Energy and the New Reality, Volume 1:
Energy Efficiency and the
Demand for Energy Services
Chapter 9: Community-Integrated Energy Systems
L. D. Danny Harvey
harvey@geog.utoronto.ca
Publisher: Earthscan, UK
Homepage: www.earthscan.co.uk/?tabid=101807
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Community-Integrated Energy Systems
consist of one or more of:
• A network of insulated underground pipes to
distribute heat from various heat sources to
where it is needed
• A network of insulated underground pipes to
distribute cold water from a central chilling
facility or natural source of cold water to where it
is needed
• Cogeneration or a central heating plant
• Trigeneration or a central cooling plant
• Seasonal underground storage of heat or
coldness
Figure 9.1 Pre-insulated district heating
pipes in Copenhagen
Source: www.logstor.com
The world’s largest district heating
systems are in:
•
•
•
•
•
•
•
•
•
•
St Petersburg
Moscow
Prague
Warsaw
Bucharest
Seoul
Berlin
Copenhagen
New York
Stockholm
237 PJ/yr
150 PJ/yr
54 PJ/yr
38 PJ/yr
37 PJ/yr
36 PJ/yr
33 PJ/yr
30 PJ/yr
28 PJ/yr
27 PJ/yr
Figure 9.2 Percent of total space heating requirements
met by district heating
Finland
Denmark
Sweden
Germany
France
USA
Norway
Netherlands
Canada
0
20
40
60
Percent of Space Heating
Source: Maker and Penny (1999, Heating Communities with Renewable Fuels: The Municipal Guide to
Biomass District Energy, Natural Resources Canada and USA Department of Energy,
www.nrcan.gc.ca)
District Heating
• Highly worthwhile if waste heat from electricity
generation can be captured and used
(cogeneration)
• This will reduce GHG emissions only if fossil fuel
electricity production elsewhere is displaced, or if
biomass is used
• If the absence of cogeneration, district heating can
save energy through the more efficient operation
of central boilers than of individual boilers (usually
at part load) in individual buildings, although this
advantage will not exist if new, condensing boilers
are used in individual buildings
Other sources (besides electricity
production) for heat for district heating are
• Sewage water (16oC in February in Tokyo)
• Steam from incineration plants powering heat
pumps to extract heat from the exhaust of the
incineration plants (done in Sweden)
• Low-grade geothermal heat in the return branch
of a geothermal heating system, upgraded with
heat pumps
Heat can be distributed either as steam or as hot
water. Hot water provides a number of efficiency
advantages due to its lower temperature than
steam:
• Less heat loss to the surroundings
• Less sacrificed electricity production
• Lower temperature heat sources can be used (in
the absence of a heat pump, the heat source
has to be warmer than the temperature at which
heat is distributed)
The lower the temperature at which heat is
distributed, the better from an efficiency point of
view
Figure 9.3 Variation in electricity generation efficiency, overall
efficiency and electricity-to-useful heat ratio in a gas combinedcycle system as the temperature of the heat supplied to the district
heating system varies.
1.00
2.0
1.5
Total Efficiency
Efficiency
Power:Heat Ratio
0.50
1.0
Electrical Efficiency
0.25
0.5
0.00
0.0
40
90
140
District Heating Supply Temperature ( oC)
Source: Spurr and Larsson (1996)
Power:Heat Ratio
0.75
Strategies for reducing the required
supply temperature:
• Upgrade building thermal envelopes so that
radiators do not need to be as hot as otherwise
while still providing sufficient heat, and (in new
developments) use radiant floor heating (which
reduces the required temperature to 30oC or so)
• Use larger and more effective heat exchangers
between the district heating hot water flow and the
building hot water flow, so that there is less of a
temperature drop between the two
• Increase the flow rate and/or use a peaking plant to
boost the supply temperature during the coldest
month
Upgrading old District Heating Systems
In many countries of Eastern Europe and the Former
Soviet Union
• Distribution systems are steam-based and lose up to 30
per cent of the heat that they carry
• The systems are also over-sized and so tend to operate
at partial capacity (which lower efficiency)
• The buildings that they serve often lack thermostatic
controls and are poorly insulated
Thus, in these countries the large potential energy savings is
not in the expansion of district heating with cogeneration, but
in the improvement of the efficiency of the existing system
and of the buildings that they serve
Summary for District Heating
• Comparing modern district heating systems with
modern on-site heating (using condensing boilers),
there is little if any efficiency gain if generating heat
centrally
• What little gain there is could be offset by
distribution losses
• Reducing the losses in existing systems can yield
large savings
• Taking heat from the generation of electricity with
fossil fuels (cogeneration) will not reduce fossil fuel
use if the alternative for electricity generation is to
use renewable energy sources (such as hydroelectric power)
DH Summary (continued)
District heating can still provide a number of energy
advantages over on-site heating
• Easier opportunities to store heat (within the network
and in insulated thermal reservoirs or underground)
• Easier to switch fuels for heating (potentially
important if we make the transition to hydrogen in
the future)
• Easier to directly use renewable energy
- biomass for cogeneration
- electric heat pumps to use excess wind-generated
electricity when it is available
- geothermal heat
Non-Energy Advantages
of District Heating
• Quiet and vibration-free heating at building sites
• No need for emission of exhaust gases at the
building site
• Reduced upfront costs for the building
developed
• Savings in space, maintenance and insurance
costs
District Cooling
• Centralized operation of multiple chillers, each
running at the fraction of full load that maximizes
overall efficiency, can yield large energy savings
compared to operation of equal sized chillers in
individual buildings
• The use of larger chillers in centralized plants
yields further savings, as larger chillers are more
efficient than small chillers
• Savings from these two factors of up to 45%
have been achieved
Non-energy advantages
of district cooling
• Quiet and vibration-free cooling at building sites
• Savings in space, maintenance and insurance
costs
• Reduced upfront cost for the building developer
• No need for cooling towers on individual
buildings (reducing on-site costs and risks
related to legionnaires disease)
• Frees up roof space for roof-top gardens or solar
collectors (PV or thermal)
Societal Cost of District Cooling
Systems Compared to
On-site Chillers
The total cost of district cooling can be less than for
cooling of individual buildings, in spite of the cost of
installing a district cooling pipe network, because
• Less total cooling capacity is needed (the peak
loads of individual buildings do not all occur at the
same time, so the system peak is less than the sum
of the peaks of all the buildings in the system)
• Less total backup capacity is needed (it is not
uncommon to have 2 equal-sized boilers or chillers
in a given building, with one serving as backup. In a
centralized system with 10-12 heating or cooling
units, there might be only 1-2 backup units)
• Unit purchase costs are less for larger boilers and
chillers
• Smaller operation and maintenance costs per unit of
heating or cooling capacity
Cogeneration and Trigeneration
Cogeneration Sizing Options
• For one case study, energy savings is maximized if
the cogeneration unit is about half the size that
maximizes the area under the load duration curve
• Net energy savings initially increases as hours of
operation increases and some heat is thrown away,
because more less-efficiently generated electricity is
displaced with more hours of operation
• As more heat is thrown away, the effective
(marginal) efficiency of electricity generation
decreases. When the marginal efficiency drops to
that of the central powerplant x transmission
efficiency, a further increase in the hours of
operation will increase the overall energy use
Figure 9.4a,b Cogeneration sizing strategies: (a) maximizing the useful heat
delivered (by maximizing the area of the rectangle under the load duration
curve), (b) reducing the size of the cogeneration unit but maximizing the
number of hours that it operates per year.
(a)
heat
heat demand
heat cogeneration “maximum rectangle”
with thermal power Q and annual use U
Q
U
load duration
heat
(b)
load duration
Source: Voorspools and D’haeseller (2006, Applied Thermal Engineering 26, 1972–1981,
http://www.sciencedirect.com/science/journal/13594311)
Figure 9.4c,d Cogeneration sizing strategies: (c) Sizing the cogeneration
unit as in (a) but with longer operation and some wasted heat; (d) using
multiple units.
heat
(c)
heat demand
heat
heat not used
load duration
heat
(d)
heat demand
cogeneration 1
cogeneration 2
cogeneration 3
load duration
Source: Voorspools and D’haeseller (2006, Applied Thermal Engineering 26, 1972–1981,
http://www.sciencedirect.com/science/journal/13594311)
Trigeneration
• Involves taking heat from a steam turbine and using
it to drive an absorption chiller
• Taking heat results in reduced electricity output
• The sacrificed electricity could instead by used in an
electric chiller to produce chilled water for cooling
purposes
• An absorption chiller, while not needing electricity for
the basic cooling operation, requires greater
auxiliary electricity use than an electric chiller for the
operation of the cooling tower
• The net result is that it is often better to maximize
electricity production, even though this requires
throwing away some waste heat, and using the
extra electricity in large and efficient electric chillers
Pollutant emission with cogeneration
These can go up or down compared to separate
production of heat and electricity, depending on
the efficiencies and emission factors for the
cogeneration facility and the alternative systems
Differences in human exposure to emissions,
related to the location and altitude of the emission
sources, must also be considered.
Cost savings with decentralized
electricity production through cogeneration
Costs with decentralized production will tend to be lower
due to
• Reduced transmission and distribution costs (from
an average of $1300/kW to $100-200/kW)
• Less needed powerplant capacity due to lower
transmission and distribution losses
• Less needed backup capacity (4-5% instead of
15%) due to having many smaller units subject to
random failure
• Revenue obtained by sale of heat produced through
cogeneration