HomeMy WebLinkAboutAngoon Multifamily Apartments Biomass Pre-Feasability Report 08-22-2014-BIO
Angoon
Multifamily
Apartments
Biomass
Pre-‐feasibility
Report
Submitted
to
THRHA
and
AWEDTG
Greg
Koontz,
PE
Bill
Wall,
PhD
of
Alaska
Wood
Energy
Associates
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Table
of
Contents
1.0
EXECUTIVE
SUMMARY ...................................................................................................3
1.1
Acknowledgements ................................................................................................................3
1.2
Objective:...............................................................................................................................3
1.3
Sources ...................................................................................................................................4
1.4
Scope ......................................................................................................................................4
1.5
Resource
Assumptions ............................................................................................................5
1.6
Summary
of
Findings ..............................................................................................................6
Project
Performance ......................................................................................................................10
1.7
Next
steps ............................................................................................................................11
2.0
TECHNICAL
SUMMARY.................................................................................................12
2.1
Existing
Conditions:..............................................................................................................12
2.2
Wood
Fuels
/
Wood
Fired
Heating
Equipment ......................................................................13
2.3
Proposed
Conditions,
Sc
1 .....................................................................................................13
2.4
Scenarios
2
through
4 ...........................................................................................................14
Scenario
2 ...........................................................................................................................................14
Scenario
3 ...........................................................................................................................................14
Scenario
4 ...........................................................................................................................................14
2.5
Energy
Savings ......................................................................................................................14
2.6
Cost
Estimate:.......................................................................................................................15
3.0
INTERCONNECTIONS
and
Thermal
Storage ..................................................................17
3.1
Interconnections
and
the
Impact
on
Construction
Cost .........................................................17
3.2
Thermal
Storage ...................................................................................................................23
Appendix
1.
Photos
and
site
map ........................................................................................24
Appendix
2.
Brochure
for
MES
OkFen
Pellet
Boilers .............................................................28
Appendix
3.
Portion
of
Tech
Brochure
for
Pex
Piping ...........................................................30
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1.0
EXECUTIVE
SUMMARY
1.1
Acknowledgements
This feasibility study was supported by the Alaska Wood Energy Development Task Group and
administered by the Fairbanks Economic Development Corporation. The THRHA supported the field
study with information and assistance while in Angoon.
1.2
Objective:
The objective of this report is to document the results of a pre-feasibility study performed for the
Tlingit Haida Regional Housing Authority (THRHA). The target buildings are 8 multi-plex residential
buildings and a community center in Angoon, Alaska. Angoon is not on the road system, but it is
accessible by ferry and float plane from Juneau.
In this report, we distinguish between a multi-plex (duplex, triplex, etc) and a residence – when
we use the term “residence” we mean a distinct “home” within a multi-plex. A duplex has two
residences, and so on. The buildings in the THRHA are currently heated with oil, one oil boiler per
residence (and one for the community center). The primary subject of the study is the feasibility of
constructing a wood-fired heating plant to serve all nine buildings in the THRHA complex.
A secondary objective is to evaluate the installation of a wood-fired boiler into a “typical” house.
This is not part of the original scope for this work, but interest was found in the community for converting
to pellet boilers if a supply of pellets were to exist in the community. Thus, the authors decided to give an
example of costs and paybacks for a residence.
Because there are no wood chips available in the area, and the THRHA is not interested in stick-
fired boiler, this study evaluates only pellet fired boilers.
Feasibility studies are often classified as Level 1 (L1), Level 2 (L2), or Level 3 (L3). Level 1
studies consist of very rough calculations on a small number of important metrics (unit fuel costs, etc).
At the other end, L3 studies are commonly called “investment grade studies”; the level of detail and
calculation is so high that one could use the results of an L3 study to get an outside entity to fund the
implementation of the project. Level 2, then, is the bridge between L1 and L3; it is a screening study
done to determine if it is worth the time and expense to initiate an L3 study. Level 3 studies are generally
quite expensive, and thus not entered into lightly. The L2 study helps decision makers determine which
aspects, if any, of a proposed project are worth the expense of an L3 study.
An L1 study can be done remotely; an L2 study requires at least a minimum amount of site
observation of existing conditions, conversations with the affected parties, and research with second-order
parties (local foresters, vendors, local contractors, etc). This is a Level 2 study, although classified as pre-
feasibility.
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Sustainability, Inc (SI) and efour, PLLC (efour) perform L2 and L3 studies across the state of
Alaska, from cities to small villages in the bush. We use the same performance and economic models for
each type of study. The primary difference between the two studies is the quality of the inputs, which is
generally a function of how much time has been spent gathering information, and the depth of that
information.
1.3
Sources
The primary sources of information for this study are data collected on site by SI, and data
provided by the Fairbanks Economic Development Corporation (FEDC). Data collected on site by SI
include existing site conditions, equipment name plate data, current energy cost data, and, equally
important, information gathered by talking to the local stakeholders in the Village.
In addition to the site knowledge gathered by SI, additional biomass boiler performance and cost
data have been accumulated over the past several years from working with local engineers and
contractors, and from completing multiple L2 and L3 wood-fired feasibility studies. Hourly weather data
for the performance model was extracted from data collected and reported by the nearby Juneau Airport.
1.4
Scope
In Angoon, the scope of this report is limited to the THRHA complex and one typical house;
currently, each residence within each multi-plex has its own boiler. The boilers feed two or more zones
of heating within each residence, using hydronic baseboard heat.
Biomass heating systems can be expensive to install; the economics generally work better for
larger buildings, or where two or more smaller buildings can be grouped together and served by a single
biomass boiler, using buried piping between the buildings to distribute the heat. Significant parts of the
cost of a district heating plant (DH Plant) are the interconnections to the individual boilers; in this case,
the fact that each residence has a boiler (as opposed to one per multi-plex) potentially increases the capital
costs of a project. Thus, more existing boilers means more interconnections and more cost, but no more
additional savings. Interconnections are discussed in detail below.
For this report, we have modeled the performance on one DH Plant, and one individual building.
The DH Plant contains all nine buildings; subsets of smaller groups of multi-plexes were not analyzed.
Smaller DH Plants would have even less economy of scale, and the THRHA showed no interest in having
two Plants rather than one. The individual building modeled is the “typical house” referred to above.
Although only one cluster of buildings for the DH Plant was evaluated, four different variations
of that Plant (each variation is called a Scenario, abbreviated as Sc) were constructed. The performance is
nearly identical between all four; the primary difference is the capital cost. This DH Plant has three
disadvantages compared to many other DH Plants SI and efour have evaluated:
1. Not much economy of scale: The total oil consumption for the nine buildings is about 11,400 gallons per
year. In many villages, a single building (usually the high school) uses two or more times as much oil as
the entire THRHA complex.
2. High cost of wood: In most cases, we evaluate stick wood, and if feasible, wood chips. Both are less
expensive on a BTU basis than wood pellets (although not nearly as convenient as pellets).
3. Many small buildings, each with multiple interconnections required. Each building is small (in terms of
oil consumption), which makes it harder to pay for the cost of the piping required to get to the building,
and the interconnections to the multiple boilers.
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For these reasons, we looked at a number of ways to strip costs out of the DH Plant without
materially affecting performance. One could say that the Scenario 1 DH Plant is the “Cadillac”, and each
successive DH Plant (Sc 2 through Sc 4) strips out equipment and features, which, while nice, may not be
strictly required for the THRHA Plant. The differences between the Scenarios are detailed in Sections 2
and 3 below.
1.5
Resource
Assumptions
As noted above, the only form of biomass modeled in this report is wood pellets. Wood chips are
not available, and the THRHA was not interested in stick-fired boilers.
Figure 1.1 below shows the assumptions that have been made for the existing fuels in the Village
(oil and electrical energy), in the units in which they are sold:
Figure 1.1
Figure 1.2 shows the assumptions made for the cost of wood fuel, in various forms.
Figure 1.2
Because each form of fuel has different heat content and is sold in differing units, direct
comparisons of the data in Figures 1 and 2 are very difficult. To make the comparison simple, all these
energy sources are converted to a common unit, one million BTU (1 mmBTU).
To make the comparison even more relevant, the conversion efficiency of each source has been
factored in. In this case, the conversion efficiency is the boiler efficiency. It is different for each fuel –
using drier wood results in better boiler efficiency, and the oil boilers have their own efficiencies as well.
In Figure 1.3 below, therefore, the mmBTUs references are those coming out of the boiler into the space,
not the gross heat content of the fuel going into the boiler.
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Figure 1.3
As Fig 1.3 shows, electrical energy is about one half again as expensive as No.1 oil, which, in
turn, is about two and quarter times more expensive than wood pellets. The wood pellets are assumed to
contain 8,162 BTU/lb.
1.6
Summary
of
Findings
The following Figures summarize the performance and economic modeling that SI and efour
performed. The model is based on a pellet cost of $300 per ton delivered to Angoon, but that may vary.
The summary results are presented twice and include a graph, which shows the sensitivity of net simple
payback to pellet cost.
Figure 1.4 overall economic summaries with pellets at $300/ton.
Figure 1.5 shows the same metrics with pellets at $360/ton.
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Figure 1.5
As footnote (3) indicates, we have not estimated any increase in annual maintenance with the installation
of the pellet-fired boiler. This is because the fuel consistency is so high, the material handling so smooth,
and the pellet boilers so reliable that in essence it is as close to being as automatic as an oil fired boiler as
a wood fired boiler can get. We would expect that whoever currently cares for the oil boilers will also
take care of the pellet boilers, and that no significant additional time or parts expenses will be incurred.
Experience has shown that pellet boilers are reliable enough to be used in residences; this would not be
the case if there were significant maintenance and expense required.
The net present value and benefit to cost calculation assume a 20 year project life; during that
period, costs must be assumed to escalate. Figure 1.6 below shows the proposed escalation factors.
Figure 1.6
Using these factors results in the following 20 year cash flows:
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Figure 1.7
With these escalation factors, the savings increase by a factor of 2.4 over 20 years.
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Of course, a primary variable in the financial analysis is the cost of pellets. For that reason, as
noted above, we include a graph that shows the effect of pellet cost on net simple payback:
Figure 1.8
Figure 1.9 below is a summary of the cost estimate. The complete construction estimate is
contained in Section 2:
Figure 1.9
There are three notes that must be amended to Figure 1.9:
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1. In order to reduce costs, Sc 3 and Sc 4 assume that no final study is done (or a minimal update of this
study), for these Scenarios, this line item is cut by 50%, from 7.0 percent to 3.5 percent.
2. Sc 1 and Sc 2 assume an outside construction manager/administrator is living and working in the Village
during construction, actively managing the project. However, in talking to THRHA, they indicate they
may have the capacity to take on these tasks. Therefore, in Sc 3 and 4, the on-site construction
management is eliminated. There is still money for an engineer / manager to answer site questions,
review submittals, etc.
3. In order to minimize soft costs, for Sc 3 and Sc 4, the contingency was cut by 25 percent (therefore, it is
0.75 * 0.75 = 0.56 of construction cost).
Finally, Figure 1.10 shows a financial / performance summary for a typical house in Angoon.
This is based on a number of assumptions as we were not able to get an amount of oil used annually for a
large house. When individual buildings are evaluated, the same cost factors are used, but with a different
reporting format, because many elements that go into a DH Plant do not apply when looking at a single
boiler / single boiler configuration. In addition, because this is a strictly residential application, many soft
costs are not applied. There is no external construction administration, for example, and no design/study
costs (the contractor is assumed to be competent to design/execute the interconnection). The boiler is
assumed to be in the residence, which eliminates site pipe, etc. Figure 1.10 shows the typical house
results at $300 per ton:
Figure 1.10
Project
Performance
Both the DH Plant (Scenario 4) and the typical house have a net simple payback of about nine
years. For a Housing Authority looking to control energy costs, budget for the future, and create an
alternative to oil, this is likely a reasonable payback – renewable energy projects often have benefits that
extend beyond the merely financial. For a home owner, a nine year payback may be harder to swallow.
In both cases, however, the benefit to cost ratio is well over 1.0, the minimum standard (2.5 and 3.0,
respectively).
Although financial forecasts cannot be made on “maybes”, there is also possibility that as wood
pellets become more common in SE Alaska, there will be more suppliers and more economy of scale,
causing unit prices to drop below $300 per ton.
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1.7
Next
steps
THRHA must evaluate these results using their own investment metrics and criteria. However, it
appears, based on the findings of this report, that if THRHA can form a team of the right professionals
and contractors, and design a lean delivery method that maximizes the assets of the Authority, this is
certainly a viable project, financially.
The next step, therefore, is to work with the Authority to determine what elements are required
for a project that they consider a “success”, and figure out to deliver those elements within a cost and
performance structure appropriate for the community.
In addition to financial performance, SI and efour believe that wood energy projects generate
benefits to the Village beyond the obvious monetary ones; we call these VBECS (value beyond energy
cost savings), a term borrowed from the Rocky Mountain Institute. Among these VBECS are:
• Use of renewable resources
• Reliance on local, rather than remote energy sources
• Reduced carbon footprint
• Reduced secondary emissions (NOx, S, CO, etc)
• Increased fuel price stability (for future budget planning)
• Energy money spent remains in the local economy
There are, no doubt, others as well. As was noted above, a Level 2 study is a screening study,
meant to provide enough information to the stakeholders to A) determine how to proceed next, B)
determine whether to proceed, or C) halt the project until conditions improve. This study provides the
information needed for the THRHA and other stakeholders to make these decisions. The authors believe
that the project is strong enough financially and with VBECS to immediately apply to the AEA
Renewable Energy Fund for a grant to support this project.
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2.0
TECHNICAL
SUMMARY
2.1
Existing
Conditions:
The following statistics in Figure 2.1 summarize the existing conditions in the THRHA complex:
Figure 2.1
The proposed pellet-fired DH Plant would displace over 99 percent of the current fuel
consumption; however, the existing boilers would remain in place as back up in all Scenarios. In some
Scenarios, they would be the only back-up. This is explained in more detail in subsection 2.4.
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2.2
Wood
Fuels
/
Wood
Fired
Heating
Equipment
Figure 2.2 below shows the properties of the pellets that were used in this study:
Figure 2.2
The most pertinent value in the Figure is the net useable heat content, 8,162 BTU/lb. Because of
the low moisture content (4 percent), pellets are by far the most energy-dense form of wood fuel.
There are a number of manufacturers of pellet boilers; the basis of design boilers used in this
study are the PES series of boilers made by Maine Energy Systems (MES). There are eight sizes in the
PES series, ranging from 41 kBTU/h to 191 kBTU/h (output).
The basic system components include:
o A pellet bin, which holds bulk amounts of wood pellets.
o This bin is kept filled by periodic deliveries to the Village by truck and ferry
o There are a number of delivery and loading methods once within the Village
o A means of getting the pellets from the bin into the boiler (material handling)
o For MES, this is a vacuum system; the bin may be up to 66 ft away from the boiler
o The boiler
o The boiler uses onboard controls to modulate the firing rate to meet heating demand
o Will remain on and operating as long as the bin is kept filled, and no fuel fouling occurs
o Is a “hands-off” unit
o A vent or boiler stack
o This vents the products of combustion and boiler emissions into the air through an
elevated stack or vent pipe
o May or may not include additional emissions control equipment
Examples of the MES boilers and accessories are included in Appendix A
2.3
Proposed
Conditions,
Sc
1
As noted above, the thermal performance of all four Scenarios is about the same. What changes
from Scenario to Scenario is the implementation cost, the level of complexity and sophistication, and the
level of involvement of THRHA. Subsection 2.3 outlines the configuration of Scenario 1, while
subsection 2.4 details how each subsequent Scenario deviates from Sc 1. Some of the features of
Scenario 1:
• A new 8 x 34 containerized pellet boiler plant, piped and wired at the manufacturer, and delivered to the
site
• The container includes primary piping, two boilers, expansion tanks, ash container, controls as specified,
lights, and all electrical lighting and wiring – plus room for variable speed secondary pumps, secondary
pumps and a secondary heat exchanger and expansion tank
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• An oil-fired boiler is included in the new DH plant to cover failures and high peak load periods.
• A new slab constructed to house the container, with pipe and wiring routed to slab
• Distribution piping from the slab to each building
• Connection from the distribution piping to each of the 23 boilers
• One control valve, HX and flow meter per interconnection (see Section 3)
• DDC controls as required to control the Plant and interconnections
• The new piping will be tied into the existing systems in such a way that will always take the “wood heat”
before taking oil/propane heat (Figure 3.2 below)
• However, if for any reason the wood fired system cannot meet load, the existing boilers will automatically
start and fire as required to meet load
• Full design and construction administration/management services, with 7.5 percent contingency
Figure 3.1 in the section below shows the “Scenario 1” configuration, as it applies to building
interconnections. The means of interconnection is one of the primary differences between the Scenarios.
2.4
Scenarios
2
through
4
Scenarios 2 through 4 differ from Scenario 1 in the following ways:
Scenario
2
• The oil fired back-up boiler at the DH Plant is eliminated, and the existing boilers are assumed to provide
back-up
• The interconnection to each boiler is revised from Figure 3.2 to Figure 3.3 – a much less complex and
expensive configuration
• The controls are all assumed to be local, with no DDC control
• The secondary pumps are assumed to be constant speed (they are so small, there is little energy penalty)
• In Sc 2, we eliminated some manufacturer’s boiler controls, and some features in an attempt to lower
costs, but it did not get the financials to a viable point, and it eliminated some desirable features, so these
options were added back in in Sc 3 and 4 (thus the container costs are slightly higher)
Scenario
3
All of the Sc 2 modification, plus
• The interconnection detail remains Figure 3.3 – with 23 total connections
• This simplifies the connections and eliminates the need for DDC controls; however, there are still 23
connections required
• Final study and design fees are cut in half; this assumes that THRHA can allocate some resources to the
project
• External construction management is handled by THRHA
• This local control allows the contingency to be decreased by 25 percent.
Scenario
4
All of the Sc 3 modification, plus
• The interconnection detail becomes Figure 3.4 or Figure 3.5 (depending on Village needs) – cutting the
total connections to 11.
2.5
Energy
Savings
Figure 2.3 below summarizes the energy consumption, existing and proposed, on a monthly basis:
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Figure 2.3
Figure 2.4 shows the same data for Sc 2 through Sc 4 (the savings for these Scenarios does not
change, only the implementation cost):
Figure 2.4
2.6
Cost
Estimate:
The construction cost estimate is provided below in Figure 2.5. These are commonly referred to
as the “hard costs”. The remaining soft costs, fees, permits, etc, are detailed in Section 1.
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Figure 2.5
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3.0
INTERCONNECTIONS
and
Thermal
Storage
3.1
Interconnections
and
the
Impact
on
Construction
Cost
One of the most important features of a District Heating system is the interconnection between
the DH system and the existing buildings systems. These interconnections can range from complex (and
expensive), to very simple, often with one or more variations in between. The simpler the
interconnections get, the less they cost. However, even the least expensive connections constitute a
significant amount of money. The goal, therefore, is to first minimize the number of connections, and
then apply the lowest appropriate level of technology for each connection, minimizing overall
construction cost.
One thing that all possible interconnections should have in common is that no operator
intervention should be required in the event that the DH Plant fails, or that the biomass boilers cannot
meet the peak loads in very cold weather. At the same time, in periods of the very high heating load, the
system should ideally use 100 percent of the capacity from the biomass boilers first, and use the “back-
up” oil only to cover the peaks.
The following is a summary of some of the things all interconnections should have in common:
• In all systems, it is preferred that a heat exchanger is installed between the distribution piping and the
building piping. Many building systems use glycol, while the DH distribution systems use 100 percent
water. The heat exchanger provides a physical barrier between the two systems to prevent cross-
contamination, while allowing heat to cross over. A control valve is used on the distribution return line to
control the hot water return water temperature on the building side of the exchanger.
• Interconnect in such a way that the building hot water return is heated before it gets to the building
boiler(s). The basic premise is that the temperature setpoint for the building return water coming off the
heat exchanger is 5 deg F (for example) hotter than the setpoint of the boiler itself. The result is simple; if
the biomass system heats the building return water to a temperature at or above that of the boiler setpoint,
the boiler will not come on, HOWEVER,
• If for any reason, the biomass system cannot heat the building return water all the way to boiler setpoint
(failure or very cold weather), the return water temperature will begin to fall, and when it falls below the
boiler setpoint, the boiler will automatically add enough heat to make its setpoint.
• This ensures that 100 percent of the available biomass heating capacity is utilized before any back-up fuel
is used. Once the load drops to the point where the heat exchanger can heat the return water to above the
boiler setpoint, the building boiler will stop firing.
Given the list above, for any given site, there can be many possible variations in the way
buildings are connected. In general, the size of the DH Plant, the number and nature of the end-users, and
the sophistication of the individual building controls also factor into the decisions on how to interconnect
the buildings.
• For large DH Plants with extensive piping systems, the cost of the pumping energy required to distribute
the heat through the pipes is significant. For that reason, variable speed secondary hot water pumps are
used. At any load less than 100 percent, variable speed pumps cut the pumping energy by 1/4th to 1/8th
of the energy of constant volume system at the same flow. In these situations, the preference is to use a
good quality motor-actuated control valve to control the flow at each building (actually, at each
connection – so there may be more than one per building).
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• A motor-actuated valve generally pre-supposes that the building has a pneumatic or DDC control system
to control all of the HVAC systems. Larger, more sophisticated buildings tend to have such control
systems; smaller buildings use only local controls.
• For a DH Plant that serves multiple buildings with multiple owners, a metering system is installed. This
allows the DH Plant to charge the end-users for the exact amount of heat the use.
In Angoon, the existing boiler system configurations present a number of challenges for keeping
the interconnection costs as low as possible. The overall housing complex consists of duplexes, triplexes,
and quadplexes, as well as the community center. In the multi-plexes, there is one boiler per residence,
and each boiler can serve only the residence it was meant to serve. This means that instead of nine
connections (the number of buildings), the current configuration requires 23 connections:
• (4) duplexes * 2 connection = 8 connections
• (2) triplexes * 3 connections = 6 connections
• (2) quadplexes * 4 connections = 8 connections
• (1) community center * 1 connection = 1 connection
The total from above is 23 connections. Even if we reduce the unit cost to as low a value as
possible, 23 connections represent a large sum of money. For Angoon, therefore, four Scenarios were
modeled. Sc 1 was the “base case”, which represents the configuration of a large, complex, multi-user
system with above average sophistication.
For Scenario 2, some of the sophistication was reduced, some of the equipment, and thus some of
the money. In Scenario 3, there is more reduction, and In Scenario 4; the assumption is that some level of
re-piping has been done in each building, such that a single point of connection can be made (instead of 2
– 4 per building). The exception is the two north-most duplexes. The individual boilers in these
buildings are not in the same room, so they will always have two connections. Figures 3.1 through 3.5
below show some of the interconnection options for Angoon.
Figure 3.1
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Figure 3.1 shows a typical Angoon duplex arrangement. Each boiler can serve only one
residence, and there are two zones for each residence, A & B. The boilers cannot cross over and serve the
other residence in the event of a failure. There is, as noted above, one boiler per residence.
Figure 3.2
Figure 3.2 above shows the most expensive means of interconnecting; this would correspond to
the pricing assumptions made in Scenario 1. This assumes that 1) each boiler must be connected
separately, 2) that each end-user must be sub-metered, 3) an external control system exists in each
building, and 4) the secondary pumping is variable speed.
As noted above, the system is configured to heat the building hot water return before it gets to the
boiler. The 2-position valve directly below the pump would be closed, and the other two 2-position
valves open; building hot water return flows to the heat exchanger. The building HWR would be heated,
and returned to the boiler loop just above the point it enters the boiler. Because the HWR is now hotter
than the setpoint for the boiler, the boiler never fires. The modulating valves at the HX control the
building HWR temperature, and the flow meters at each HX allow the DH Plant operator to measure the
exact amount of heat consumed by each residence within the multi-plex.
This is clearly over-kill for the situation in Angoon. First, the amount of flow in the system is so
small that the secondary pumps are less than one horsepower. Thus, while electricity is still expensive,
there is no need to use expensive control valves at each HX, and no need to make the secondary pumps
variable speed. Second, there are no building control systems to control all the valves shown above.
Third, there appears to be no reason to individually meter the heat to each residence.
Item 3.2 above means that a single heat exchanger can be used for each multi-plex – however, we
do still have to hook up to each boiler individually. There are still 23 separate connections, although each
one is significantly less complex and less expensive than the interconnections priced out in Scenario 1.
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This is the basic configuration assumed in Scenarios 2 and 3 (there are other differences between 2 and 3
that affect price; these are detailed elsewhere in this report).
Figure 3.3
In Figure 3.3, all of the actuated valves have been replaced, except that the building HWR
temperature is still controlled by what we have labelled as a self-controlled valve. This valve is
controlled by the expansion and contraction of a fluid within a “sensing bulb” strapped to the pipe and a
fluid-filled line from the bulb to the actuator itself (light dashed line). The hotter the building HWR gets,
the more the fluid expands; the resulting pressure moves the actuator in the valve to modulate to control
the HWR temperature – no external power source or controller is required. The level of precision is not
as high, but is more than enough for the application.
The 2-position valves are replaced with manual valves. They would normally be left as shown
(the two horizontal valves are open, the vertical valve is closed). These valve positions would only be
reversed if, for some reason, a resident wished to isolate their boiler from the DH Plant.
A single HX is used for the multi-plex, although as noted above, there are still two connections
required, and the boilers cannot back one another up (each boiler can still only serve its original
residence).
In talking to the Villagers, we learned that there was some thought of converting the multi-plexes
such that each one had only one boiler, regardless of the number of residences. This would obviously
make the DH Plant much less expensive; in addition to using the much less expensive interconnections,
the number on interconnections decreases from 23 to 11 (the two northernmost duplexes cannot easily be
converted to one boiler, so 7 buildings * 1 connection + 2 buildings * 2 connections = 11 connections).
No indication was given on the potential timing of such a measure, but the model shows the effect
that such a re-configuration of the existing systems would have. Although Sc 2 and Sc 3 have been
reduced, the combination of 1) a small overall amount of “base” oil consumption, 2) a long distance
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between the buildings (thus lots of buried pipe), and 3) a very large number of interconnections was
making Scenarios 1, 2, and 3 appear unattractive, financially.
A final model was run, assuming only 11 interconnects. Rather than assume that the existing
boilers had been removed, and a single new boiler added, two ways were presented to reduce the number
of connections to one per building while leaving the existing boilers in place. Figures 3.4 and 3.5 show
the basic piping schematics.
Figure 3.4
In this configuration, a new building return water (HWR) header is installed. All four HWR lines
tie into the header. A new secondary building pump is shown, but may not be needed (more field work
would be required to determine this). The single heat exchanger can now heat all of the hot water for the
whole building with one connection to the new header.
At each boiler, a solenoid valve is added – the valve would be open whenever the associated
PHWP was operating. Thus if Residence 2 did not need heat, PHWP-2 would be off. The associated
solenoid would close, preventing water that should be flowing only to Residence 1 from flowing through
B-2.
Since all of the return lines flow into a common header, there is no need for two HWR lines to
each boiler, so one line is removed. This assumes the remaining line can handle the entire HWR flow. ,A
subject for future field work.
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This configuration represents the aim of reducing the number of connections to one. However,
each boiler can still only serve the original residence it “belonged to” – the supply lines off each boiler are
still separate and dedicated.
Figure 3.5, therefore, shows an added building hot water supply header as well. In this
configuration, there is a single point connection, and any boiler can serve any load in the building.
Figure 3.5
NOTE that while Scenario 4 takes credit for the cost savings associated with reducing the number of
connections from 23 to 11, it DOES NOT include the costs associated with adding the header or headers
(depending on final configuration). Since THRHA was considering something similar, the change was
modeled to show how it would affect the economics of the DH Plant.
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3.2
Thermal
Storage
When referring to a hot water heating system, thermal storage simply refers to a hot water tank, which
stores hot water (thus thermal storage). The importance of using thermal storage in a biomass-fired
heating plant varies, depending on the form the wood.
Stick fired boilers are batch fed, with an operator adding batches of fuel as needed. In this case, thermal
storage is almost a requirement. This is because once the fuel starts burning; it is impossible to modulate
the rate of burn to match the heat load. Instead, the amount of fuel added is sized to heat the thermal
storage, while the pumping/piping system extracts heat from the thermal storage as needed to match the
load. The thermal storage “de-couples” the rate of burn from the variations in heating load.
Chip fired boilers are automatically fed, and can modulate to meet load. It would seem then that they
would not need thermal storage, and in fact many chip systems are installed without storage. Where
storage really provides value in a chip system is when the heating load varies over a very large range, as
they do in Alaska. The boiler can only turn down to about 25 percent of full load capacity – below that
heating demand, the boiler will cycle off until hot water temperature drops a set amount, and then restart.
A good chip boiler will auto-restart, but they still will not cycle On and Off like an oil boiler, for instance.
Once the fuel is in a solid fuel burner, it will burn whether the heat is needed or not. They take a long
time to cool down, and an equally long time to heat back up. Finally, if the fuel is very wet, the auto-start
may take a long time, or in extreme cases, fail. A storage tank can help limit the cycling, the boiler now
modulates to keep the tank at setpoint, and as above, the system extracts heat from the tank as needed.
The thermal storage can keep the boiler running at very low levels rather than cycling.
The performance of pellet boilers is as close to an oil-fired boiler as is possible with wood. The fuel is
very dry, and easy to re-start. The boilers are generally much smaller than chip boilers, so there is not
much fuel in the unit at any given time. They are as heavy, so they heat up much quicker. While a
thermal storage tank would, again, limit cycling at low loads, pellet boilers generally do not need a tank to
modulate and follow loads. However, all good pellet boilers have an auto-cleaning feature, where they
clean the tubes, generally once a day. Many models cannot do this while the boiler is actually running, so
they shut down. Such boilers generally use thermal storage to “bridge over” the time they are off. The
Okofen boilers sold by Maine Energy Systems do not shut down while cleaning, and so while thermal
storage can be added to the MES boilers, a determination must be made for storage based on the
application, need and cost.
In a district heating application, one is likely to have two, or even three boilers. If each boiler has a 4:1
turndown, then a plant with two boilers can turn down 8:1, and a three-boiler plant can turn down 12:1.
The buried piping provides a small but constant load, and even on warm days in AK, nights can be cold.
So, in these DH situations, thermal storage is not added, the combined turndown of the boilers is
sufficient to minimize cycling.
In a small single building or residential application, a small (50 – 90 gallon) tank may be added, even for
an MES boiler, space and money permitting.
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Appendix
1.
Photos
and
site
map
Figure
1.
Site
plan
with
potential
location
of
the
Pellet
District
Heat
Plant.
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Figure
2.
Each
building
has
as
many
boilers
as
there
are
apartments
in
the
building.
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Figure
3.
Example
of
a
duplex
with
two
boilers,
two
oil
tanks
and
two
stacks
on
top
of
the
building.
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Figure
4.
Example
of
a
four
plex.
See
the
4
stacks
on
right
from
adjacent
4-‐plex
and
the
boiler
room
for
the
building.
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Appendix
2.
Brochure
for
MES
OkFen
Pellet
Boilers
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Figure
1.
Two
page
Brochure.
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Appendix
3.
Portion
of
Tech
Brochure
for
Pex
Piping
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