District Energy - 2010 4th Quarter

potentially supply the total district heating supply temperature requirement for an ‘ultra-low-temperature’ district hot water system. However, economic considerations (district system supply/return temperature difference or “delta T”) will push the recommended district heating supply temperature up. The economically optimal level depends on case-specific factors, but in many situations a district heating system designed for supply/ return temperatures of 200/115 F could be economically feasible, particularly where the customer base is comprised largely of new buildings designed for low temperatures. Even if the peak district heating supply temperature is higher, it can be feasible to capture low-grade heat for pre-heating and/or to meet off-peak supply temperature requirements.

Figure 1. Comparison of Achievable Low-Temperature District Hot Water Supply and Return Temperatures to Potential Sources of Heating Energy.

1,100 1,000

900

800

Degrees;F

700

600

500

400

300

200

100

0

District heating supply
District heating return
R e cipro ca tin g e n gin e lu bric atin g oil
Pulp/paper mill process waste heat
Reciprocating engine jacket water
Conventional chiller condenser heat
Flat-plate solar collectors
Treated s m elter exh aust gas
Lo w-te m perature ge otherm al
B oiler flu e g as
P a ra b olic tro u g h solar colle ctors
Reciprocating engine exhaust gas
Gas turbine exhaust gas

Figure 2. Impact of Heat Recovery Temperature on Power Output in Steam-Cycle CHP.

Extraction;COP;(k W;thermal;recov-ered;per;kW;power;output;foregone)

11

10

9

8

7

6

5

4

3

200

225 250 275 300 325 350

Temperature;of;hot;water;or;saturated;steam;(F)

Source: FVB Energy Inc. project files.

Opportunities for Heat Recovery and
Renewable Energy

Figure 1 compares these potentially achievable low-temperature district hot water supply and return temperatures to potential sources of heating energy. By dropping the district heating temperatures, you can pick up a range of waste heat sources, including a variety of sources of industrial process energy. Lower temperatures also open up the potential to access renewable resources such as geothermal (there are many more sources of low-temperature geothermal as compared with high-temperature) and cost-effective solar (flat-plate solar collectors are relatively inexpensive compared with parabolic trough or other high-temperature solar technologies used for solar power generation).

Lower district heating temperatures also can improve the economics of CHP. In steam-cycle or combined-cycle CHP, the lower the temperature of the recovered heat, the higher the output of electricity. For example, as illustrated in figure 2 for steam-cycle CHP, at 200 F about 10 units of heat can be recovered for every unit of electricity production reduced as a result of the heat recovery. At 300 F (about 50 psig saturated steam) only five units of heat can be recovered for every unit of electricity production reduced. Thus, a lower heating temperature helps make CHP more feasible because it allows higher production of electricity, which has a higher market value than heat.

Heat Pumps Further Expand
Temperature Range

Heat pumps can further extend the temperature range on the low end of energy resource temperatures for low-carbon heat recovery. For example, heat from chiller condensers, typically at 95 F, has been recovered for years in Sweden in integrated district heating and cooling systems. The relative warmth of the earth can be tapped through ground water or boreholes and ‘pumped up’ to 170 F. Or the heat in treated sewage effluent or even raw sewage can be similarly accessed and made usable with heat pumps. Sweden has also led the way in extracting useful heat from the earth or sewage treatment systems.

Heat pumps can further extend the temperature range
on the low end of energy resource temperatures for
low-carbon heat recovery.

Use of heat pumps in North American district energy systems is increasing. The Southeast False Creek Energy Centre in Vancouver, B.C, recovers heat from sewage to provide the base heat load for a development that includes the Olympic Village, home of the recent 2010 Winter Olympics. The heat pumps work by pumping filtered but otherwise untreated warm sewage through the evaporator side of the heat pump. This heat, coupled with the mechanical compression of the refrigerant, produces 175 F hot water on the condenser side of the heat pump.