Conventional district heating systems are generally designed around the load. However, an alternate approach would be to design around potential heat sources, including the waste heat from power plants. Many steam turbines are designed for backpressure of 5 in. of mercury, which allows heating water to at least 120F, suitable for district heating.
With the recent emphasis on renewable energy, thermal energy has been neglected, even though fossil fuels (natural gas, fuel oil, and propane) make up 46% of the energy used by buildings in the U.S. Most of these fuels are used for space heating and domestic hot water. This is an energy load that has largely been ignored in developing district heat in the U.S., although in Europe, district heating has been used for apartment buildings.
An Ignored Heat Source
While cogeneration plants have been used to supply heat for district heating for many years, most of the ones in the U.S. have been special purpose plants, using extraction turbines to supply high-pressure steam to district steam systems with limited cogeneration.
There are two principal conventional technologies that have higher cogeneration potential. There is European technology, using 180F water provided by central power plants with extraction turbines, and smaller-scale plants, using internal combustion engines or combustion turbines to make 180F to 220F hot water for local use.
Systems using 180F water suffer from several limitations. Special purpose plants are required, which are useless when heat demand is low in the summer—the plant investment sits idle during the summer peak. Furthermore, water at 180F requires pre-insulated steel pipe with welded joints, expansion joints, careful joining to maintain casing water tightness, and contractors familiar with the pipe. All of these factors add to the cost of the system, and 180F water is not really needed to heat buildings.
There are numerous ways for buildings to use lower-temperature water. They include:
- ■ Radiant slabs, which can use water at 90F to 100F, but these are expensive for retrofits.
- ■ Conventional radiators, with increased radiation.
- ■ Heat pumps to boost the heating supply temperature and reduce district heat return temperature.
- ■ Use of cooling coils in air-conditioned buildings for heating.
In these low-temperature water systems, the district heating supply water temperature is about 120F, with a return temperature of about 90F. The significance of water at 120F is that almost all power plant turbines are capable of heating water to 120F, as the standard maximum condensing pressure is 5 inches of water, equivalent to 134F. Power plants with air-cooled condensers (ACCs) are necessarily equipped with turbines that will condense at higher temperatures. The plants can be operated at slightly higher than normal condensing temperatures to provide 120F water for heating buildings, at slight extra fuel cost. This heat is normally rejected, and so by recycling it for heating, almost all power plants can become combined heat and power (CHP) plants.
Obtaining Heat from Power Plants
Ways to obtain heat at 120F from power plants vary with the type of cooling system and plant operating conditions, but in all cases, they are simple and require no modifications that would affect the operation of the power plant when heat is not required.
Power plants with ACCs only require an auxiliary condenser to supply heat to the district heating system, and the necessary pumps to circulate district heating water through the condenser (Figure 1). The cooling fans on the ACC would be selectively shutdown to obtain the desired hot water temperature, at a slight increase in heat rate.
1. This schematic shows a method of obtaining heat from a power plant for district heating purposes. Courtesy: Robert W. Timmerman, PE
Obtaining heat from power plants using a wet cooling tower is similar to the dry cooling case, differing only in requiring a water-to-water heat exchanger to isolate the district heating system from the tower water. Calculations on the benefits of running wet tower fans in winter for one particular power plant showed that the additional power produced by lower condensing temperatures with the fans was almost equal to the additional power used by the fans.
Power plants using once-through cooling require a more elaborate way to obtain heat. They would need a water-to-water heat exchanger to isolate the once-through cooling water from the district heating water, which isolates both pressure levels and water quality. Raising the temperature could be done by reducing circulating water flow to the point where the velocity through the condenser becomes unacceptable. Below that point, water could be recirculated from discharge back to the intake. Some power plants are equipped with recirculation, either to prevent icing in the intake or to control marine growth in the cooling system.
Transmission of Heat
The low temperatures at which this system operates (120F supply, 90F return) permit a dramatic simplification of the piping system. Calculations have shown that the cost of insulating pipe sizes larger than six inches usually exceed the economic benefit. Lack of insulation reduces both the initial cost and maintenance cost of the system. The relatively small amount of thermal expansion can be handled by the bell and spigot joints in cast iron pipe or by installing polyethylene pipe in a sinuous path. A further contributor to low installed cost is contractor familiarity with these types of pipe—cast iron is standard water main pipe and polyethylene is widely used for natural gas distribution. Because these are not prefabricated, pre-insulated pipes, they can be easily re-routed to adjust for unforeseen underground obstacles.
There are a number of commonly available piping materials that could be used for a low-temperature district heating system. They include:
- ■ Ductile iron water main pipe, which is available pre-insulated.
- ■ Cross-linked polyethylene.
- ■ For lower-temperature applications, PVC (polyvinyl chloride) water main pipe, which was used to convey 95F water in a prototype system that is referenced below.
Because the cost of heat from this CHP system is small compared to the system capital cost, both the user interface and the rate schedule differ from conventional district heating practice. Quantity of heat would be measured by total quantity of water used per month. This is cheaper than measuring flow and temperature difference to calculate thermal energy. The capital cost portion of the system would be recovered by selling customers a fixed flow quantity determined with flow restrictors. The fixed revenue from this would pay the fixed charges.
Possible Customer Interfaces
There are a number of ways for customers to connect to the district system and use low-temperature heat. The simplest is a radiant slab, which can use water from 90F to 100F directly, although installing a radiant slab in an existing building is expensive. Buildings with existing hot water heating systems can be modified with more radiation to use 120F water, but the hot water heating system could not cool the return water to 90F. A hybrid system could be used, with the first stage being radiators and convectors operating from 120F to 105F or so. The second stage would use a heat pump to cool the 105F return water from the building heating system down to 90F, while heating water to 120F (Figure 2). Finally, buildings with central chilled-water systems could use the cooling coils for heating.
2. This schematic shows a heat pump building interface in which hot water (HW) is pumped to the building (BLDG) heating system. Courtesy: Robert W. Timmerman, PE
In today’s utility industry, most of the waste heat would come from combined cycle plants, which usually have only one or two steam turbines; plants with multiple turbines are rare. Newer plants are often designed for quick starting to supply power when intermittent renewables are not available. These characteristics do not make them a reliable source of heat for district heating.
There are several ways to supply heat to a district heating system with a source that is intermittent. Short term, water can be stored in large tanks. A 48 million-gallon tank would store 12 hours of full heat output from a 200-MW steam turbine, which would be a typical size for a 600-MW combined cycle plant. Tanks of this size are available.
A second alternative is simply to sell thermal energy, not firm heating capacity. This is easily done in retrofit situations, where the customers already have their own boilers. When there is no heat available, the customers simply run their boilers until district heat is available.
The Massachusetts Municipal Wholesale Electric Co. (MMWEC), a consortium of municipal utilities, wanted to heat its new administrative office with waste heat from its nearby 400-MW combined cycle power plant, as a model of energy efficiency. A system was designed as a prototype of low-temperature district heating. It was a constrained design, with a low budget, an unknown operating schedule for the power plant, and the office building itself was to be built by a design-build contractor.
The system as designed used the 95F water from the power plant cooling tower as the primary heat source, transmitted via a 1/2-mile-long, underground, uninsulated PVC pipe from the power plant to the building. The building heating, ventilation, and air conditioning (HVAC) system used 17 fan-coil units for heating and cooling, each with a main coil and a supplementary coil for additional heating.
In summer, the main coil was supplied with 45F chilled water for cooling; the water chiller was cooled with water from an 11 million-gallon makeup water tank at the power plant. The heat from the chiller was stored in the tank for backup heating when heat from the power plant was not available.
In winter, the main coils were supplied with 95F water from the power plant, and the booster coils were supplied from a heat pump operating from 95F water to provide 120F water for the booster coil loop. When heat from the power plant was not available, the main water chiller operated as a heat pump, recovering heat stored in the tank to heat primary loop water to 95F. There was also a backup boiler.
This successful project proved the concept. Several additional studies have been conducted on other systems and all have demonstrated worthwhile benefits. ■
—Robert W. Timmerman, PE (RWTimmerman@gmail.com) holds Bachelor’s and Master’s degrees in Mechanical Engineering from Cornell University. He is a Certified Energy Manager, and has done independent research in energy conservation, district heating and cooling, and CHP.