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XYLENE POWER LTD.

FNR DISTRICT HEATING

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

INTRODUCTION:
Elsewhere on this website liquid sodium cooled Fast Neutron Reactors (FNRs) have been identified as the primary sustainable source of energy for meeting mankind's future needs. This web page focuses on use of FNRs for urban district heating.

In Canada use of urban sited nuclear reactors for both district heating and electricity generation is both a major opportunity for fossil CO2 emission reduction and is a major opportunity for reducing the cost of electric heating. By effectively using waste heat from electricity generation for comfort heating urban sited reactors reduce the nuclear reactor capacity required for urban comfort heating by at least a factor of four.

However, one of the financial challenges of nuclear district heating is that system customers need a backup heat source for use when the reactor is shut down. That backup heat source is likely to be natural gas / hydrogen. Until there is a very large fossil carbon tax applied to natural gas, there will be a tendancy for building owners to preferentially choose natural gas rather than clean electricity and reactor waste heat for normal building heating.
 

DISTRICT HEATING MARKET:
Most existing European district heating systems supply water at about 120 degrees C and return water at about 80 degrees C. That temperature choice enables connection to older hydronic heating systems that need to be pressure isolated and also need to deliver heating water to terminal radiators at about 80 degrees C. Thus there is 20 degrees C available for the temperature drop across the isolation heat exchanger and there is 20 degrees C available for district heating system distribution system temperature losses.

The fundamental problem with conventional existing district heating systems is that at 120 degrees C turbogenerator coolant discharge water is too hot for efficient electricity generation. In the market place electricity is worth about 5X as much as heat, so it pays to improve electricity generation efficiency at the expense of thermal distribution system efficiency. Further, in modern highrise construction high temperature heating systems are not compatible with air conditioning which is increasingly a necessity due to global warming.

The market place contains both buildings without central air conditioning built pre-1970 that use heating systems which need 80 degree C heating water going to terminal radiators and more modern buildings with central air conditioning that use 60 degree C heating water going to terminal fan-coil units. In the more modern buildings in the summer the supply water temperature going to the terminal fan coil units is cooled to about 10 degrees C to provide air conditioning.

The solution to these problems is to install a heat pump in every building to usefully use waste heat from electricity generation for comfort heating. In each building there is also a backup electric or natural gas/hydrogen fired unit to provide heat when the nuclear reactor or district heating piping are shut down or when supplementary heat is required in extremely cold weather.
 

HEAT DISSIPATION REQUIREMENT:
A constraint on the size of any nuclear reactor located in the center of a city is its ability to dissipate heat, especially in the summer. In the winter local cooling towers are effective and there is the district heating load. However, in the summer local cooling towers are less effective, and there is little district heating load, so it is necessary to use remote cooling towers connected to the district heating mains to achieve the required heat removal capacity. Thus an important feature of a nuclear district heating system is its ability to use the district heating pipe network to reject heat via remote cooling towers.
 

DISTRICT HEATING WATER TEMPERATURE:
Nuclear reactor based district heating systems realize economy by utilization of waste heat from electricity generation that would otherwise be discarded. However, for most existing building heating systems the turbogenerator condenser coolant water temperature is too low for effective winter comfort heating unless each building is fitted with a heat pump.
 

EMERGENCY OPERATION:
During reactor shutdowns and during severe cold weather building heat pumps need supplementary natural gas/hydrogen heat or electric resistive heat.
 

FNR THERMAL DISCHARGE:
The FNR thermal energy discharge is primarily in the forms of steam produced in steam generators intended for electricity generation and warm water produced in turbogenerator condensers as a result of condensing turbogenerator steam.

The steam used for electricity generation is at a dry bulb temperature of about 440 degrees C and is saturated at 320 degreess C, 11.25 MPa. This steam is at too high a pressure and too high a temperature for economic comfort heating, although it may be appropriate for use in nearby industrial processes.
 

SUPPLY OF CONTINUOUS LOCAL HIGH TEMPERATURE HEAT FOR INDUSTRY:
The best way to apply FNRs to supply continuous high temperature heat for nearby commercial use is to dedicate selected secondary sodium heat transport loops for that purpose instead of for electricity generation.

However, this heat delivery methodology reduces FNR electricity generation capacity and hence is only economic for high load factor heat loads.
 

DISTRICT HEATING SYSTEM OVERVIEW:
The geographic area around each FNR is divided into four approximately equal heat load regions. These regions may be at different elevations.

Each FNR is nominally able to produce about 900 MWt of heat of which about 300 MWt is used for electricity generation. The remaining 600 MWt is used to supply four independent district heat distribution loops, one per regon, each rated at about 150 MWt. During the peak of the winter the combined electricity load imposed by all of the in-building heat pumps and related equipment will be 150 kWe to 200 kWe corresponding to a COP in the range 4.0 to 5.0.

Each of the four 150 MWt district heat distribution loops consists of:
a) 14 intermediate heat exchangers;
b) 2 heat exchange galleries;
c) 14 secondary sodium induction pumps;
d) 14 steam generators;
e) 4 high pressure steam pipes;
f) 4 turbogenerators and condensers;
g) 4 condenser coolant circulation pumps for 24 inch diameter pipe;
h) 8 X 24 inch diameter isolation valves;
i) 1 X 48 inch diameter buried supply water header;
j) 1 X 48 inch diameter buried return water header;
k) 1 local dry cooling tower with 24 inch diameter water service pipes;
l) 3 remote dry cooling towers, each with 24 inch diameter water service pipes;

The district heating system mains are engineered to operate at a nominal water flow velocity of 2 m / s and at a nearly constant dT of 20 degrees C. At full load with a dT of 20 degrees C a 48 inch OD pipe transports about 160 MWt and a 24 inch OD pipe transports about 40 MWt. A 4 inch pipe transports about 1 MWt, a 6 inch pipe about 2 MWt and an 8 inch pipe about 4 MWt. Thus building thermal service capacity can be estimated from its connection pipe size.

Note that the thermal load and the loop flow rates at the FNR are high all the time to enable continuous full power electricity generation. The buildings use variable speed circulation pumps to maintain a constant dT across the building of 20 degrees C. Thus in the summer their flow rates are low whereas in the winter their flow rates are high. The cooling towers are connected in parallel with the buildings and operate in a heat balancing mode. Whatever heat the buildings to not absorb the cooling towers must reject. Hence in the winter the cooling tower water flow rates are low whereas in the summer the cooling tower water flow rates are high. The cooling tower flow control systems attempt to maintain a constant dT of 20 degrees C.

The various building loads might valve themselves off the system in the summer when they do not require heat but are permitted to draw heating water in the winter at their maximum contracted rate when they do require heat. Hence the volumetric heating water flow through the building loads is proportional to the time dependent heat load.

Each building must be fitted with a heat meter that measures both water flow and the differential temperature.

In order to run the electricity generation at maximum capacity the turbogenerators require a constant flow of cooling water to sink rejected heat. Hence the heat distribution loops are operated at a water flow rate which is at least sufficient to absorb the turbogenerator waste heat. The turbogenerator waste heat must always be greater than the sum of the building heat loads, which limits the turbogenerator turndown in the winter.

To the extent that the building thermal loads do not draw heat the cooling towers must reject an equal amount of heat. Hence the water flow rate through the cooling towers must be adjustable to balance the building load water flow rate, so that the thermal distribution loop water flow rate remains sufficiently high to absorb all of the turbogenerator waste heat. The cooling towers can automatically adjust their water flow rates to maintain their design dT of 20 degrees C.

The cooling towers draw water from the supply line and discharge water to the return line. The cooling towers must be spaced along the thermal distribution loop to distribute their exhaust heat over a wide area and to keep the net loop water flow rate nearly uniform.

At very low outside air temperatures the water flow rate through the cooling towers will be low so the cooling towers must close protective air flow dampers to prevent coil freezing.

The heat pump working fluid is butane (C4H10). The key butane properties are:
Liquid density = 604 kg / m^3
Vapor pressure at 25 deg C = 35.4 psia
Specific Heat Cp = 1675 J / kg deg K
Cp / Cv = 1.096
Latent heat of vaporization = 386,000 J / kg
Freezing point = -138 deg C
Critical temp = 152 deg C

Each building heat pump should be fitted with a temperature sensor that will turn on the boiler to prevent the heat pump from freezing of its isolation heat exchanger.
 

NORMAL OPERATION:
The turbogenerator condensers operate at an inlet cooling water temperature of about 20 degrees C above the ambient air temperature and at a condenser cooling water discharge temperature of about 40 degrees C above ambient air temperature. The condenser cooling water discharge temperature becomes the district heating supply water temperature and the cooling tower inlet water temperature. The district heating return water temperature is about 20 degrees C less than the condenser cooling water discharge temperature and becomes the condenser cooling water inlet temperature.

When the outside air temperature is above 15 degrees C the building heat load is small and almost all the waste heat from electricity generation must be dissipated in the 4 cooling towers associated with each thermal distribution loop. Thus under these circumstances each of 4 cooling towers must dissipate at least:
150 MWt / 4 = 37.5 MWt.

Assume that at full air and water flow:
the cooling tower rising air temperature is 20 degrees C above ambient.

Assume that the district heating supply water temperature = cooling tower inlet water temperature is 40 degrees C above ambient.

Assume that the cooling water temperature increase across the turbogenerator condenser is one pass at 20 degrees C.

At an outside air temperature of - 20 degrees C:
Engage system freeze protection by completely closing cooling tower dampers, setting cooling tower water flow rate to preset minimum, engage hydrogen burners at each load to keep building return water temperature above freezing.

At an outside air temperature of - 10 degrees C:
cooling tower dampers are slightly open;
cooling tower rising air temperature = 10 degrees C,
district heating return water temperature = condenser cooling water inlet temperature = 10 degrees C;
condenser cooling water discharge temperature = district heating supply water temperature = 30 degrees C.

At an outside air temperature of 0 degrees C:
cooling tower dampers about half open;
cooling tower rising air temperature = 20 degrees C,
district heating return water temperature = condenser cooling water inlet temperature = 20 degrees C;
condenser cooling water discharge temperature = district heating supply water temperature = 40 degrees C.

At an outside air temperature of 15 degrees C:
cooling tower dampers full open;
cooling tower rising air temperature = 35 degrees C;
district heating return water temperature = condenser cooling water inlet temperature = 35 degrees C.
condenser cooling water discharge temperature = district heating supply water temperature = 55 degrees C

At an outside air temperature of 30 degrees C:
cooling tower dampers full open;
cooling tower rising air temperature = 50 degrees C,
district heating return water temperature = condenser cooling water inlet temperature = 50 degrees C;
condenser cooling water discharge temperature = district heating supply water temperature = 70 degrees C.

At cold outside air temperatures there is less surplus heat to reject so the water flow rate through the cooling towers diminishes. In these circumstances it is necessary to partially close the cooling tower dampers to keep the cooling tower discharge water temperature above 0 degrees C.

In hot weather at an outside air temperature of 30 degrees C the turbogenerator working fluid (steam) is condensed and cooled to about and 75 degrees C. In these circumstances the electricity generation is less efficient than in cold weather when the turbocondenser inlet temperature is lower.

Thus in the summer waste heat from electricity generation is partially dumped by the on-site cooling towers and is partially dumped by the remote cooling towers.

In mid-winter little or no heat is dumped because almost all the turbogenerator waste heat is used for comfort heating. In the event that the grid electricity load should drop in mid-winter it is necessary to ensure that the electricity load seen by the district heating system is maintained to ensure continuous availability of sufficient waste heat. This conditionis normally met if there is certainty that the electricity generated is delivered to the buildings for operating their heat pumps. Thus it might make sense for the reactor to supply the electricity used for heat pump operation.
 

THERMAL DISTRIBUTION PIPE CROSS SECTIONAL AREA:
600 MWt = A V Cp Rho dT
or
A = 600 MWt / [V Cp Rho dT]

Assume that:
V = 2 m / s
Cp = (1 cal / gm deg C) X 4.18 J / cal X 1000 gm / kg
= 4180 J / kg deg C
dT = 20 deg C
Rho = 1000 kg / m^3
Then:
A = [600 X 10^6 J / s] / [2 m /s X 4180 J / kg deg C X 1000 kg / m^3 X 20 deg C]
= 600 m^2 / [2 X 4.18 X 20]
= 3.5885 m^2

Thus the total district heating loop supply and return pipe cross sectional areas are:
3.5885 m^2 / 4 loops = 0.897125 m^2 / loop

Let R be the inside radius of the main header pipes. Thus:
Pi R^2 = 0.897125 m^2
or
R^2 = 0.8971 m^2 / Pi
= .28556 m^2

R = 0.53438 m

ID = 2 (.5344 m)
= 1.0688 m
= 1.0688 m / (0.0254 m / inch)
= 42.077 inch

Hence use 48 inch OD pipe for the main loop supply and return headers. Use 24 inch OD service pipes for each cooling tower.
 

MECHANICAL POWER REQUIRED PER FLUID ACCELERATION:
(mass flow / unit time) X V^2 / 2
= Rho A V X V^2 / 2
= Rho A V^3 / 2
= 1000 kg / m^3 X 3.5885 m^2 X (2 m / s)^3 X (1 / 2)
= 3588.5 X 4 kg m^2 / s^3
= 14,354 J / s
= 14.354 kW

Thus each main pipe loop needs:
14.354 kW / 4 loops = 3.5885 kW of mechanical power per fluid acceleration. Viscosity flow resistance and corner resistance is additional.

Each pipe loop involves four circ pumps, one per condenser. Thus each circ pump must supply:
3.5885 kW / 4 = 0.897 kW of mechanical energy per fluid acceleration. For practical purposes the fluid velocity drops to almost zero in each condenser, so each of the 4 condensers requires a 2 HP pump before consideration of the external thermal distribution loop pump load.
 

WATER FLOW DESIGN:
The central circ pumps operate at nearly constant flow. They must be designed to cancel the flow pressure drop across the condensers. These central circ pumps each have flow direction check valves as well as isolation valves.

The building loads have variable speed pumps that operate to maintain a loop dT of 20 degrees C. As the building thermal load increases the water flow through the buildings increases.

The cooling towers have variable speed pumps that operate to maintian a loop dT of 20 degrees C. As the building thermal load increases the water flow throungh the cooling towers decreases.

If during the shoulder seasons the electricity load decreases then the central heat suppply will drop. Under these circumstances there is little water flow through the buildings. the cooling towers are at full water flow. Hence the district heating return water temperature will decrease which will improve the turbogenerator efficiency. Provided that the outside air temperature is above freezing there is no danger of any equipment damage.
 

BUILDING AND COOLING TOWER CIRC PUMPS:
The building and cooling tower circ pumps see not only the pressure drop across the building but also the pressure drop across the heat distribution loop. Each building circ pump has a check valve to prevent the water flow through that building being disturbed by the water flows through other nearby buildings. Note that near the buildings the district heating return line pressure will be higher than the district heating supply line pressure. It is essential to have enough head pressure on the district heating supply line to ensure that circ pump suction head requirements are met. The best way of ensuring this suction head complianced is with a common gravity tank located above the highest point in the thermal distribution loop.
 

HEAT PUMP OPERATION:
In the proposed nuclear district heating system each building is fitted with a central heat pump which uses butane as a working fluid. The heat pumps for buildings that need 80 degree C heating water are of slightly different design from the heat pumps for buildings that need 60 degree C heating water. The different heat pumps will use compressors and heat exchangers that are engineered for optimal operation at different working fluid temperatures and pressures. For explanatory purposes herein we will concentrate on heat pumps for buildings that are designed to operate with 60 degree C heating water supplied to terminal fan coil units. This building design is consistent with use of the same terminal fan-coil units for providing air conditioning in the summer.

The heat pump consists of two heat exchangers, a working fluid, a compressor and an orifice. The cool working fluid temperature (10 degrees C) lower working fluid pressure (1.5 bar) heat exchanger is referred to as the evaporator. The hot working fluid temperature (80 degrees C) higher working fluid pressure (10 bar) heat exchanger is referred to as the condenser.

Under the typical operating circumstances described herein the district heating water that has been heated by NPP turbocondenser cooling enters the evaporator at 40 degrees C and exits at 20 degrees C. Typically in the evaporator the district heating water is contained in copper fin tubes that are surrounded by butane vapor at about 1.5 bar.

In the evaporator heat from the district heating water warms the liquid butane causing it to evaporate. This process causes the butane latent heat of vaporization to flow from the district heating water to the butane vapor. The cool butane vapor exits from this heat exchanger at about 10 degrees C, 1.5 bar.

This cool vapor flows through a compressor cooling coil and into an electric compressor which increases the butane vapor pressure to about 10 bar. The act of compressing the butane changes compressor electric energy into heat which increases the butane temperature. The butane vapor temperature at the compressor discharge is over 80 degrees C.

The compressor electric motor should be coupled to the mechanical compressor by a magnetic coupling to prevent the compressor leaking butane vapor.

The condenser is a more robust shell and tube heat exchanger because it must be rated for working with both high pressure butane (10 bar) and high pressure building heating water (11 bar).

The hot compressed (10 bar) butane vapor flows into the condenser, which uses building heating water at 50 degrees C in, 60 degrees C out to condense the butane vapor at about 80 degrees C. In so doing the butane latent heat of vaporization plus the compressor power flows from the butane vapor to building heating water. The resulting 80 degree C liquid butane collects at the bottom of the condenser, and flows out through a presssure reducing orifice and back into the evaporator.

The evaporator is at a low pressure (~ 1.5 bar) as compared to the condenser. That drop in pressure immediately causes partial evaporation of the butane which causes an immediate drop in the remaining liquid butane temperature.

The remaining liquid butane is evaporated by absorbing butane latent heat of vaporization from the district heating water. This low temperature heat absorption must take place above 0 degrees C to prevent local freezing of the district heating water. Each building has its own variable speed district heating water pump that should be operating at maximum design flow under heavy heat loads to prevent local freezeup.
 

PRESSURE ISOLATION HEAT EXCHANGERS:
Every building is pressure isolated from the district heating pipe network by an isolating heat exchanger which is integral with the heat pump. The temperature of the isolation heat exchanger district heating water inlet varies from 70 deg C on a hot summer day to 30 degree C in the winter. The temperature of the isolation heat exchanger district heating discharge water varies from 50 degrees C in the summer to 10 degrees C in the winter.

If the isolation heat exchanger primary discharge temperature approaches freezing a natural gas/hydrogen burner is enabled to prevent freezing. If the reactor is shut down these burners maintian the district heating water temperature which indirectly provides heat for summer domestic hot water supply.
 

HEAT PUMP EFFICIENCY:
The heat pump working fluid absorbs thermal energy at about 10 degrees C (283 degrees K) and discharges thermal energy at about 80 degrees C (353 degrees K). The electrical energy input to the heat pump raises the butane temperature by:
(80 - 10) = 70 degrees.

The heat pump coefficient of performance (COP) is limited by the actual behavior of the heat pump working fluid butane in this application. The latent heat of vaporization is about 386,000 J / kg. The Cp is about 1675 J / kg deg K. The change in butane temperature is about 70 deg K. Thus the mechanical energy that must be added between the two butane phase transition points is:
1675 J / kg deg K X 70 deg K = 117,250 J / Kg.
Thus the crudely estimated heat pump COP under typical operating condidtions is:
(386,000 + 117,250) / (117,250) = 4.29.
Note that waste heat from the heat pump motor should be captured by the working fluid upstream from the compressor. Hence ongoing heat removal from the heat pump room is not an issue. However, detection of butane in the room air is an important fire prevention issue.

Typically the compressors must be sized to provide about 25% of the building's peak heat load. Even if there are two heat pumps per building these compressors are still major motor loads. For a single family home with a single heat pump that must deliver 25 kWt to the home the compressor motor size has to be ~ 10 HP.
 

REVERSE RETURN DISTRIBUTION PIPE CONFIGURATION:
The district heating thermal distribution in each region is laid out in a reverse return pipe configuration. Near the reactor site on the supply side the return pipes terminate and on the return side the supply pipes termiante. This feature simplifies the piping close to the reactor site.

Each thermal load building has its own variable speed circulation pump for controlling the water flow through its isolation heat exchanger primary which determines the rate at which the building draws heat from the district heating system. Those flows will tend to reduce the pressure in the regional heat loop supply side pipe as compared to the regional heat loop return side pipe. Hence all the building circ pumps and cooling tower circ pumps operate against an external differential pressure.

Thus in the service area under every street are two pipes, a heat supply water pipe and a heat return water pipe, but the flow direction is the same in both pipes. These pipes should be laid out in loops so that a reverse return configuration is achieved with a minimum length of buried pipe.

In close proximity to the reactor site there 4 major heating loops which require 8 radial pairs of such pipes. As these pipes converge on the reactor site the under road piping becomes quite congested. Near the reactor site half of the pipes terminate so that at under each of the road intersections adjacent to the reactor block there are just two 48 inch diameter X 90 degree pipe elbows. Under the perimeter roads each 48 inch pipe main splits into 4 X 24 inch pipes going into or out of each turbine hall.
 

BURIED PIPE LAYOUT:
Four parallel 24 inch OD district heating water supply pipes exit from a turbogenerator hall, combine into one 48 inch OD supply pipe, round the nearest intersection corner, are mated with an associated return pipe, go out in a big loop and then return to the to the vicinity of the reactor. At that point it the supply pipe terminates and the 48 inch OD return pipe passes through a 48 inch X 90 degree elbow around an road intersection corner, splits into 4 X 24 inch OD return pipes and enters a turbogenerator hall.

In the turbogenerator halls there are isolation valves which permit taking any turbogenerator condenser out of service while maintaining heating with three other turbogenerators. Each such loop has one on-site dry cooling tower and three associated remote dry cooling towers.
 

In the turbine halls these pipes pass through 4 isolation valves, 2 circ pumps, 2 check valves, 2 condensers and then wrap around the inner side of the shared adjacent cooling tower and water tank and then pass into and through the adjacent turbine hall also containing 4 isolation valves, 2 circ pumps, 2 check valves and 2 condensers.
 

BURIED PIPE SIZES:
This description applies to each of four identical buried heat distribution loops. a) At the reactor site there are 4 X 24 inch OD supply water pipes. Under the adjacent perimet road theses pipes combine into one 48 inch pipe.
b) This pipe exits the area via a 48 inch 90 degree elbow around the adjacent street corner. c) After passing the corner the 48 inch supply water pipe is paired with a 24 inch return water pipe.
d) About (1 / 4) of the way along the line is a remote cooling tower. At this point the supply pipe decreases to 42 inches and the return pipe increases to 36 inches.
e) About (1 / 2) way along the line is another remote cooling tower. At this point the supply pipe decreases to 36 inches and the return pipe increases to 42 inches,
f) About (3 / 4) way along the line is another remote cooling tower. At this point the supply pipe decreases to 24 inches and the return pipe increases to 48 inches.
g) Close to the reactor the 24 inch supply pipe terminates. The 48 inch return pipe contines.
h) The return pipe goes through a 48 inch X 90 degree elbow.
i) The 48 inch return pipe splits into 4 X 24 inch pipes under a reactor perimeter road.
j) The 4 X 24 inch return pipes enter a turbine hall.

Note that the load head seen by the circ pumps installed in buildings asd at remote cooling towers increases with distribution heating loop length. Hence these circ pumps will have different flow versus pressure load characteristics depending on where they are installed. This may lead to a spare parts issue. Perhaps use a common pump and different sealed balancing valve settings so as to set the building's maximum water flow rate.
 

BILLING:
The instantaneous thermal load at any building is the product of the circulated water differential temperature and the loop flow. There must be a demand charge proportionsal to the annual thermal load drawn from the district heating system.

Recall that at dT = 20 degrees C, V = 2 m /s:
A 4 inch pipe transports about 1 MWt, a 6 inch pipe about 2 MWt and an 8 inch pipe about 4 MWt. Thus building thermal service capacity should be rated by annual peak thermal load.

Each building and each cooling tower has a magnetic flow meter which is also used for heat metering.
 

CUSTOMIZATION:
The district heating distribution pipes typically operate with a supply water temperature in the range 30 degrees C to 70 degrees C and a return water temperature in the range 10 degrees C to 50 degrees C.

In general each building is fitted with heat pumps which are chosen to match the temperture and heat flux requirements of the building heating system to the available district heating water temperature. These heat pumps are comparable in size to Trane Centrivac air conditioning chillers. They need suitable basement space and a suitable electricity service. Buildings should use speed control of their district heating circ pump(s) to maintain the building dT = 20 degrees C. This control strategy prevents water flow problems with other customers and to maintains electricity generation efficiency. It is necessary to keep the sum of all the building water flow rates less than the sum of the cooling tower maximum water flow rates.

Buildings usually control their internal heating water temperature by on/off cycling of their heat pump compressors.

A natural gas/hydrogen burner or independent electric heater is required to prevent isolation heat exchanger freezeup in extremely cold weather or when the reactor is shut down.
 

SUMMARY:
This district heating system design likely requires rearrangement of existing buried services under the perimeter roads adjacent to the reactor block. It also requires 12 remote cooling towers, each of which requires real estste equivalent to two adjacent suburban single family homes, each with 20 m frontages and 40 m deep. However, this system provides 600 MWt of district heat and 200 MWt of supporting electricity for heat pumps for a peak heating capacity of 800 MWt. In addition, when comfort heat is not required this system provides over 200 MWe of electricty.

The pipe network is designed so that each heating zone is heated by waste heat from four independent turbogenerators, each of which can be individually taken out of service. When a reactor is out of service each building relies on supplementary electric or natural gas/hydrogen heating to maintain its heat pump source temperature.
 

DISTRICT HEATING SERVICE RADIUS:
In very dense high rise residential developments (eg St. James Town, Thorncliffe Park) the peak heating load per unit area is about 15 MWt / city block,or:
15 MWt / (134 m)^2 = 835.4 Wt / m^2

In suburban single family home residential developments with quarter acre lots the peak heating load may be as low as:
25 kWt / 1000 m^2 = 25 Wt / m^2

Thus the real estate area that can be served by a single FNR varies from a low of:
800 MWt / (835. 4 Wt / m^2) = 957,625 m^2
to a high of:
800 MWt / (25 Wt / m^2) = 32,000,000 m^2

In terms of a service radius for uniform property developments that radius varies from about 0.5 km for very high density developments to about 3 km for residential suburbs. Clearly in design of a district heating system the future makeup of the thermal load must be carefully assessed. In lower density suburbia where individual properties are 1 acre or more rather than (1 / 4) acre, district heating may not be economic as compared to other heating alternatives.

This web page last updated February 21, 2021

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