Sidebar: In the Pipeline: District Energy and Green Building
Environmental Benefits
Cleaner combustion. Producing heat at large, central facilities (using large boilers or CHP plants) is cleaner, per unit of heat delivered, than operating smaller boilers or furnaces at individual buildings. Higher efficiencies. Larger, central boilers generally operate at higher efficiency than individual systems in buildings. With central heat production, it is common to install several boilers so that individual units can be brought on as needed, and they operate at full-load for highest efficiency. Boilers at individual buildings (residential or commercial) are sized to meet peak heating loads, but most of the time they operate at part-load, which reduces efficiency. CHP plants, which are well suited to district heating, offer much higher source-energy efficiency than conventional power generation. Ability to utilize renewable energy and waste-fuel sources. With a district heating system, the heat source can be shifted to renewable-energy sources relatively easily. For example, if a gas-fired CHP plant is serving a district heat network, additional capacity can be delivered by adding a renewable-energy-fired plant (woodchips or landfill methane, for example) on the same heat distribution network. Or the gas plant can be converted to renewable energy. Reduced risk of fuel spills in and around buildings. Avoiding the combustion of heating fuel in buildings eliminates the risk of fuel spills (primarily a concern with heating oil). With district heating, only hot water enters buildings, not fuel. At central boiler or CHP plants, provisions to contain spills can be incorporated into the facility design. Reduced pollution from trucking fuel. Heating oil and propane are distributed by truck; replacing those energy sources with district heat replaces the trucking energy and pollution with much less (and cleaner) electrical pumping energy. Opportunities for use of exterior space. Eliminating chillers, cooling towers, and packaged air-conditioning equipment may reduce local opposition during permitting, while making that space available for green roofs or green space. Improved health and safety. Keeping fuel combustion out of buildings will protect occupants from combustion products, which cause health problems.Economic Benefits
Lower first cost to building owners. Eliminating onsite boilers, chillers, or air conditioners from buildings can reduce first costs. Lower maintenance costs. Eliminating mechanical equipment from buildings eliminates the cost of maintaining that equipment. Lower cost of heat to the building owner. Particularly if heat is derived from a CHP plant—in which the distributed heat is a byproduct of power production—the cost per unit of delivered heat may be lower than if heat were generated in individual buildings. Value of space not lost to mechanical equipment in buildings. Heating, cooling, and water-heating equipment takes up valuable space in buildings, especially in commercial buildings, that could otherwise be utilized or rented. Rooftop space not required for cooling towers or packaged air-conditioning equipment can be rented for cell towers or used as high-value penthouse or restaurant space, for example. Increased reliability. District energy systems have a history of superb reliability and minimal downtime. Even with earthquakes and other natural disasters, district energy systems have usually maintained uninterrupted operation. Lower cost of fuel at central plant. Central boilers and CHP plants may be able to use less expensive fuels, such as wood chips (which cannot be used in smaller boilers in buildings). Reduced peak electricity demand. In commercial building, replacing electric chillers or air-conditioning systems with district cooling can significantly reduce electric demand charges. Fuel flexibility. District heating systems offer the potential to switch fuels based on cost. Potential to benefit from renewable energy credits. With CHP plants using renewable fuel (wood chips, landfill methane, etc.), electricity can be sold at a premium to utility companies providing renewable energy credits (see EBN Vol. 15, No. 10).Drawbacks and Challenges
High first cost. The initial cost of building central heating or CHP plants and burying district energy pipes is high. Land disruption. Burying district energy pipe disturbs land and may necessitate costly ecological restoration. May diminish the incentive to conserve energy. Inexpensive thermal energy can reduce the motivation to build highly energy-conserving buildings. (In fact, with such buildings, it may be harder to justify district energy systems.) May reduce a building’s “passive survivability”. Despite a track record of excellent reliability, dependence on centrally distributed energy can increase vulnerability to fuel shortages or distribution system failure, particularly if the source of inexpensive thermal energy has reduced the motivation to build highly efficient buildings. (For more on passive survivability, see EBN Vol. 15, No, 5.) Need for coordinated planning and cooperation. Implementing successful district energy systems necessitates a higher level of planning and coordination than is common in North America (except with the electric utility infrastructure). Inherent resistance to losing control. District energy systems are very successful with “captive audiences” such as university campuses and hospital complexes, but there may be resistance in the U.S. to giving up full control of one’s sources of heating and cooling.
