than it left. It will then flow to the engine radiators. If it is already cooler than the
radiator setpoint temperature, the radiator fans will not come on – the water continues
back to the engine jacket to start the cycle over. If the water from the DH Plant is
warmer than the radiator setpoint, the fans will come on as needed to cool the water, and
send it back to the jacket.
3.7 Design Considerations for the use of Stick Fired Boilers
Some of the material below has also been presented above. The intent is for this section
to be a stand-alone description of some of the implications of using stick-fired boilers
without reference to the remainder of the report.
As the name implies, stick-fired boilers burn round or split wood in relatively straight
pieces. The wood is minimally processed, being selected for a range of diameters and
trimmed only for length. If the diameter of the wood is too large, the wood may be split.
Although the processing is minimal (compared to chipping), it is generally all done
manually (some splitting may be done with a machine). Nevertheless, at the assumed unit
costs, stick-wood is the cheapest energy source available to the villages for generating
thermal energy.
However, utilizing stick-wood means that much of the available biomass cannot be used.
Wood that is too large or too small, smaller tops and limbs that are bent and/or tangled, or
tops, which contain leaves, cones, or needles, are generally too difficult to handle in a
stick-fired boiler. The burn chamber of the boilers (see figure 3.5 below) is designed for
straight stick-wood of a given length. The wood used is generally air-dried, not
mechanically dried; mechanical drying would be very expensive on such small scales.
The Garn stick-fired boilers used as the basis of evaluation for this study are scheduled
above in this section and in Section 2.3.
3.7.1 Sizing, Boiler Control, and Utilization Rate.
A primary feature of the Garn boiler is the built-in thermal storage. Physically, this is a
large hot water tank that surrounds the combustion chamber. Functionally, the tank
“decouples” the burn rate of the boiler from the actual heat load requirements. In
essence, the process of combustion heats the tank and the tank serves the load (through
pumps and a piping system), but not at the same rate. This is illustrated in figure 3.2
above. In the WHS 3200, for instance, the process of combustion generates up to 925
KBTU/h. The storage tank can hold 2,064 KBTU. So, if the “burn” lasts a little over two
hours, it will completely charge the tank. If at the same time the building-heating load is
500 KBTU/h, it will take a little over four hours to deplete the tank – thus the rate of
charging the tank is decoupled from the rate of extraction (serving the heating load) by
the storage tank.
Heating a building is a continuous process, heating the tank in a Garn is a “batch”
process. The thermal storage tank bridges the gap between the continuous process of
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heating and the batch process of burning. A batch process is one in which an event takes
place at intervals – for instance, every eight hours, one fills the Garn with a “batch” of
wood, and burns it.
This decoupling effect eliminates the need for sophisticated combustion controls that
would allow the boiler to track the load; that is, to match the burn rate to the heating load.
The boiler is manually fed, and manual started. This results in a very simple boiler,
which holds down first cost. The primary control function of the Garn is combustion
control – simply ensuring that the combustion air is controlled such that the wood burns
hot and clean.
The decoupling effect also means that sizing is less of an issue than it would be with a
chip-fired boiler. If more capacity is needed to meet load, the operator can simply
conduct more “burns” per day (within the limits described above). When less capacity is
needed, fewer burns are performed.
There are limits to this, of course. An operator would not want to have to feed the boiler
once every three hours round the clock, especially in the -50 deg F temperatures that can
occur in the interior of Alaska. In this study, the assumption was that if more than four
burns per day were required to meet peak heating load, a larger boiler would be used, or
another boiler would be added to the installation. Four burns per day implies a minimum
of six hours between burns. Adding another boiler increases the time between burns, but
it adds significant cost as well.
In addition, the number of stick-fired boilers per installation is generally limited to three.
Beyond this limit, the installations would get too large and too expensive. (None of the
combinations of boilers in Chalkyitsik would require three boilers.) Because of the
thermal storage, the boilers are quite large, and they require at a minimum a covered roof
and flat slab floor; ideally they would be completely enclosed. Equally important, the
utilization rate of the equipment drops as the number of boilers increases. If one boiler is
adequate in “warm” weather, two required for “cool” weather, and all three for “cold”
weather, then the overall utilization rate of the plant is probably no more than about one
half (50 percent). Installing equipment in the interior of Alaska is expensive; the higher
the utilization rate, the more cost-effective the installation.
In the summer, heat loss from the tank may become a significant factor. The seasonal
range of heating loads in the interior of Alaska is the highest in the country. The heat
load at -60 deg F is 20 times higher (or more) than the load at 80 deg F (when the load is
probably only domestic hot water). So a burn that only lasts six hours at peak
theoretically lasts 120 hours in the summer. Obviously, in 120 hours, more heat is going
to be lost through the tank insulation than is going to be actually used. It might therefore
be more practical to run the existing oil-fired boilers when the load drops too low.
However, for the purposes of this study, although insulation losses were accounted for, it
was assumed that the Garn boilers were not turned off in summer.
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3.7.2 End‐user Issues.
All of the facilities included in this study already exist, thus any installation of a wood-
fired boiler would by necessity be a retrofit to an existing heating system. The intent is
that the boilers be installed in such a way as to be transparent to the end-user. That is, the
occupants cannot tell whether the heat is coming from the existing oil-fired equipment or
from the proposed wood-fired equipment. Moreover, the mechanical heating system
must operate the same way regardless of heat source; switching from one source to the
other must be as simple as opening and closing valves. Finally, the intent is that the
systems will be installed in such a way that a failure of a wood-fired boiler automatically
starts the oil-fired back-up, and ideally, notifies the operator of the failure.
The Garn boilers do have one major limitation in terms of end-user transparency; they
cannot control the hot water supply throughout the burn cycle. When a burn finishes, the
storage tank is at design temperature (200 deg F is the design temperature for Garn).
However, as the hot water is pumped through the heating system, it gives up heat to the
space. As a result, when it gets back to the tank it is colder than when it left – the
difference between supply and return temperature, called the delta T (or change in T)
depends on the type of heating equipment (air handling unit, baseboard heat, radiator, etc)
and the heating load.
The cooler return water immediately begins to dilute the 200 deg F water, cooling it.
Once the burn is done, no more heat is being added to the tank, but heat is continuously
being removed to heat the space – thus the tank temperature falls throughout the tank’s
“draw-down” cycle. Garn considers the tank to be “depleted” when it reaches 120 deg F.
The basis of the heat storage capacities listed in figure 3.2 is the assumption that the tank
is heated to 200 deg F, and then heat is extracted until it reaches 120 deg F, at which
time, another burn is initiated.
However, in a retrofit situation, 120 deg F hot water may not be suitable. Many hot water
heating systems are designed to use hot water at 180 deg F or more when at peak load.
For instance, the heating coil in an air handling unit may have been sized to provide the
required peak heating output using 180 deg F supply water (180 deg F is a very common
coil temperature). In such a case, with the heating load at or near peak, the Garn boiler
will be able to meet load as long as the storage tank temperature equals or exceeds 180
deg F, but as it falls below that value, the air handling unit may no longer be able to meet
the load. By the time the supply temperature falls to 120 deg F, the air-handling unit will
be operating significantly below design capacity.
As a specific example, assume an air handling unit that requires 180 deg F supply water
at peak load (i.e., there was no spare capacity in the coil at peak load), then in effect the
storage capacity of the Garn boiler would be reduced by 3/4: 1 - [(200 – 180) / (200 –
120)] = 0.75. The WHS 3200 that has been used as an example above would have a
storage capacity of only 516 kBTU, rather than 2,064 kBTU. At the same time, the time
between burns would also be cut to 1/4 of the calculated time, although each “burn”
would be much shorter, since the burn only had to raise the temperature of the tank by 20
deg F.
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Practically speaking, most heating systems use hot water in the 140 deg F – 200 deg F
range. Only radiant floor systems typically use hot water as low as 120 deg F. Thus, in
almost all cases, it would appear that the storage capacity of the Garn units would have to
be de-rated. This study did not de-rate capacity of the Garn for three reasons: 1) there
was not sufficient time to survey all the existing equipment, and related drawings and
specs, to determine the design supply temperatures, and 2) In all cases, at load conditions
not at or close to peak, 120 deg F water may suffice – thus the number of hours per year
when the de-rate would be applied may be quite small (however, this is when the weather
is coldest, and the most labor is required to maintain the fuel supply and burn rate), and
3) direct observations of systems in Alaska have shown that many are so over-sized that
they operate even with low hot water supply temperatures. It was therefore assumed that
the specified storage capacity could be used in full; before any installation design is
finalized, however, this assumption should be confirmed.
In Chalkyitsik, there is one system of specific concern – the clothes dryers in the
washeteria. Hot water coils served by the existing oil-fired boilers heat the inlet air for
these dryers. Drying clothes is a function of not just BTUs, but also of air temperature.
The boiler in the washeteria was set to maintain 210 deg F during our last site visit (in the
summer). It is believed this supply water temperature was set this high to accommodate
the dryers. If this is in fact the case, a Garn boiler would not be a good application to
serve the dryers. It is likely that a small dedicated boiler would be required to serve very
hot water to the dryers as needed. This would alter the economics of serving this
building with heat from wood, but the effect is not likely to be large enough to make it
infeasible – the dryers are not large and do not run continuously.
3.7.3 Material Handling.
As noted above, the Garn boilers are manually fed. For each burn, the operator must
clean out any ash as needed, load the combustion chamber with new stick-wood, and
manually start the fire. Once the fire is lit, the chamber door is shut, and the fire burns
until all the fuel is consumed.
However, as noted above, it would take a little over two hours of burn to fully heat the
storage tank (using the WS 3200 as an example again). A single load of wood will not
burn for two hours, meaning that each burn must consist of more than one load of wood.
In addition, during that time that the burn is taking place, heat is being extracted from
tank to meet the heating load. So although a “burn” is treated as a single event in this
study, it is important to note that at or near peak load, a burn could take as long as three
hours to complete, and require two to three “reloads” of the combustion chamber. (A
complete burn is defined herein as burning enough fuel to raise the storage tank from 120
deg F to 200 deg F, even as heat is being extracted from the tank for ongoing heating.)
Thus although the number of burns is limited (in this study) to no more than four a day,
this could still imply roughly 9 - 10 hours a day of loading and cleaning the combustion
chamber in cold weather.
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As with any stick-fired appliance, the fuel should be kept dry, and should be located close
to the point of use. Therefore, any building or structure constructed to house the boiler
should have sufficient space to stack cordwood. The amount of wood to be stored within
the building (as opposed to in a wood yard) depends on the site conditions. In harsh
conditions, it may be desirable to store several days’ worth of cordwood (at peak load
consumption rate) in the boiler building, in case weather keeps the operator from being
able to re-stock the building from the wood yard. On the other hand, in all cases the
existing oil-fired system is assumed to be left in place as back-up, so this may limit how
much wood the operator chooses to store in the boiler building.
Regardless of how much wood is stored in the boiler building, considerable manual labor
would be required to get the sticks from the wood yard to the building; labor to load,
unload, and stack the wood. Because no equipment is required once the stick-wood
reaches the yard, the material handling, though labor intensive, is not subject to
equipment breakdowns.
There are a number of options for discarding the ash. It is likely the ash would be
collected in a small bin or dumpster, and emptied only as this gets full.
2.7.4 Emissions Controls/Efficiency.
Garn boilers have no active emissions controls. The boiler uses an induced draft (ID) fan
to ensure that enough air is present to provide complete combustion. This alone helps
eliminate or mitigate many emissions. It helps prevent the formation of carbon monoxide
(CO), which forms as a result of incomplete combustion. It minimizes smoke and
particulates (but cannot extract any particulate formed and emitted) by burning clean and
hot and leaving very little behind but incombustible ash.
Using more air than is strictly necessary simplifies the control, and makes for a clean
burn, but it also reduces efficiency. Excess air cools the boiler down as it enters and
brings additional moisture with it, both of which require excess heat to bring them up to
combustion temperature.
The Garn does provide good transfer from the stack gas to the hot water storage tank.
The stack gas essentially passes through the tank five times (a five-pass heat exchanger);
see Figure 3.5 below. There are four horizontal passes and one vertical pass. Overall, the
efficiency of the Garn is good – the study assumed 75 percent of the net useable heat
content of the wood was transferred into the tank.
3.7.5 Maintenance.
There is very little maintenance required on a Garn boiler, and in fact, there is not much
that an operator could do. Figure 3.5 below shows a cross section of a Garn boiler. The
wood is burned in the primary combustion chamber, “E”. In the secondary combustion
chamber, “F”, only hot gases are burned. As long is the ash is removed from “E” before
each burn, there is not much to maintain. The ID fan (“H”) must be repaired or replaced
if it fails.
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Figure 3.5 also shows the “tubes” that transfer heat to the storage tank. The tubes (from
the end of “F” through the end of “I”) must be cleaned; if not, then any scaling or fouling
of the tubes is not removed, and these will gradually erode the efficiency of the boiler (or
even cause the tubes to fail). Running a wire brush through them can generally clean the
tubes. The frequency of cleaning depends in part on how clean the wood is; clean forest
wood should have no inclusions, while scrap and construction debris often do. If these
inclusions (adhesives, preservatives, etc) do not burn completely, they often plate out on
the tubes, degrading performance.
Between cleanings, efficiency will slowly degrade as deposits accumulate, until the next
cleaning. Figure 3.5 also shows that the boiler has two vertical tubes sections – these are
more difficult to clean. All feeding, de-ashing, and cleaning are manual. The timing of
the feeding is manual, although that could be partly automated (i.e., a control system
could alert the operator when the tank is nearly depleted).
Figure 3.5 Cross Section through a Garn Boiler
3.8 Siting Issues.
As noted above, the Garn boilers are quite large as a consequence of the storage tank.
The WHS 3200, the largest Garn boiler considered in this study, is 14’ – 3” long, 7’ -2”
wide, and 7’ – 9” high. Each unit (full of water) weighs 34,500 lb. The largest Garn
plant “allowed” in this study would include three WHS 3200 units (we arbitrarily limited
the number of boilers per Plant to three, but no Plant at Chalkyitsik required three
boilers). Not including interior wood storage (but including clearance around each unit),
this would require a minimum of 678 square feet (20’ – 3” long by 33’ – 6” wide) with an
average floor loading of 153 lb/sf. This floor loading will likely require a relatively thick
reinforced concrete slab to prevent differential settlement.
The recommended sites are shown in Appendix C & D with all of the buildings to be
heated labeled. The Chalkyitsik Village Council who also operates the power plant owns
the area behind the Tribal Office for the lower boiler site. This area is a large enough site
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to house both the dual Garn boiler plant and a small wood yard large enough to store over
25% of the annual wood supply needed to operate the plant. This site makes piping
distances minimal for the lower plant. The plant and wood yard will be secured with a
chain link security fence with a gate large enough to allow access to wood hauling
equipment.
The upper plant will benefit the school mostly and will potentially be located on the
school property. It will be associated with the current powerhouse in order to capture the
heat from the generators. This site will need to be negotiated with the school in the final
design phase of the project. There is room for wood storage for about 25% of the annual
wood needed.
SECTION 4
4.1 Limits
As with any performance evaluations, the quality and validity of the outputs and
subsequent conclusions depends on the quality of the inputs and the methodology.
Methodology is discussed in Section 4.2 below. The input data gathered for use in the
analyses performed as a part of this study include:
Building specific data
Historical village PCE (electrical consumption) data
Proposed equipment data
Annual oil consumption, by building
Annualized weather data (bin data) from the Fort Yukon airport
Village maps and plans
Interviews with Civil Engineers, contractors, and consultants with significant experience
in the interior of Alaska
Pricing data from boiler manufacturers, piping suppliers and other AK vendors
Performance data from Garn
Input from other Alaska Wood Energy Associate team members
Detailed measurements of building heat loads and existing equipment performance were
not part of this analysis. A building heating load profile is central to predicting annual
fuel consumption. Ideally, this would be generated by directly measuring heating load
throughout the year. At the same time, the actual operating efficiency of the existing
boilers and distribution system would be measured. This would provide a highly detailed
profile of heating load and the energy required to meet that load, for any condition
throughout the year.
In practical terms, however, the required measurements are difficult to perform, and not
cost-effective. The equipment needed to make these measurements is not present at any
of the installations in the village, and would have to be flown in and installed. The
measurements would need to continue from winter to summer, to generate a complete
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load profile. The resulting incremental increase in the accuracy of the load profile cannot
justify that level of cost. As Section 4.2 explains, even without the measurements, the
data that were collected limit the load profiles to within a narrow range of values.
Also missing from this analysis is a detailed analysis of electrical load profiles (and thus
available recovered heat). The only data available was monthly. Ideally, at least some
hourly or daily data would be included in any Level 3 analysis.
4.2 Methodology
4.2.1 Energy Savings.
The performance of the existing and proposed heating systems was modeled using
spreadsheets; the type of model used is known as a “bin model”. In this case, the bins are
ranges of outside air temperatures (OATs). Temperature bins are used because heating
load is very closely correlated to OAT. Each “bin” of OAT is 2 deg F wide – for
instance, 40 – 42 deg F is a bin, with the midpoint temperature of 41 deg F. For each
OAT bin, the heating load profile assigns a heating load to that temperature bin. The
actual “bin data” is the number of hours per year that the outside air temperature falls into
each specific bin.
Bin weather data is published for numerous sites, including many in Alaska. However,
no such data are published for Chalkyitsik. Therefore, actual hourly temperature data
from the Fort Yukon airport was used to construct a bin table for this site. The weather
data came from calendar years 2007, 2008, 2009 and 2010. The data were combined to
come up the average number of hours per year that the OAT in the Yukon Flats area falls
into each bin. This was done a on a monthly basis – for instance, the data in Figure 4.1
shows the spread of temperatures in the first thirteen bins; from the 85 deg F bin to the 61
deg F bin (each 2 deg F bin is labeled by the midpoint temperature).
Figure 4.1. Fort Yukon Airport Bin Data
In the calculations performed within the model, individual calculations are completed in a
series of tables that have the same format at the original bin temperature data (see
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Appendix A for sample calculations). Figure 4.2 below shows a portion of a calculation
used in this study (in this case, the calculations involved with the School in Chalkyitsik).
Figure 4.2. Partial Bin Model, Chalkyitsik School
The basis of all calculations is the heating load profiles for each building included in the
study. These load profiles reflect all the information available about the building, such as
heating equipment capacity, operating data, historical oil consumption data, size and type
of building, etc. As noted above, it is difficult to measure heat load directly, but simple
to measure oil consumption. So the first profiles generated are oil consumption profiles.
These profiles assign a specific oil consumption rate to each OAT bin. Using the
calculation format above, the model calculates the amount of oil required to heat each
building, and then compares that to known consumption – obviously, the model must be
able to back-predict known consumption if it is to be used to predict future consumption.
Subsequent calculations are done to determine how much oil/wood/recovered heat is
required to meet the predicted heating load – one table for each energy source (complex
rules determine which source is the primary, secondary, or tertiary source in each load
condition). Once the required oil (for instance) is calculated for each spot in the table, the
SUMPRODUCT function sums up all the oil required for each month. This is the basic
format of the calculations.
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Once the oil consumption profiles are verified, the oil consumption profile is converted to
a space heating load profile, by multiplying BTU of input heat (oil) times the efficiency
of the boiler/furnace to arrive at the actual heat to the space. Once these space load
profiles are generated, they are fixed. A visual example of oil consumption versus OAT
load profile is shown in Figure 4.3 below.
Figure 4.3. Oil Consumption versus OAT for Fort Yukon School
No matter what heat source is used, or how great any parasitic or piping losses are, any
proposed mini plant or biomass boiler must deliver that same amount of BTUs to the
space as the current oil-fired appliance does. Once the space load profiles are
established, the spreadsheet models the various mini plants and boilers to determine how
much energy they would consume to provide this required space heat.
As noted above, in addition to producing a set amount of BTUs for space heat, a mini
plant must produce enough additional BTUs to heat the plant itself (parasitic loss) and to
overcome the heat lost from the distribution piping into the ground (piping losses).
Finally, the model must calculate how much pumping (electrical) energy must be used to
get the heat to the buildings. Once inside the buildings, the electrical energy used for
pumping is the same for the existing systems as it would be with a mini Plant in place, so
this energy is not calculated or accounted for in the model.
Additional key load profile assumptions:
Space heating load varies linearly with OAT (a 10 deg F drop in OAT results in twice the
increase in load that a 5 deg F drop causes). There is an OAT at which space heating
stops – the OAT combined with the internal loads in the building (people, lights,
equipment) are such that no additional heat is required; above this temperature, the only
load is DHW. This value can be set individually for each building. However, in many
buildings, there is heating required above this point, for heating domestic hot water. This
DHW heating value can also be set individually for each building.
Figure 4.4 below shows the table where the user inputs their values.
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Figure 4.4. Building Characteristics
The column “min as a % of max” has to do with the space-heating load. A value of 0.020
for instance, means that at the temperature at which space heat begins (the last column),
the space-heating load at that temperature is 2.0 percent of the maximum space-heating
load. If the heat goes to a process (such as heating village water to keep it from freezing),
this value can be set to 1.000 – in other words, the load is the same at all temperatures.
The column “% of peak to DHW” is the percent of peak heating load assigned to heating
domestic hot water. Some buildings use their boilers to heat DHW, and some have
separate heaters. If the boiler heats the DHW, this column is an attempt to estimate how
much of the boiler heat goes to DHW.
4.2.2 Recovered Heat.
Just as with heating loads, a “recovered heat” profile is generated. This profile assigns a
specific village power requirement to each temperature bin. This is less straightforward
than assigning heat loads versus OAT, because the correlation between village power and
OAT is not as strong – there is also a time-of-day component to power output. However,
bin models predict long-term average performance, not hour-by-hour performance. A bin
model that predicts consumption accurately on a monthly basis is generally as specific as
is required – most utilities bill and/or report consumption on a monthly basis
A preliminary temperature versus power curve was developed for Chalkyitsik that
accurately predicts monthly consumption. However, in a Level 3 analysis, this
relationship would need to be better understood and defined.
4.2.3 Cost Estimates.
The other component required to calculate the payback of any given scenario is the
project cost. One estimate was prepared for each proposed mini plant and individual
boiler installation. The current cost estimates contain the best knowledge of the team
members regarding construction in bush Alaska. As the study level proceeds, these costs
estimates will be constantly updated to reflect the current design.
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Ultimately, actual vendor quotes and contractor’s estimates or bids will be used for the
final cost estimates.
4.3 Business Structure
A sustainable business structure is critical for a successful project. This report
recommends that the business structure be based on a for-profit model. That is the
District Heating Utility takes in more funds than it expends and it sets up a fund that will
cover the costs of annual wood harvest, delivery, boiler operations, administration,
equipment maintenance and an infrastructure and equipment replacement fund. Cost of
BTUs should be calculated based on enough funds to make the business viable.
The NSP calculation in the feasibility model demonstrates the potential amount of
savings $124,124 annually for the upper installation and $93,268 for the lower
installation. This model includes a cost of $300 per cord fed into the boiler displacing
$7.00 per gallon diesel fuel. We estimate approximately $50 per cord of labor to cut the
wood into appropriate sized sticks and to feed the boilers. So that leaves a potential to
pay $250 for a delivered cord of wood. The model also assumes that the boilers are fed
up to four times per day on extremely cold days and they are fed seven days a week when
needed. That is the only way the maximum amount of fuel oil can be displaced. Any
time the optimum amount of wood based on the demand of the system is not fed into the
boiler, then a less than optimum amount of fuel is displaced and the business revenues are
less than optimum. However, a realistic expectation would be to capture 90% of the
optimum values as there will be times when each buildings current oil heat systems will
be used.
The feasibility analysis demonstrates that a for-profit heat utility is fully feasible. The
basic benefits include almost doubling the efficiency in the use of diesel generators for
making electricity and heating buildings, keeping a total of $217,392 in the community
for jobs, infrastructure maintenance and stabilizing the cost of heating fuel for primary
public buildings if both heating systems are operated at an optimum level.
The most practical business structure, since the CVC owns the power plant and the heat
recovered from that facility is a key part of the of the Upper District Heating System, is
for the CVC to own the heat system as well. There are some business synergies that
make a lot of sense. The powerhouse operator could easily pick up the responsibility of
managing the upper and lower boiler plants. This does not mean that person will be the
one to feed the boiler; but that the person is responsible to make sure that the boiler is
fed. Feeding the boiler could be part of a wood gathering contract or be contracted
separately. Another way of operating the plant itself would be to contract that aspect as
well.
Billing of customers will be based on BTU meters in each connected building just as the
electric meters are used. This can be done in a prepay manner or as you go. Both ways
the meters must be read and billing must occur. The same office that does the electric
billing could accomplish the heat billing and bookkeeping.
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Wood delivery can be accomplished in several ways. Probably the most effective would
be for the CVC to contract for wood delivery with key wood gathers as well as be willing
to buy wood from independent wood haulers on an as needed basis. A prime contractor
would lease any procured harvest equipment from the CVC at a minimal cost, but with
key issues of insurance and maintenance spelled out in the contract and lease.
The discussion above is meant to start the process of creating a business structure. Once
a decision is made to move forward with a biomass project and the CVC has worked out
a general business structure a full-blown business plan will need to be developed with the
final design process, costs and structures.
4.4 Project Permits
The project is small enough that no permit is needed operations of the boilers. Garn
boilers are extremely efficient and have been used as the model for developing emissions
specifications for boilers of this size by EPA.
The amount of effected acres for harvest is minimal annually for the327 cords of wood
required operating both of the boiler plants. However, it is not known whether the
Alaska State Forest Practices Act for wood harvest on private lands will be enforced at
this scale of harvest or whether this will be considered subsistence wood gathering. A
discussion between AEA and DNR Forestry Department is currently underway to
determine how and when Best Management Practices (BMP) will be enforced. If AFPA
notification and BMP implementation is required then a Fish Habitat permit will be
required for all crossings of anadromous fish streams when hauling wood during the
winter.
If federal funds are used for the construction of the project or for purchase of harvest
equipment an Environmental Assessment will be required under NEPA.
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Appendix A. Wildland fires in the vicinity of Chalkyitsik.
!
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Appendix B. Harvest Equipment Examples.
Tractor pulling log trailer with grapple arm loader.
Self-loading log trailer.
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Tractor with a harvesting head for cutting, limbing and cut to length
Forcat with self-loading trailer for transporting wood to the village
Example of an armored articulated tractor
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Appendix C. Lower boiler site.
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Appendix D. Upper boiler site.