WO2003033964A1

WO2003033964A1 - Central heating system with controlled charging of heat storage vessels

Info

Publication number: WO2003033964A1
Application number: PCT/EP2001/011904
Authority: WO
Inventors: Peter J. Collet
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-10-16
Filing date: 2001-10-16
Publication date: 2003-04-24
Anticipated expiration: 2004-04-16

Abstract

A central heating system for a plurality of separate heat users, for example houses or apartments, each of such users having a relatively small stratified heat storage vessel containing heat transfer liquid of the circulation system as heat storage medium, such circulation system including branch conduit systems each supplying heat transfer liquid to a group of heat users, such branch conduit system (7) comprising main delivery conduits 10D and 11D having good heat insulation and connected to end distribution subsystems (12, 13), that supply heat transfer liquid to a smaller group of heat users (8), such subsystems comprising delivery and return conduits, such as 15D, 15R, having little or no heat insulation.

Description

Central heating system with controlled charging of heat storage vessels

The present invention relates to a heating system for a plurality of heat users, such as houses, apartments, and the like, such heat users having a domestic hot water demand and premises requiring a space heating demand, comprising at least one heat source for heating a heat transfer liquid, a circulation system for delivery of heat transfer liquid to each of such users having a stratified heat storage vessel containing heat transfer liquid as heat storage medium.

Said circulation system comprises delivery conduits for supplying hot heat transfer liquid to said heat users and return conduits for returning cooler heat transfer liquid back to the heat source. Delivery and return conduits comprise main conduits and service conduits, said service conduits connecting main conduits to each of said heat users. Said circulation system includes branch conduit systems that each supply heat to a group of heat users.

Patent application PCT/EP 00/03443 describes such a heating system having an operating method characterized by cooling down of stagnant liquid contained by de livery and return conduits o f such a branch conduit system, during time intervals in between deliveries o f heat transfer liquid for charging heat storage vessels. Such coo ling down of conduits causes conduit heat losses to be decreased in comparison with such conduits having a continuously high temperature, particularly when said time intervals are relatively lo ng, for examp le more than 3 hours, thereby improving the energy efficiency of the heating system. The described operating method further aims at realizing such long cooling down time intervals by concurrently charging heat storage vessels of a plurality of heat users connected to a branch conduit system, so as to increase the likelihood for a long time to elapse before one such heat user has a heat demand causing depletion of its heat storage vessel, particularly during periods that heat users have low heat demand, for example nighttime periods o f days that heat users have no space heating demand.

The described heating system further aims at reducing the co sts of the circulation system by providing heat users with relatively small heat storage vessels, having a liquid content of, for example, between 40 and 80 liters, and by adapting branch conduit subsystems to include low co st conduits having relatively little heat insulation.

A problem for achieving long coo ling down time intervals is however, that heat users, having high heat demands at different times, may require individual charging of such small heat storage vessels in between concurrent charging operations, causing average cooling down time intervals to be considerably shorter, particularly for conduits supplying relatively many heat users, therefore having a relatively larger number of through flows for individual charging operations .

Shorter cooling down times and an increased number of charging operations cause the energy effic iency o f such a heating system to be decreased for two reasons.

One reason is that the average conduit temperatures in between through flows o f heat transfer liquid are higher, causing a corresponding increase in conduit heat losses. A second reason is that the temperature and the vo lume of heat transfer liquid that is returned to the heat source without having been delivered to heat users, may be very much higher. Such heat transfer liquid, contained by delivery conduits having temperatures at the end o f coo ling down time intervals that are be low a selected minimum temperature for delivery to heat users, for example below 80 degrees Ce lcius, is conducted through bypass valves and return conduits back to said heat source.

Such bypassed heat transfer liquid , not having been cooled by useful heat transfer, may considerably increase the heat content of heat transfer liquid returned to the heat source, particularly when the temperature o f main delivery conduits having a relatively high liquid content, drops only slightly below said minimum temperature for delivery to heat users, such condition becoming more likely as cooling down time intervals are decreased.

Such increase in temperature o f bypassed heat transfer liquid causes the heat losses o f return conduits to increase and such increase in heat content of bypassed heat transfer liquid causes the energy efficiency of heat production to decrease, particularly when said heat source comprises energy saving means of heat production having an energy efficiency and/or heat recovery effectiveness that benefits from low return temperatures, for example heat pumps, so lar panels, low temperature heat recovery means, etc..

Such disadvantageous effects for the energy efficiency of the heating system are particularly large when charging of individual heat storage vesse ls cause a large increase in the number of charging operations. An object of the invention is to improve the energy efficiency of such heating systems comprising relatively small heat storage vessels, by adapting the operating method so as to minimize such disadvantageous effects. Another object of the invention is to provide a low cost circulation system in accordance with such adapted operating method.

According to the invention concurrent charging operations are controlled to occur, on days that heat users have little or no space heating demand, at times that a period of relatively high heat demand changes into a period of relatively low heat demand. Such timing maximizes the likelihood that when all heat storage vesse ls contain a relatively large vo lume of hot heat transfer liquid at the beginning of such a low heat demand period, such period may be bridged without any heat user having a fully depleted heat storage vessel during such period.

Such high heat demand periods are likely to include early morning periods, when domestic hot water withdrawals include high volume withdrawals for a hot shower or bath, midday and early evening periods when frequent low volume withdrawals for kitchen or washbasin usage predominate, and a late evening period when such demand is also likely to include high vo lume withdrawals for hot shower or bath.

During low demand periods in between such high demand periods, the occurrence of domestic hot water demands causing a heat storage vessel to be fully depleted, is likely to be incidental.

Further according to the invention individual charging o f heat storage vessels during such high heat demand periods is controlled to prevent the temperature of selected main delivery conduits of such branch conduit systems from dropping below a selected minimum temperature for delivery to heat users, thereby avo iding bypassing o f at least part of the heat transfer liquid contained by such conduit at the beginning of each charging operation, thereby increasing the energy efficiency of the heating system during such high heat demand periods.

According to the invention such high conduit temperature is maintained by controlling time intervals between charging operations to not exceed a selected maximum time interval. When such maximum time interval has elapsed without charging of a fully depleted heat storage vessel, a charging operation is initiated for a heat storage vessel that is not yet fully depleted.

A preferred embodiment of a heating system according to the invention comprises conduits having heat insulation that are adapted to provide a highly promising combination of high energy efficiency, when operated according to the invention, and a low cost circulation system having high reliability. Such combination is particularly promising for increasing the market penetration of heating systems comprising large scale energy saving means of heat production and requiring such heat to be distributed to a plurality of heat users.

Such low cost circulation system is provided by features such as heat users having relatively small heat storage vessels, branch conduits systems including conduits having little or no heat insulation, and not having to exclude connecting heat users located in areas that require relative ly long conduit lengths per heat user.

These and other features and advantages of the present invention are described with reference to the accompanying drawings, for an example of an embodiment of a heating system according to the invention.

Fig. 1 is a schematic representation o f a heating system comprising a heat source and a circulation system for supplying heat transfer liquid to a plurality of heat users.

Fig. 2 is a schematic representation of some o f the apparatus for receiving and utilizing heat transfer l iquid by a heat user.

Fig. 1 schematically shows a circulation system comprising delivery conduits that are represented by full lines and indicated by numerals having an affix D, and return conduits that are indicated by broken lines and like numerals having an affix R, such circulation system further including a heat source 2 comprising circulation pump 3 providing circulation of heat transfer liquid through delivery and return conduits, and heat production means 4, 5, for heating such circulating heat transfer liquid.

Heat source 2 preferably comprises energy saving means of heat production that include means 4, having an energy efficiency and/or heat recovery effectiveness that benefits from low return temperatures, such as heat pumps, so lar panels, low temperature heat recovery means, etc., and means 5, having such effic iency and/or effectiveness that is little influenced by heat transfer temperature, for example high temperature heat recovery means o f a gasmotor driven cogeneration installation.

Said delivery conduits include main ring delivery conduit I D having at least two connections with heat source 2. During periods that little or no heat transfer liquid is delivered to any o f the heat users of said heating system, circulation pump 6 provides circulation of heat transfer liquid through said main ring de livery conduit I D and high temperature heat production means 5 , so as to maintain the temperature of such circulating heat transfer liquid above a selected minimum temperature, for example above 85 degrees Celcius.

Main ring delivery and return conduits I D, 1 R, are connected to branch conduit systems that each supply a group of heat users. Branch conduit system 7 is an example o f such a branch conduit system, comprising delivery and return conduits supp lying heat to houses 8 of a group comprising two rows of houses 9.

Branch conduit system 7 comprises main delivery conduits 10D and 1 1 D that are provided with good heat insulation, such delivery conduits being selected to have temperatures that are maintained at a high level during high heat demand periods. The specific heat lo ss coefficient of such conduits is preferably less than 0,2 W/m. K for conduits having carrier p ipes with inner diameters less than 30 mm. Main return conduits 1 0R and 1 1 R are advantageously provided with less heat insulation, so as to reduce costs while only marginally increasing conduit heat losses.

Said main delivery and return conduits are connected to end distribution subsystems 1 2, 13 , that each supply a subgroup of heat users, and comprise delivery and return conduits that have little or no heat insulation. Such subsystems shown in the drawing are examples o f conduit arrangements whereby only service conduits are laid within the property boundaries of individual heat users, indicated by dotted lines 8a.

Subsystem 12 is an example of a subsystem comprising a prefabricated distribution section 14D connected to service delivery conduits 15D. Subsystem 13 is an example of a subsystem comprising a prefabricated distribution section 16D including main delivery conduit 17D, that is connected to service delivery conduits 1 8D. Service delivery conduits are provided with relatively little heat insulation, having a specific heat loss coefficient of, for example, more than 0, 5 W/m. K for a conduit having an inner carrier pipe diameter between 12 and 15 mm. Main delivery conduit 17D, having a relatively higher number o f through flows for charging o f individual heat storage vessels, may advantageously be provided with relatively more heat insulation.

The arrangement of return conduits of such end distribution sections, shown less extensively in the drawing, is similar to the shown arrangement for delivery conduits. For heat users having space heating systems operated with relatively low return temperatures, such return conduits may advantageously co mprise simple carrier pipes having no further heat insulation. The arrangement of return conduits according to subsystem 12 may advantageously be combined with the arrangement of return conduits according to subsystem 1 3 , so as to provide lower costs without causing any appreciable increase in conduit heat lo sses.

Branch conduit systems comprising such end distribution subsystems having little heat insulation, have much lower costs than branch conduit systems comprising delivery and return conduits that are all provided with good heat insulation.

Reasons for such decrease in co sts include lower material costs because o f less heat insulation, and lower installation costs because o f easier handling o f such conduits having a much smaller outer diameter and reduced we ight per meter length, particularly in the case of flexible conduits. Installation costs are further reduced by simpler and more reliable connections between conduit s having little heat insulation, such connections indicated by single cro sses such as 1 8, particularly when such connections are made above ground. Subsequently laying who le conduit sections into trenches also provides co st reductions because of greatly reduced trench widths.

The costs of such branch conduit systems are further reduced by conduits 10, 1 1 , requiring fewer connections than in the case of each user being connected to such conduits, such arrangement being particularly advantageous when flexible conduits having good heat insulation are delivered in long lengths on rolls.

Such branch conduit systems comprising end distribution subsections with conduits provided with little heat insulation, cause conduit heat losses to be increased and therefore the costs of heat production to be increased. However, for a heating system operated according to the invention, such increase is only marginal, and generally very much lower than the reduction in investment and maintenance costs provided by conduits having much reduced heat insulation.

Fig. 2 is a simplified schematic representation o f apparatus for receiving and utilizing heat transfer liquid by a heat user, the shown parts being limited to what is relevant for a description of the present invention, leaving out other parts and their function that are known to persons skilled in the art.

Such apparatus includes a relative ly small heat storage vessel 23 having a liquid content of, for example, 60 liters. The advantages of having smaller heat storage vessels particularly include lower costs and reduced space requirements. The latter advantage is particularly important when space availability is restricted, which may be the case when the heat storage vessel is installed near the main entrance o f a house or apartment, such lo cation be ing advantageous for minimizing the length and thereby the cost and the heat losses of in-house conduits connecting storage vessels to service conduits.

Bypass valve 21 , connecting service delivery conduit 1 5D to service return conduit 1 5R, is controlled to open at a signal for starting an individual or concurrent charging operation. When temperature sensor 22 signals the temperature o f the bypass flow to exceed a selected minimum temperature for delivery to stratified heat storage vessel 23 , bypass valve 21 is governed to clo se and valve 24 open, thereby causing heat transfer liquid to be conducted to the hot top end o f stratified storage vessel 23 , thereby causing cooler heat transfer liquid to be driven out of the bottom end. Valves 2 1 and 24 may advantageously be replaced by a single three way valve.

Stratified heat storage vessel 23 contains heat transfer liquid having a stratified temperature distribution, whereby an upper zone of hot liquid is separated from a lower zone of cooler liquid by a thermal separation zone 25 having a relatively steep vertical temperature gradient, such thermal separating zone moving downward during charging of the heat storage vessel, and moving upward when circulation pump 26 is operated, so as to circulate heat transfer liquid through a through flow heat exchanger 27 for heating domestic water passing through the secondary side of such heat exchanger, or through heat emitters such as radiators 28 for providing space heating. An individual charging operation, whereby only one heat storage vesse l is charged, may be initiated when temperature sensor 29 senses an upward movement of said thermal separating, signifying the heat storage vessel to be depleted by containing less then a selected minimum vo lume o f hot heat transfer liquid. Such condit ion is further referred to as a fully depleted storage vessel, although the volume of hot heat transfer liquid contained by said heat storage vesse l may be further decreased during the time after initiation o f the charging operation and before delivery o f heat transfer liquid to said heat storage vessel.

Charging of a heat storage ve ssel may also be initiated before such vessel is fully depleted, for example when a central controller, not shown in the drawing, initiates a charging operation whereby a plurality of heat storage vessels connected to a branch conduit system are charged concurrently. Such charging is preferably restricted to such storage vessels that are depleted to more than a selected minimum depletion level. Such level o f depletion, further to be referred to as minimum partial depletion, may be indicated by temperature sensor 30 detecting an upward passing of thermal separating zone 25. The level o f depletion past such minimum partial depletion may be more roughly indicated by the accumulated operating time of circulation pump 26 after such detection o f the minimum partial depletion level.

The preceding description of features of an such example of a heating system according to the invention provides a background for the following description o f operating such a system according to the invention.

Such operating method includes timing of co ncurrent charging operations to occur when a period of high heat demand changes to a period of low heat demand, particularly on days that heat users have little or no space heating demand. During such charging operation, all heat storage vessels connected to branch conduit system 7 that are depleted past minimum partial depletion level, are concurrently charged, such depletion level sensed by temperature sensor 30.

Such a concurrent charging operation may for example be initiated at around 9 a. m. on a day that heat users have little or no space heating demand, such time being more or less at the end of a high demand period that has started at around 6 a.m.. Heat demands in between such times are like ly to include large vo lume domestic hot water withdrawals for hot shower or bath, such withdrawals causing small heat storage vessels to be fully depleted and requiring individual charging at different times.

During said high demand period, and starting with the first individual charging of a fully depleted heat storage vessel, the temperature of delivery conduit 10D is contro lled to remain above a selected minimum temperature for delivery o f heat transfer liquid for heat storage, for examp le above 80 degrees Celcius. According to the invention such operating method includes initiating charging of a not yet fully depleted heat storage vessel when the time that has elapsed since a preceding charging operation exceeds a selected maximum time interval. Preferably such charged heat storage vessel has the highest depletion level at such time, such level being detected by means described in the foregoing.

Such maximum time interval, for example 20 minutes, is selected so as to prevent the temperature of stagnant heat transfer liquid contained by delivery conduit 10D from dropping more than for example 5 degrees Celc ius during such cooling time interval. As the space heating demand o f heat users increases, so that the frequency of individual charging of fully depleted heat storage vessels increases, such method of operation may be adapted so as to maintain such selected temperature level for delivery conduit 1 1 D, so as to further decrease the heat content of bypassed heat transfer liquid. Such higher space heating demand is required to provide a sufficient number of charging operations for maintaining such high temperature, such delivery conduit having a higher cooling rate because o f a lower liquid content per meter length, and because the lower number of connected heat users decreases the frequency o f individual charging operatio ns.

The heat content of bypassed heat transfer liquid may further be reduced by delivering to heat users heat transfer liquid having a temperature that is lower than said selected minimum temperature for heat storage, for example below 80 degrees Celcius, but high enough for utilization for space heating, for example above 60 degrees Celcius. During delivery of such lower temperature heat transfer liquid circulation of heat transfer liquid through radiators 28 is co ntrolled so as to prevent heat transfer liquid having such lower temperature from entering heat storage vessel 23. This may for example be effected by operating circulation pump 26 in a higher speed mode, so as to provide a circulation flow through radiators that is at least as high as the delivery flow to the heat user.

Such adaptation of the operating method according to different heat demands, particularly space heating demands, is preferably controlled by a central system controller, not shown in the drawings, connected to sensor means for sensing the outside ambient temperature. Such adaptation further inc ludes controlling the daily total time that temperatures of main delivery conduits 10, 1 1 , are maintained above said minimum delivery temperature level for heat storage, to increase as the daily space heating demand increases. Such total time may be being extended to the whole of the day when the space heating demand exceeds some higher level.

Such adaptation further includes controlling the temperature of heat transfer liquid circulating through main ring delivery conduit I D to be higher during times that conduit temperatures are maintained above a selected minimum temperature for heat storage, than during times that heat storage vessels are charged concurrently, so as to compensate for the temperature drop during cooling down time intervals and a greater temperature drop during delivery of heat transfer liquid at a flow rate that is lower during individual than during concurrent charging operations.

Claims

1. Method for delivering heat to a plurality of heat users, using a heating system comprising at least one heat source for heating a heat transfer liquid, a circulation system for delivery of hot heat transfer liquid to each of such users having a stratified heat storage vessel containing heat transfer liquid as heat storage medium, such heat storage vessel comprising operating and control means for charging such heat storage vessels, such means including means for detecting the depletion level of such heat storage vessels, the circulation system comprising main and service delivery and return conduits, the circulation system further comprising at least one branch conduit system supplying a group of heat users, c h a r a c t e r i z e d by controlling the charging of heat storage vessels connected to a selected main delivery conduit of said branch conduit system, during periods that heat users have a relatively high heat demand, so as to prevent the temperature of heat transfer liquid contained by said main delivery conduit from dropping below a selected minimum temperature.

2. Method according to claim 1, c h a r a c t e r i z e d by controlling charging of heat storage vessels to include charging of a heat storage vessel that is not yet fully depleted when a selected maximum time interval has elapsed since the preceding charging of a heat storage vessel supplied by said main delivery conduit.

3. Method according to claim 2, c h a r a c t e r i z e d by said time interval being selected to prevent the temperature of heat transfer contained by such delivery conduits from dropping below a selected minimum temperature for delivery of heat transfer liquid to said heat storage vessel.

4. Method according to claim 3, c h a r a c t e r i z e d by controlling the temperature of heat transfer liquid exiting from said heat source to provide a temperature of heat transfer liquid contained by said selected main delivery conduit at the end of said maximum time interval that is above said selected minimum temperature for delivery to heat storage vessels.

5. Method according to any of the preceding claims, c h a r a c t e r i z e d by charging operations, whereby a plurality of heat storage vessels connected to a branch conduit system are charged concurrently.

6. Method according to claim 5, c h a r a c t e r i z e d by controlling said charging operations to occur at selected times when a high heat demand period changes to a relatively long low heat demand period.

7. Method according to claim 5 or 6, c h a r a c t e r i z e d by said concurrent charging operation to only include heat storage vessels that are depleted to more than a selected minimum depletion level.

8. A method according to any of the preceding claims, c h a r a c t e r i z e d by delivery of heat transfer liquid to include heat transfer liquid having a temperature that is below said selected minimum temperature for heat storage, and controlling the whole of such heat transfer liquid delivered to the heat user to circulate through space heating emitters.

9. Method for operating a heating system according to claim 8, characterized by adapting operation of a circulation pump speed of the space heating system to provide such circulation of heat transfer liquid through said heat emitters.

10. A heating system according to claim 1, and operated according to any of the preceding claims, c har ac t e r i z e d by said branch conduit systems comprising said selected main delivery conduits provided with good heat insulation.

11. A heating system according to claim 10, c har ac t e r i z e d by said branch conduit systems comprising end distribution subsystems comprising delivery and return conduits provided with relatively little heat insulation.

12. A heating system according to claim 11, c har ac t e r i z e d by said end distribution subsystems having return conduits comprising carrier pipes having no further heat insulation.