In the February 2022 issue of POWER, an overview of existing district heating systems that utilize nuclear power for their energy source was presented and best practices of successful installations were reviewed (see “District Heating Supply from Nuclear Power Plants“). In this article, the technical and economic aspects of these combined heat and power systems are examined.
The heat delivery systems from nuclear power plants are based on a tertiary (in relation to the reactor core) circuit. Pressure in this circuit is kept higher than the maximum possible pressure of the extracted steam. This prevents radioactive products from getting into the district heating water.
District Heating Design Considerations
District heating piping size is determined by the heat load and the water temperature drop in the system. In most cases, the required amount of heat is provided to the customer by varying the temperature in the supply network at the power station as a function of outdoor ambient temperature. A substantial temperature drop between the supply and return lines reduces the flow rate of the circulating water and, as a result, piping size and pumping power. However, a high supply temperature at the nuclear combined heat and power (NCHP) plant reduces electrical generation.
To maximize electric power production, the optimum supply temperature is generally set at about 250F; this is increased to as much as 300F on the coldest days by peaking boilers. Return temperature is usually about 160F. For long-distance transport, supply temperatures up to 400F have been considered.
Typically, 60-inch-diameter pipes will carry about 2,400 MWth, and 40-inch-diameter pipes will carry about 700 MWth. Installation of underground district heating piping in the U.S. typically costs from $800/ft. to $1,500/ft. However, despite the high piping cost, it may be economical to transport hot water as far as 100 miles to cities with heat loads of 1,500 MWth to 2,500 MWth.
One-way heat transport pipelines with use of large electric driven heat pumps to increase the quantity of useful heat is also possible. In this case, at the user end the water can be cooled to temperatures acceptable for discharge.
Optimum Heat Load
It is generally not economical to supply district heat to areas with heat densities less than 90 MWth per square mile (MWth/mi2). Commercial and high-rise residential areas with heat densities of 130 MWth/mi2 to 180 MWth/mi2 are most suitable for district heating.
The problem of heat load is especially important when NCHP systems are being considered. Such plants would usually be far from the urban area and require long pipelines. Because large quantities of heat transported reduce the specific cost of heat transmission, large cities are the most favorable for NCHP supply systems. However, large district heating systems in U.S. cities, such as New York (Figure 1), Philadelphia, Boston, and Indianapolis, are mostly medium-pressure steam systems (150 psi to 400 psi), and extraction and long-distance transport of steam with those parameters is not economical.
1. Diagram of a nuclear combined heat and power (CHP) plant with extraction/condensing steam turbine. Courtesy: Joseph Technology Corp.
District Heating Turbines
To heat water in an NCHP system, it is necessary to regulate the amount of heat extracted from the cycle to follow load, maximize electric power production at each extraction rate, and use standard turbine equipment insofar as possible. The most satisfactory modification to extract heat is the use of multiple extraction points, including extraction from the crossover of the low-pressure sections. This scheme can be used to heat water to about 300F in a three-stage process, with the total heat extracted equal to about half of the heat normally condensed in the full condensing mode. The electric power loss is close to 0.2 kWe per kWth extracted.
Alternatively, two stages of extraction can be used to heat water to about 250F. In this case, the electrical power loss is about 12% of the thermal extraction. Single-point extraction from the crossover is substantially less efficient that multi-point extraction. Studies on steam cycles for NCHPs indicate that existing turbines could be modified to provide efficient combined outputs of electricity and heat, compatible with variable load demands.
CHP Coefficient and End-Use Systems
The CHP coefficient is the ratio of heat load supplied by the steam turbine to the maximum peak demand in the district heating system (Figure 2). To increase the capacity factor, the CHP turbine is usually designed for baseload operation and the peak load is satisfied by another heat source. Because of the high cost of the NCHP, maximum utilization of the reactor at rated output must be ensured.
2. This graph shows an annual heat load duration curve for a combined heat and power (CHP) district heating installation. Courtesy: Joseph Technology Corp.
The turbine should operate with close to constant steam flow, and variable electrical and heat loads. However, if the heat load is disconnected, electricity is generated in the condensing mode. Therefore, the low-pressure cylinder of the turbine should be designed for full steam flow to the condenser. Optimization studies indicate that the CHP coefficient for nuclear power plants should be approximately 0.5–0.6. In some cases, throttle steam may be used for peaking purposes.
End-use systems for space and hot water district heating are readily available. When introducing district heating systems, feasibility often depends on the convertibility of existing building heating units to hot water, if they are not already designed in such fashion. The changeover can cost between $50/kWth and $150/kWth. Hot water district heat can also be used with absorption chillers for air conditioning.
District Heating System Development
In evaluating the feasibility of district heat supply, the rate of heat load increase is one of the major factors. The appropriate time for introducing an NCHP is very important from an economic viewpoint. Premature introduction of such a plant or delay in the development of the network system would be uneconomical. To prevent such a condition, either of two approaches can be used.
One is a step-by-step approach in which a fossil fuel–based heating-only plant accommodates the early heating loads. As the system grows, a fossil CHP plant, and finally an NCHP plant, can be added. The heating-only plant can then serve as a peaking and standby unit.
Another approach is to build an NCHP plant with provisions for the addition of a future district heat supply system. NCHP turbines with condensing exhaust ends can be installed. In the power-only mode, these turbines are capable of providing the same net electric output as conventional units. They are also capable of supplying the district heating load when it is developed. At that time, the required district heat exchangers, pumps, and related equipment can be installed.
Provisions for a 1,500 MWth heat supply in the design of a standard 1,000 MWe nuclear power plant will generally increase plant cost by less than 5%. Extracting steam for district heating involves a reduction in electrical generation. Depending on the steam flowrate, pressure, and number of extractions, the ratio of the electricity lost to the heat-supplied for hot water–based systems can be about 1 kWe per 5 kWth to 10 kWth.
Challenges to Implementation
Issues to be resolved relative to the use of heat supplied by nuclear energy include public acceptance of nuclear plant sites in relative proximity to populated centers, development of economical heat transport methods, and availability of large dual-purpose turbines for production of electricity and extraction steam at suitable temperatures and pressures. In the U.S., future use of district heating supplied from NCHP will require increased commitments to district heating and greater acceptance of nuclear plants sited relatively close to cities to limit transmission costs.
Although costs and technology status of nuclear-based district heating are favorable and there are environmental advantages, institutional barriers are deterrents to implementation. These barriers must be overcome before the energy conservation potential of this approach can be realized on a significant scale.
—Ishai Oliker, PhD, PE (email@example.com) is principal with Joseph Technology Corp. He has been involved in power plant development and design in the former USSR, Korea, China, and the U.S. for more than 30 years.