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 low grade heat rejected by 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.

REFERENCES:
5th Generation District Heating

District Energy Digest No. 2

Beyond Electricity_The Economics of Nuclear Cogeneration

Increasing the Sustainability of Generation Assets

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, when the outside air temperature is very cold or when there is a heat pump compressor failure. That backup heat source is likely to be a synthetic fuel produced using interruptible electricity that is available during periods of relatively low electricity demand.

Until there is a large fossil carbon tax applied to natural gas, there will be a tendancy for building owners to preferentially choose natural gas rather than choose clean electricity and reactor low grade reject heat for normal building heating.
 

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 400 degrees C and is saturated at 310 degreess C, 10.0 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.

The best way to apply FNRs to supply continuous high temperature heat for nearby commercial use is to dedicate selected NaK heat transport loops for that purpose instead of for electricity generation. For long heat transport runs the heat transfort fluid should be an oil with a low vapor pressure to avoid problems relating to low temperature solidification of the heat transport fluid.

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

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 cooling towers are more effective and there is the district heating load. However, in the summer 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 an urban nuclear district heating system is its ability to use the district heating pipe network to reject surplus heat via remote cooling towers.
 

DISTRICT HEATING MARKET:
Most existing older 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 up to about 80 degrees C.

A 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 chilled to about 12 degrees C to provide air conditioning.
 

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 nuclear reactor's turbogenerator condenser coolant discharge water temperature is too low for effective winter comfort heating.

The solution to this problem is to install a water source heat pump in every building to usefully use waste heat from electricity generation for comfort heating. In each building there must also be compatible backup electric resistance or synthetic fuel fired heating to provide heat when the nuclear reactor or district heating piping network are shut down or when supplementary heat is required in extremely cold weather.
 

AVAILABLE HEAT SOURCE TEMPERATURE:
Typically turbogenerator condensers are rated to work with inlet cooling water temperatures in the range 5 degrees C to 25 degrees C and discharge water temperatures up to 45 degrees C. Assume that the district heating loop has a dT of 20 degrees C. Then at condenser cooling water discharge temperatures higher than:
25 + 20 = 45 degrees C
the electricity generation efficiency will be significantly detrimentally affected. Modern air conditioned buildings typically require a heating water supply temperature of 60 degrees C and older buildings without air conditioning require a heating water supply temperature of 80 degrees C. Thus in modern buildings heat pumps must raise the supply water temperature by:
60 C - 45 C = 15 C
and in older buildings heat pumps must raise the supply water temperature by:
80 C - 45 C = 35 C.

Note that in reality at full thermal load there is about a 10 degree C temperature drop across each heat pump heat exchanger, so the for modern buildings the full load differential temperature across the heat pump compressor is at least:
2(10 C) + 15 C = 35 C
and that for older buildings the full load differential temperature across the heat pump compressor is at least:
2(10 C) + 35 C = 55 C.
This issue severely limits the full load heat pump efficiency in older buildings. However, in modern buildings at half load the differential temperature across the heat pump compressor falls to:
2(5 C) + 15 C = 25 C
which is managable with minimum impact on the heat pump coefficient of performance (COP).
At full load there may be an operating cost tradeoff between heat pump electricity including peak demand charges and combustion of synthetic fuel.
 

DISTRICT HEATING SYSTEM OVERVIEW:
The geographic area around each FNR should divided into four approximately equal heat load zones. These zones may be at different elevations to minimize requirements for high pressure rated heat distribution pipes.

Each FNR is nominally able to produce about 1000 MWt of heat of which about 300 MWt is used for electricity generation. The remaining 700 MWt is used to supply four independent district heat distribution loops, one per zone, each rated at about 175 MWt.

Each of the four 175 MWt district heat distribution loops consists of:
a) 12 intermediate heat exchangers;
b) 2 heat exchange galleries;
c) 12 secondary sodium induction pumps;
d) 12 three way valves;
e) 12 NaK-salt heat exchangers;
f) 12 molten nitrate salt loops;
g) 12 steam generators;
h) 12 Steam Pressure Regulating Valves (PRVs);
i) 2 turbogenerators and condensers;
j) 8 X 24 inch diameter isolation valves;
k) 1 X 48 inch diameter buried supply water header;
l) 1 X 48 inch diameter buried return water header;
m) 1 local dry cooling tower with 24 inch diameter water service pipes;
n) 3 remote dry cooling towers, each with 24 inch diameter water service pipes;

COOLING TOWER TYPES:
Cooling towers are of two general types, wet and dry.

In a wet cooling tower incoming outside air is raised about 10 degrees C in drybulb temperature and then sprayed water is added to to the warmer air stream to raise its water vapor content. Thus the wet cooling tower discharge gas contains latent heat of vaporization of the added water vapor that is not present in the incoming air stream.

In a dry cooling tower to achieve the same heat removal capacity the incoming outside air temperature is raised about 20 degrees C and there is no addition of water.

As is shown herein, in the context of a nuclear district heating system to obtain efficient electricity generation it is necesary to to operate the cooling towers wet in the summer with dT = 10 degrees C and dry in the winter with dT = 20 degrees C. The change in dT requires a change in district heating loop water flow rate. Thus there is a winter mode and a summer mode. The mode is a function of the outside air temperature. Outside air temperatures above 5 degrees C indicate summer mode and outside air temperatures below 5 C indicate winter mode.
 

Each connected building has both a heat pump and a supplementary synthetic fueled heater. On the building air side the sequence is:
1) Circulating fan;
2) Heat pump;
3) Supplementary heater.

NORMAL SUMMER OPERATION - COOLING TOWER WITH 10 DEGREE C AIR TEMPERATURE RISE:
Assume that natural draft wet cooling towers and water flow rates are sized so that if the cooling tower inlet water temperature is 20 degrees C above the outside air temperature then the cooling tower discharge water temperature is 10 degrees C less than the cooling tower inlet water temperature.

Further assume that near the bottom of the cooling tower the contained air temperature rises by about 10 degrees C. Then the air and the water both undergo a 10 degree C temperature change. However the air flow has an addition of water vapor. These parameters in turn establish the required cooling tower throat diameter and height.

Note that absent building heating loads the cooling tower discharge water temperature becomes the turbogenerator condenser inlet water temperature and the turbogenerator condenser discharge water temperature becomes the cooling tower inlet water temperature.

Note that in the summer mode there must beconstant addition of water to the district heating loops to replace the water that is evaporated.

Then one important operating point is:
Condenser inlet temperature = 25 C = cooling tower discharge water temperature
Condenser discharge temperature = 35 C = cooling tower inlet water temperature
Outside air temperature = 35 C - 2(10 C) = 15 C

From this design operating point one can project a summer mode low temperature operating point of:
Condenser inlet temperature = 15 C = cooling tower discharge water temperature
Condenser discharge temperature = 25 C = cooling tower inlet water temperature
Outside air temperature = 25 C - 2(10 C) = 5 C

From the design operting point we can reasonably project a high temperature summer operating point of:
Condenser inlet temperature = 45 C = cooling tower discharge water temperature
Condenser discharge temperature = 55 C = cooling tower inlet water temperature
Outside air temperature = 55 C - 20 C = 35 C

Note that on hot summer days there is reduced efficiency in electricity generation due to the 45 degrees C condenser inlet water temperature which is 20 degrees C above nominal condenser specification.

From this data one can in principle project a wet cooling tower winter operating point of:
Condenser inlet temperature = 5 C = cooling tower discharge water temperature
Condenser discharge temperature = 15 C = cooling tower inlet water temperature
Outside air temperature = 15 C - 2(10 C) = - 5 C

However, there is a big problem. When the outside air temperature is below 0 degrees C water vapor discharged by the cooling tower can condense and form an unwelcome film of ice over everything. To avoid that problem it is necessary to operate the cooling towers dry when the outside air temperature can go below 0 degrees C. That dry cooling tower operaation results in a much larger cooling tower for removing the same amount of heat at the same temperature because the heat contained in the cooling tower discharge gas flow is much less.

To keep the cooling tower size reasonable we can increase dT from 10 degrees C in the summer to 20 degrees C in the winter. That dT increase is not practical in the summer because it leads to too high a condenser inlet temperature.

SUMMARY:
A basic problem with a dry cooling tower with a 10 degree C internal temperature rise in dry cooling towers is that the required cooling tower is physically very large. To make the cooling tower smaller it is necessary to operate the tower wet in the summer and to use a larger internal air temperature rise in the winter. The larger internal air temperature rise requires a smaller district heating loop water flow rate.

Thus the nuclear district heating system must have two operating modes. When the outside air temperature > 5 degrees C the system operates in the summer mode where the district heating loop water flow rate is approximately twice the corresponding winter flow rate.
 

WINTER MODE OPERATION - COOLING TOWER WITH INTERNAL 20 DEGREE C AIR TEMPERATURE RISE:
Assume that natural draft dry cooling towers and water flow rates are sized so that if the cooling tower inlet water temperature is 40 degrees C above the outside air temperature then the cooling tower discharge water temperature is 20 degrees C less than the cooling tower inlet water temperature.

Further assume that near the bottom of the cooling tower the flowing air temperature rises by about 20 degrees C. Then the air and the water both undergo a 20 degree C temperature change and the relative mass flows of air and water are established. These parameters in turn establish the required cooling tower throat diameter and height.

Note that absent building heating loads the cooling tower discharge water temperature becomes the turbogenerator condenser inlet water temperature and the turbogenerator condenser discharge water temperature becomes the cooling tower inlet water temperature.

Then an important operating point is:
Condenser inlet temperature = 25 C = cooling tower discharge water temperature
Condenser discharge temperature = 45 C = cooling tower inlet water temperature
Outside air temperature = 45 C - 2(20 C) = 5 C

Note that at an outside air temperature of 5 degrees C the winter mode is slightly less efficient at electricity genertion than is the summer mode, but the wintermode requires less electericty for district heating loop pumping.
 

OTHER OPERATING POINTS:
1) Condenser inlet temperature = 20 C = cooling tower discharge water temperature
Condenser discharge temperature = 40 C = cooling tower inlet water temperature
Outside air temperature = 40 C - 2(20 C) = 0 C

2)Condenser inlet temperature = 15 C = cooling tower discharge water temperature
Condenser discharge temperature = 35 C = cooling tower inlet water temperature
Outside air temperature = 35 C - 2(20 C) = -5 C

3) Condenser inlet temperature = 10 C = cooling tower discharge water temperature
Condenser discharge temperature = 30 C = cooling tower inlet water temperature
Outside air temperature = 30 C - 2(20 C) = -10 C

4)Condenser inlet temperature = 5 C = cooling tower discharge water temperature
Condenser discharge temperature = 25 C = cooling tower inlet water temperature
Outside air temperature = 25 C - 2(20 C) = - 15 C

Note that at condenser inlet temperatuires below 5 C it is necessary to use dampers to modify the cooling tower heat transfer characteristics to prevent water freezing.
 

DEEP WINTER OPERATION WITH dT = 20 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.

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 reject heat. This condition is 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:
Assume that the maximum rejct heat supplied by the FNR is 700 MWt.
Assume that (1 / 4) of the reactor heat output is dissipated by local cooling towers on the reactor site. Assume that A is the cross sectional area of 4 X 48 inch outside diameter pipes serving extended district heating loops. Then:
(3 / 4) 700 MWt = A V Cp Rho dT
where:
V = water flow velocity
Cp = heat capacity of water
Rho = sensity of water
dT = district heating loop water temperature differential
A = cross sectional area of 4 X 1.2 m inside diameter pipes. Note that a pair of 1.2 m diameter pipes is believed to be the largest that most roads can accommodate considering the need for other buried services such as storm sewers, sanitary sewers, freshwater, natural gas, electricity and telecommunications.

Rearranging the formula gives for dT = 10 deg C:
V = 525 MWt / [A Cp Rho dT]
= {525 MWt / [4 Pi (0.6 m)^2 (4.18 kJ / kg-deg C)(1000 kg/ m^3) (10 deg C)]}{1 kJ /s-kWt}{1000 kWt / MWt}
= {525 / [4 Pi (.36)(4.18)(10)]} m / s
= 2.776 m / s
for the summer mode and for:
dT = 20 deg C:
V = 1.388 m / s
for the winter mode. These are acceptable water flow velocities for the summer and winter modes. However,the pumping power required in the summer mode will likely be 4X the pumping power required in the winter mode.

Hence use 48 inch OD pipe for each of the four main loop supply and return headers. Use 24 inch OD service pipes for each of 16 cooling towers.
 

At full load one 48 inch OD pipe transports about 131.25 MWt and a 24 inch OD pipe transports about 32 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 the approximate 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 specified mode dependent dT across the building. In the summer the load water flow rates are low whereas in the winter the load water flow rates are high. The cooling towers are connected in parallel with the buildings and operate in a heat balancing mode. Whatever water flow and heat the buildings do not absorb the cooling towers must absorb. 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 also attempt to maintain the specified mode dependent dT.

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

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

In order to run the electricity generation at maximum capacity the turbogenerators require a constant flow of condenser cooling water to sink rejected heat. Hence the heat distribution loops are operated at a water flow rate which must be sufficient to absorb the turbogenerator reject heat. The turbogenerator reject heat must always be greater than the sum of the building thermal loads.

To the extent that the building thermal loads do not draw all the available heat the cooling towers must reject that heat. Hence the water flow rate through the cooling towers must be adjustable to balance the total building 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 the specified dT value.

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 to the atmosphere 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 might be so low that the cooling towers must close protective air flow dampers to prevent coil freezing.
 

HEAT PUMP ELECTRICITY LOAD:
At maximum thermal load the combined electricity load imposed by all of the in-building heat pumps and related equipment will be 150 MWe to 200 MWe corresponding to a COP in the range 4.5 to 5.7.
 

MECHANICAL POWER REQUIRED FOR FLUID ACCELERATION OUT OF THE CONDENSERS:
Independent of viscosity the circulated water must be accelerated twice on each trip around the loop, once on exiting the turbogenerator condenser and once on exiting the load or cooling tower. The theoretical power required for a single fluid acceleration when dT = 10 degrees C is:
(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 4 Pi (0.6 m)^2 m^2 X (2.776 m / s)^3 X (1 / 2)
= 48,388 kg m^2 / s^3
= 48,388 J / s
= 48.388kWe

Since each loop requires at least dtwo fluid accelerations, the theoretical minimum pump electrical load is 96.777 kWe

With real pumps that are seldom more than 50% efficient there is a minimum pump load of 200 kWe before consideration of load pressure drop or viscosity or corner resistance.

The thermal distribution system pump load is assigned to the cooling towers and the buildings.
 

WATER FLOW DESIGN:
The central circ pumps operate at constant but mode dependent 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 the required mode dependent loop dT. As a building's thermal load increases the water flow through that building increases.

The cooling towers have variable speed pumps that operate to maintain the required mode dependent a loop dT. As the buildings' thermal load increases the water flow throungh the cooling towers decreases.

If during the shoulder seasons the electricity production decreases then the condenser sourced 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 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.

HEAT PUMP PARAMETERS:
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.

Each 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 condenser 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 backup synthetic fuel burner is enabled to prevent freezing. If the reactor is shut down these burners can maintain 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 leaking into 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 ~ 7 kWe.
 

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 and 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 = 1.39 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 estate equivalent to two adjacent suburban single family homes, each with 20 m frontages and 40 m deep. However, this system provides 700 MWt of district heat and 150 to 200 MWe of supporting electricity for heat pumps for a peak heating capacity of 850 to 900 MWt. In addition, when comfort heat is not required this system provides 300 MWe of electricty.

The pipe network is designed so that each heating zone is heated by waste heat from two independent turbogenerators, either of which can be individually taken out of service. When a reactor is out of service each building must rely 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, Liberty Village) 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 plus associated street width 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 0.75 km for very high density developments up to about 4 km for residential suburbs. Clearly in design of a district heating system the future geographic distribution of the thermal load must be carefully assessed. In low density suburbia, where single family residential properties are greater than (1 / 4) acre or (20 m X 40 m), nuclear district heating might not be economic as compared to other heating alternatives.

This web page last updated January 5, 2023