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By Charles Rhodes, P.Eng., Ph.D.

If mankind is to survive on Earth with anything close to the present population the temperature in the lower latitudes must remain low enough for human habitation which means that humans living at the higher latitudes will require comfort heating in the winter. Meeting that heating load without consumption of fossil fuels will require nuclear energy. The required amount of nuclear power can be greatly reduced if the comfort heat requirement is met by direct delivery of heat via district heating pipes rather than via electricity, because then low grade waste heat from the electricity generation process can be usefully used. Typically the waste heat production from a nuclear power plant exceeds the electricity production by 2X to 3X. On this web page we examine some of the practical constraints on the design of nuclear district heating systems.

Today the overwhelming majority of high rise residential suites rely on fan-coil units for both heating and cooling. For several practical reasons these fan-coil units in their heating mode are designed to operate with an inlet water temperature of about 60 deg C and a discharge water temperature of about 50 deg C. Hence any practical district heating system has to one way or another provide these circulated water temperatures.

However, a practical district heating system also requires a heat exchanger interface to each building to provide pressure isolation. Driving su‪fficient heat through that heat exchanger under worst case conditions requires a primary fluid inlet temperature of about 75 deg C and a primary fluid discharge temperature of about 65 deg C.

Practical operating experience has shown that the optimum high temperature in buried heating water pipes has a high limit of about 120 deg C. This limit arises from two sources. One is pressure safety. The other is electricity generation efficiency.

The maximum pressure in the buried pipe system is the sum of the hydraulic head plus the steam head. At 120 deg C the steam head is about 15 psi. For reasonable capital cost economy assume use of schedule 40 steel pipe and fittings, which are generally rated for a maximum working pressure of 160 psi. Allowing for circulation pump pressure differentials that allows for hydraulic heads above and below the heat source location of about 140 psi. Thus without further pressure isolation in theory a suitably located nuclear district heating plant can serve thermal loads spanning an elevation range of about 600 feet. For communities located on the side of a mountain two such district heating systems may be required to meet the pressure constraints.

The second reason for limiting the circulated water temperature to 120 deg C maximum is efficiency of electricity generation. As the circulated water temperature increases the efficiency of electricity generation decreases. It is essential to generate sufficient electricity to operate heat pumps located near the district heating system outer perimeter. Hence the discharge water temperature from the heating plant cannot be too high.

The thermal loads can be divided into two categories. For loads that are close to the nuclear power plant the isolation heat exchanger primary inlet water temperature is in the range 120 deg C to 75 deg C and the discharge water temperature is in the range 110 C to 65 deg C. For loads that are further from the nuclear power plant the isolation heat exchanger inlet water temperature is in the range 75 deg C to 30 deg C and the discharge water temperature is in the range 65 deg C to 20 deg C. From a heat pump perspective the worst case is:
Primary inlet temperature = 30 C
Primary discharge temp = 20 C
Secondary discharge temp = 75 C
Secondary inlet temp = 65 C

Operation in this mode requires a heat pump that can raise the water temperature by about 45 deg C. The efficiency of heat pumps decreases fairly quickly as the temperature rise increases. A 45 degree C temperature rise is about the practical limit for heat pumps. A standard deep freeze cooler is a heat pump in reverse and it provides about a 40 degree C difference between the outside air temperature and the interior temperature. The same is true for ice rinks, cold storage facilities, etc.

The benefit of using heat pumps is that they double the thermal load that can be attached to district heating system branch pipe circuit. In circumstance of lower density loads they greatly extend the maximum radius between the nuclear heating plant and the furthest thermal load.

An average high rise suite needs 5 kWt of heat to meet peak winter conditions. Since suites vary by size and exposure it is common for engineers to provide 8 kWt to 10 kWt of peak heating capacity per suite. Similarly average single family homes typically need 20 kWt to meet the peak winter heating load but due to uncontrolled variations between homes most homes in southern Ontario have furnaces rated to deliver 30 kWt to 40 kWt of peak heating capacity.

When used to provide both heat and electricity a 1000 MWt FNR type nuclear plant will provide about 250 kWe of electricity and about 750 MWt of heat. If there were no transmission losses in theory that reactor could supply heat for up to:
750,000 KWe / (5 kWe / suite) = 120,000 suites.

However, if the system is maximally built out so that the transmission losses equal the intended thermal load one 1000 MWt reactor can service up to 60,000 high rise suites or perhaps 30,000 single family homes. Another practical rule of thumb is that the total average power requirement is about 10 kW / person. Thus one 1000 MWt reactor can meet the combined energy needs of about 100,000 people.

Another important consideration is that for overall heating system reliability under worst case conditions adjacent district heating systems sharing the same elevation should be pipe coupled together so that if one reactor has to be shut down adjacent reactors can provide some heat. This is yet another valid argument for limiting the design thermal load to less than the reactors maximum thermal capacity, so that in emergency situations there is sufficient spare thermal capacity available to meet the thermal load of a nearby reactor. It is practical to import electricity from distant electricity generators. It is a lot more difficult to import heat from distant nuclear heating systems.

Another important potential application of nuclear reactors is for desalination of sea water for production of potable water. When a FNR type nuclear reactor is used to generate electricity it typically throws off two units of heat for every unit of electricity generated. If that heat cannot otherwise be sold it can be used for large scale water distillation. Potable distilled water is relatively easy and inexpensive to store for long periods of time and to pipe considerable distances. While this application may not be important near the great lakes, in many places around the world where there are chronic shortages of potable water this application makes business sense, especially if the heat is otherwise going to be discarded.

Note that if there is spare electricity generation capacity there are other more energy efficient means of desalinating water. For example, evaporation occurs at a lower temperature if the ambient air pressure is reduced by suitable pumping. Alternatively various heat pump or reverse osmosis processes can be used. However, these alternate processes all require electrical or mechanical energy. Simple distillation requires very little mechanical energy.

An important additional application of the buried district heating piping is for distributed heat rejection in the summer. If a nuclear reactor is located in the center of an urban area it may not be a good idea to dump surplus waste heat to the atmosphere close to the reactor. A better idea is to use the buried radial piping to move the surplus heat away as far as practical and then dump that heat to the atmosphere via remote cooling towers and/or roof mounted fan-coil units, or dump the heat into a nearby large body of water.

This web page last updated July 17, 2019.

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