Off-peak electric liquid heating system employing regulatable heat pipe

A liquid heating system, for example a central heating system, including a heat store in the form of a stack of bricks heated to a relatively high temperature by electric heating elements embedded in the bricks and energized by off-peak electricity is thermally connected to a vessel containing the liquid to be heated by a controllable heat pipe. The heat pipe includes an evaporator zone in thermal contact with the heat store and a condenser zone in thermal contact with the vessel, which could be a water tank. The zones are joined by at least one duct to form a hermetically sealed unit containing a small quantity of a volatile liquid which, in use, is a totally evaporated in the evaporator zone so that the rate of heat transfer to the condenser zone from the evaporator zone is determined by the return flow rate of condensed volatile liquid to the evaporator zone. A return flow control device responsive to the pressure within the condenser zone is provided for controlling the return flow rate of the condensed volatile liquid to the evaporator zone.

This invention relates to liquid heating systems and is applicable 
particularly, but not exclusively, to the use of a heat pipe for the 
transfer of heat from an off-peak electrically heated store to a supply of 
water. 
According to one aspect of the invention, there is provided a liquid 
heating system, for heating a first liquid, including a heat store to be 
heated to a relatively high temperature by off-peak electricity thermally 
connected by a heat pipe to a vessel to contain the first liquid which is 
to be heated to a temperature lower than said relatively high temperature, 
the heat pipe including an evaporator zone in thermal contact with the 
heat store and a condenser zone in thermal contact with the vessel, the 
zones joined by one or more ducts, the evaporator zone, condenser zone and 
the duct or ducts being hermetically sealed and containing a predetermined 
quantity of a volatile second liquid, arranged so that after the second 
liquid is evaporated in the evaporator zone, it passes through the duct or 
one of the ducts to the condenser zone where it is condensed, and from 
whence the second liquid returns to the evaporator zone through the duct 
or through another of the ducts, the quantity of second liquid being 
chosen to be sufficiently small so that, in use, the rate of heat transfer 
from the evaporator zone to the condenser zone is determined by the rate 
of return flow of the second liquid toward the evaporator zone rather than 
the rate of flow of the evaporated second liquid towards the condenser 
zone, or the heat transfer to the evaporator zone or from the condenser 
zone. 
Conveniently the liquid heating system includes control means arranged to 
collect a predetermined volume of the second liquid, said volume being 
variable in response to the pressure or temperature within the heat pipe, 
so as to reduce the amount of second liquid circulating in the heat pipe 
as the pressure or temperature therein rises. Preferably the liquid 
heating system includes reservoir means positioned to collect said second 
liquid up to a predetermined level, and means to alter the volume of the 
second liquid in the reservoir means in response to changes in pressure or 
temperature in the heat pipe. 
The heat store may comprise a mass of solid material to be heated by an 
electrical resistance element, for example bricks. Furthermore, there may 
be metal fins in thermal contact with the evaporator zone, the fins 
extending among the solid material. Such fins may be cast iron plates. 
Preferably the outside of the condenser zone, within the vessel, is 
provided with heat exchange fins, corrugations or other heat exchange 
surfaces. 
The wall of the condenser zone above the reservoir may be of downwardly 
convergent shape, whereby condensate forming thereon will run down and 
drop into the reservoir. 
Conveniently the reservoir means includes a sealed capsule which contracts 
in volume as the pressure in the condenser rises. 
The liquid heating system may include a valve means arranged to constrict 
the flow of the evaporated second liquid along the duct as the temperature 
or pressure in the condenser increases. 
The first liquid may be water and the vessel may be connected into a hot 
water central heating system.

FIG. 1 shows, in diagrammatic form, one embodiment of the invention in 
which heat is extracted from an off-peak electrically heated store 10 and 
transferred to water circulating through a system, such as a domestic 
central heating installation. The heat store 10 comprises a rectangular 
stack of bricks 11 embedded in which are electrical resistance heating 
elements, shown schematically at 10A in known manner. Normally, 
electricity is used to heat up the bricks 11 at a time when electricity 
can be purchased cheaply and heats up the bricks 11 over a period of 
hours. The whole of the heat store 10 is surrounded by insulation, which 
is unshown apart from the base portion 12. 
Heat transfer through the bricks 11 is assisted by horizontally extending 
plates 13 having a higher thermal conductivity than the bricks and aligned 
central bosses 14 through which a vertical heat pipe 15 is fitted. The 
heat pipe 15 is not necessarily attached to the bosses 14 but is at least 
a close fit thereto, so as to ensure good heat transfer. Heat is conducted 
through the bricks 11 for a relatively short distance to the nearest plate 
13. It then passes inward along the plates 13 to the bosses 14 and hence 
into the heat pipe 15. 
The heat pipe 15 comprises a vertical hermetically sealed evaporator tube 
16, closed at its base and opening at the top through the downwardly 
sloping base 17 of a condenser 18, which has a cylindrical wall 19 and top 
wall 20. 
The condenser 18 is immersed in a water tank 21, having an inlet pipe 22 
and outlet pipe 23. 
In a typical domestic central heating system, water is pumped through the 
inlet pipe 22, through the tank 21 and out through the pipe 23 to the 
radiators in various rooms. To assist heat transfer from the condenser 
wall 19, it is formed with heat exchange fins 24, or other appropriate 
heat exchange surfaces such as corrugations 24D, FIG. 1A, which depicts 
diagrammatically a cross-sectional plan view of a condenser 18D having a 
wall 19D in which the corrugations 24D are formed, in the water of the 
tank 21. 
The heat pipe 15 is generally evacuated apart from a few cc of an 
appropriate volatile liquid, such as water, so that the interior of the 
heat pipe 15 contains water and water vapour only. In use, heat from the 
bricks 11 heats the wall of the evaporator 16 and evaporates the water 
which rises up the centre of the evaporator 16 and into the condenser 18, 
where it condenses on the walls thereof and runs, as shown by the arrows, 
back down the walls of the evaporator 16, where it is re-evaporated, thus 
forming a continuous cycle. This principle of the heat pipe 15 is well 
known and enables high rates of heat transfer to be made between the 
evaporator 16 and the condenser 18. 
The temperature of the bricks 11 will rise during the period in which 
electricity is supplied thereto, reaching a maximum of some hundreds of 
degrees centigrade, from which it falls during the period when heat is 
being extracted. Thus, the potential heat supply to the evaporator 16 
varies with time. 
Similarly, the water passing out of the tank 21 to the pipe 23 will 
normally be required at a temperature somewhat below 100 degrees C. 
although the temperature of the water entering through the pipe 22 may 
vary from cold up to nearly the temperature of the outgoing water through 
the pipe 23, depending on the amount of heat extracted through the rest of 
the central heating system On some occasions, the pump may be stopped so 
that little water flows through the tank 21. In the circumstances, there 
may well be a mis-match between the heat fed into the evaporator 16 and 
that removed by the water through by the pipe 23. Particularly, there may 
well be a tendency for the water in the tank 21 to boil unacceptably. 
This situation is alleviated by the automatic control of the rate at which 
heat is transferred up the heat pipe 15. In the condenser 18 there is 
provided an open topped reservoir 25 rigidly located from the condenser 18 
by structure 26. An evacuated bellows capsule 27 is fastened within the 
reservoir 25. As the pressure of vapour in the condenser 18 rises and 
falls it will cause the capsule 27 to shorten and lengthen respectively. 
Over a short period of time the heat input to the evaporator 16 from the 
bricks 11 and plates 13 will tend to remain substantially constant, 
whereas if the water passing through the pipe 23 is reduced in flow or 
increased in temperature, the heat removed by the water leaving through 
the pipe 23 will be reduced. In this situation the temperature of the 
vapour in the evaporator 16 and condenser 18 will tend to rise, and the 
vapour pressure therein will also rise, and thus, the capsule 27 will 
shorten. In normal operation, the cooling of the vapour within the 
condenser 18 causes water to collect in liquid form within the reservoir 
25, but outside the capsule 27. Therefore, as the vapour temperature and 
pressure rise and the capsule 27 shortens, there is space within the 
reservoir 25 to collect more water. Consequently, the amount of water in 
circulation through the evaporator 16 and condenser 18 is reduced which 
reduces the rate of heat transfer from the evaporator 16 to the condenser 
18, thus providing a compensating effect for the reduced amount of heat 
which is required to be withdrawn through the pipe 23. If the capsule 27 
is arranged to retract axially to the point where the reservoir 25 will 
accommodate all the liquid in the heat pipe 15, at a given temperature, 
transfer of heat thereby will be virtually stopped at that temperature. 
The liquid in the heat pipe 15 is chosen such that its boiling point, at 
the relevant working pressure within the condenser 18, is only a little 
above the maximum required temperature for the water leaving the pipe 23. 
Thus, the heat source is easily able to evaporate the liquid arriving in 
the evaporator 16. The result is that the heat transferred is insensitive 
to the temperature of the bricks 11 and plates 13, which can vary widely 
over time with little effect. Furthermore the operating pressure is much 
closer to the vapour pressure of the working fluid at the temperature of 
the condenser 18 than at the temperature of the evaporator 16, so the 
pressure is also insensitive to the temperature of the bricks 11 and 
plates 13. Therefore, a much more volatile working fluid can be safely 
used than would normally be permitted by the envisaged maxmum of the 
temperature of the bricks 11 and plates 13. 
The large surface area of the condenser 18 in contact with the water in the 
tank 21 ensures that the temperature at which the water condenses in the 
condenser 18 is higher than the temperature of the water in the tank 21 by 
a modest amount, this is necessary for stable operation of the heat pipe 
15. 
It is convenient to arrange the capsule 27 such that it displaces as much 
as possible of the liquid out of the reservoir 25 when the pressure in the 
condenser 18 is around 0.4 an atmosphere absolute. It is also arranged to 
allow the reservoir 25 to collect all of the liquid (typically only a few 
cubic centimeters) when the pressure is around 0.85 atmosphere absolute. 
The heat pipe 15 therefore transfers a maximum quantity of heat (several 
hundred watts to a few kilowatts) up to a radiator water temperature of 
about 75 degrees C., and then progressively less until the heat transfer 
is zero at around 95 degrees C. If there is no water flow around the 
condenser 18, the stagnant water reaches 95 degrees C. and nothing further 
happens until cooler water enters from the radiators. 
FIG. 2 shows an alternative to the upper part of FIG. 1 in which the 
bellows 27A is arranged to sense the difference in pressure between the 
condenser 18A and the pressure of the water in the tank 21A, which will 
generally be a small fixed amount above atmospheric pressure. For this 
purpose, the interior of the bellows 27A is vented through loose fitting 
cylindrical guide elements 28, 29 to the interior of the condenser 18A, 
whilst the whole of the capsule is immersed in the water in the tank 21A. 
In further, unshown, embodiments, the bellows capsule 27 may respond to the 
difference between the pressure in the condenser 18 and atmospheric 
pressure, by locating the bellows capsule 27 in the air external of the 
tank 21. In addition to the automatic regulation of temperature of the 
water leaving the pipe 23, the actual temperature achieved can be adjusted 
by applying an appropriate external axial force to the bellows 27, for 
example through a spring. Alternatively, the relative axial location of 
the reservoir 25 and capsule 27 in FIG. 1 can be adjusted, so as to change 
the pressure in the condenser 18 at which all of the liquid becomes 
trapped in the reservoir 25. 
In FIG. 3 the reservoir 25B is formed partly between the bellows capsule 
27B and a further coaxial bellows capsule 30. The capsules 27B and 30 are 
sealed together at the bottom whilst the capsule 27B is sealed at the top 
to the condenser 18B and capsule 30 is sealed at the top to the evaporator 
16B. The upper part of the evaporator 16B fits within the lower part of 
the condenser 18B, with a small gap, so that liquid condensed on the walls 
of the condenser 18B trickles down through the gap and maintains the 
reservoir 25B full of liquid, to the level of the top edge of the 
evaporator 16B. In this embodiment, when the operating pressure increases, 
more room for liquid is created in the reservoir 25 so that the amount of 
liquid in circulation is reduced rapidly, the mechanism being the flow of 
all the condensed liquid into the reservoir 25B, rather than the flow of 
some condensed liquid and condensation of vapour therein. 
All the arrangements described above ensure that the corrugations of the 
bellows are beneath the level of the liquid in the reservoir. If they are 
not, liquid can then condense in the corrugations and become trapped 
there. 
The variation of heat transfer with radiator water temperature, and the 
stability of operation of the heat pipe 15 can be altered by arranging 
that the component which displaces liquid from the reservoir 25 and/or the 
reservoir 25 itself have horizontal cross-sectional areas which vary with 
height. 
The sensitivity of heat transfer to the temperature of the bricks 11 could 
be further reduced by various means, for example, spiral grooves in or 
wires on the wall of the evaporator 16 or multiple circular corrugations 
in the wall of the condenser 18, forcing the liquid film to flow at a 
small angle to the horizontal, thus slowing the flow and increasing the 
water content of the condenser 18. 
An alternative use to the system described above, instead of transferring 
heat from high temperature bricks 11 to water in a tank 21, is where it is 
required to maintain an object or a space at a given temperature against 
variable heat losses. The object or space is connected in thermal contact 
with the condenser 18 of the heat pipe 15 of the type described above. 
Heat can be applied to the evaporator 16 through a simple on-off control 
which would cause the temperature surrounding the evaporator 16 to rise 
and fall significantly. However, for the reasons explained above, the 
object or space would approach the desired temperature smoothly and would 
tend to stay at that temperature. 
In the embodiments described above, the condenser 18 is located above the 
evaporator 16, so that condensed liquid can flow under gravity down to the 
evaporator 16. Other dispositions of the components may be utilised if the 
condensed liquid is returned from the condenser 18 to the evaporator 16 by 
means of capillary action, such as by the use of a wick or porous means as 
is well-known in heat pipes. 
In all the above embodiments the quantity of fluid in the heat pipe 15 is 
so chosen that the liquid running down the wall of the evaporator 16 is 
evaporated before it reaches the bottom. The heat transfer from the 
evaporator 16 to the condenser 18 is therefore determined by the rate at 
which the liquid film can fall from the condenser 18 to the evaporator 16. 
The small diameter evaporator 16 means that, for any given quantity of 
water in circulation, the thickness of the water film and therefore the 
heat transfer are less sensitive to brick temperature than they would be 
if the evaporator 16 were as large as the condenser 18. This is because 
only a small fraction of the water in circulation is in the evaporator 16. 
In FIG. 4, the top wall 20C of the condenser 18C has a downwardly 
convergent conical form, so that vapour condensing on the wall 20C runs 
downwardly and inwardly to fall into the reservoir 25C which is rigidly 
fastened to the condenser 19. 
Within the reservoir 25C, a bellows capsule 27C has its ends sealed 
respectively to a cap 32 and the base 33 of the reservoir 25C. As the 
pressure within the condenser rises the capsule 27C shortens until the 
tube 31 rests on base 33 and the cap 32 abuts the top end of the tube 31. 
When the cap 32 is thus fully lowered towards the base 33, a shield 34, 
suspended by 3 rods 35 from the cap 32, is arranged to just close the 
upper part of the heat pipe 15C, to prevent heat transfer by convection of 
water vapour. 
As the pressure in the condenser 18C falls, the cap 32 will rise, which in 
turn raises the shield 34, to restore full flow of vapour between the heat 
pipe 15C and condenser 18C. 
The interior of the condenser 18C is evacuated and charged with water 
through a vacuum seal-off fitting 36. 
In FIG. 4 the outlet pipe 23C is below the level of the top of the wall 
20C. During steady operation of the system most of the heat transfer 
occurs through the side walls 19C of the condenser 18 and the heat 
exchange surfaces 24C. Thus most of the condensate will form on the side 
walls and not pour into the reservoir 25C. However, when the temperature 
of the water in the tank 21C starts to rise, the temperature of the water 
within the conical wall 20C, being somewhat isolated from the main stream 
leaving by the outlet 23C, will lag behind so that condensate continues to 
form thereon and runs into the reservoir 25C, which will have had its 
capacity increased by compression of the bellows capsule 27C in response 
to the increased pressure in the heat pipe 15C.