DISCHARGE SYSTEM FOR MASSIVE-CORE STORAGE HEATER

Abstract: The present invention aims to achieve a discharge system for a massive-core storage heater (1) in which system the water flow via the heater (1) and the condenser (4) is controlled in such a manner that the water recirculation rate depends only on the heat output extracted via the condenser (4). The invention is based on a closed recirculation of water in the system and on the control of water recirculation by means of a diaphragm-type expansion vessel (7). The storage heater (1) is discharged via a condenser (4) connected to one or more evaporation channels (2) and the water recirculation circuit of the system is comprised of a diaphragm-type expansion vessel (7) having a variable-volume water volume (8), a water pipe union (6) which connects the water volume (8) of the expansion vessel (7) to the evaporation channel (2), and a condensate pipe (5) adapted to the condenser (4) so as to connect the water volume (8), the union (6), the evaporation channel (2) and the condenser (4) into a closed circuit.

Description: Discharge system for massive-core storage heater The present invention relates to a discharge system in accordance with the preamble of claim 1 for controlling the discharge of the heat store of an evaporating massive-core storage heater. Various types of massive-core storage heaters are most typically used as the heating system for small buildings such as single-family or multiple-family row houses. A massive-core storage heater is an apparatus, which has a core with high heat acceptance to act as the heat-storage medium. Materials typically employed in such cores are various metals, ceramics or liquids. Most commonly used heat- storage media include water, ceramic masses and iron. The control element described in this application is best suited to such storage heaters in which the core is made of a solid material such as steel. The storage heater operates so that its core is heated to a very high temperature, and the heat store of the charged core is discharged by means of an evaporator. The charging is preferably performed using cheap energy alternatives such as night-rate electricity when the tariffs are generally appreciably lower than those of times of peak demand. In heating with electricity, the core is heated by resistive elements placed inside the core material. The operating cycle of a storage heater in night-rate heating application is 24 hours, with the charging of the storage heater starting at the onset of the lower tariff. The core is heated to approx. 500...600 °C, then the charging is disconnected when the stored heat content of the core is sufficient. Typically, the core is charged to maximum storage acceptance. When the stored heat is required for, e.g., heating an apartment or hot water, water is pumped into the evaporating channels of the core in which it is evaporated, thus permitting the transfer of the core's thermal energy by means of the steam and a condenser to heating use. During the evaporation of water, pressure in the core channels increases rapidly. Due to the pressure increase, water feed must be implemented using an effective pump which can deliver water against a high backpressure. Simultaneously while water is fed into the core of the heater, steam is removed from the core and its heat content is recovered in a condenser. The steam is condensed into water in the condenser, whereby the pressure in the core is lowered by the condensation. The condensation rate depends on the heat output extracted via the condenser, and thereby, the heat energy discharged from the core correspondingly is related to the amount of water pumped into the core. Consequently, the amount of water pumped into the core of the heater should be able to rapidly follow the changes in the heat output via the condenser in order to maintain the recircuiation system in equilibrium. In other words, the water recircuiation system must be designed for an effective dynamic response, causing an increase in the required pump output capacity and complication of the control system for the equipment. The required pump output is further increased by pressure losses in the condenser. Due to these reasons, the water recircuiation systems of massive-core storage heaters are implemented using various types of pressure relief valve equipment and expansion vessels which serve to dampen the system's operation. Such arrangements result in a slow system response. In simple heating use the discharge rate via the condenser changes slowly, but the tapping of hot water causes large peaks of relatively short duration in the heat output extracted via the condenser that the system response must cope with. The continuous heat load of a single-family house is approx. 4 Kw, but the heating of hot water can increase the heat load by 20...40 kW. Furthermore, charging of the heater core simultaneously with its discharge affects the control of the water recircuiation system. Of course, the heat output cannot be cut off for the duration of the charging, and charging of the heater core must be possible, even during the daytime at peak loads, if the heat energy charged in the core during cheap tariff is insufficient to cover the daily requirement. Due to the above described reasons, the discharge systems for massive-core heaters have had a complicated, and thereby, costly design, whereby the implementation of a storage heater suited to use in a single-family house has been impossible by current means. The complicated construction of the equipment increases the need for maintenance and decreases system reliability. It is an object of the present invention to achieve a discharge system for a massive-core heater in which the water flow via the heater core and the condenser is controlled in such a manner that the water recircuiation rate depends only on the heat output extracted via the condenser. The invention is based on a closed recircuiation of water and control of water recircuiation by means of a diaphragm-type expansion vessel. More specifically, the control system according to the invention is characterized by what is stated in the characterizing part of claim 1. The invention offers significant benefits. The control system has an extremely fast response and eliminates use of external energy for pumping the water. The system settles rapidly to an equilibrium state corresponding to each load situation and has excellent dynamic characteristics. By virtue of the large pressurized area of the expansion vessel, even small pressure changes produce an immediate control system response. Changes in core temper- ature do not affect the heat output rate extracted via the condenser, because the water recircuiation rate and thus the heat output from the core are determined by the heat output rate delivered via the condenser to systems external to the condenser. Thus, the heat store of the storage heater can be discharged linearly when desirable. The control system avoids steam pressure peaks, since water is not pumped into the core, but rather, the system operates at a constant pressure level. The system is entirely closed and has only one moving component. All elements which could be subject to damage are omitted, and the need for system maintenance is minimal. A storage heater system with its storage heater dimensioned for family-house applications has a small size and requires approximately the same space as a large refrigerator. Because the system has no pumps or other noise-generating components, the entire equipment can be located even in actual living spaces. The invention is next examined with the help of the attached drawings, in which Figure 1 shows diagrammatically the basic system construction according to the invention. Figure 2 shows a graph plotted during an operating test performed on an equipment according to the invention. An expansion vessel in the context of this application refers to an apparatus whose fluid volume changes automatically in relation to the ratio of pressures prevailing in the vessel. Such an apparatus is, for example, the diaphragm-type expansion vessel described in the exemplifying embodiment below. As is evident from Fig. 1, the storage heater implemented using the control system according to the invention has an extremely simple construction. The storage heater is designated by reference number 1. The core of the storage heater 1 is provided with an evaporation channel 2, illustrated herein in a simplified manner. The form of the channel 2 can be varied, and its structure is not essential to the function of the invention. The evaporation channel 2 is joined via a steam pipe union 3 to a condenser 4, via which a piping 11 of, e.g., the central heating system of a building is adapted to pass. A condensate pipe 5 connected to a water pipe union 6 exits the condenser 4. The water pipe union 6 connects the evaporation channel 2 of the condenser 1 to the water volume 8 of a diaphragm-type control element 7, and the condensate pipe 5 is connected to the water pipe 6 in the section between the storage heater 1 and the control element The diaphragm-type control element 7 comprises two volumes separated by a diaphragm 9, namely a water volume 8 and an air volume 10. The condenser 4 has a condensing volume for the steam and an air- venting valve 12 for the system. During system start-up, entrapped air is released from the system via the air-venting valve 12 and the valve is then closed, thus making the system fully closed during use. The function of the control system is as follows: The core of the storage heater 1 is heated by appropriate means using, e.g., night-rate electricity. The heat store of the storage heater 1 can be discharged independently of its heating if the core temperature is sufficiently high. When the storage heater 1 is not being discharged, the evaporation channel 2 and the condenser 4 are filled with steam, while the rest of the recirculating water is contained in the water volume of the diaphragm-type control element 7 above the diaphragm 9. When the heat store of the storage heater 1 is to be extracted, water is circulated via the secondary circuit piping 11 of the condenser 4, thereby cooling the steam contained in the condenser 4. Resultingly, the steam contained in the condenser 4 is condensed into water, thereby lowering the pressure in the condenser 4 and allowing steam to enter the condenser 4 from the evaporating channel 2 of the storage heater 1. Thus, the pressure in the evaporation channel 2 tends to decrease. Since the pressure within the water volume of the diaphragm-type control element 7 now becomes lower than the pressure within the air volume, the water level in the water volume 8 of the control element 7 becomes higher due to the deflection of the diaphragm 9 to the water volume side under the air pressure exerted from within the air volume 10. As the water level in the control element 7 rises, water is expelled into the evaporation channel 2 of the storage heater 1, wherein it is evaporated. The water condensed in the condenser 4 returns via the condensate pipe 5 into the water pipe union 6, from where it can continue into either the evaporation channel 2 of the storage heater 1 or the water volume of the control element 7, depending on the heat load situation. The greater heat output rate from the system, that is. the more steam is condensed in the condenser 4, the more water is expelled into the evaporation channel 2 of the storage heater 1 by the air pressure imposed on the diaphragm 9. When the entire heat store of the storage heater 1 has been discharged, the evaporation channel 2 is filled with the entire volume of water that can be recirculated in the system. If no heat is being extracted, the entire volume of liquid water is contained in the water volume 8 of the diaphragm-type control element 7, while the other parts of the primary circuit channels are filled with saturated or superheated steam. A storage heater dimensioned for a single-family house has a heat acceptance of approx. 100 kWh during a day. The water volume of a control system for such a heater is approx. 1...4 liters, and as a rule, the same ratio of heat acceptance to required water volume is also applicable to larger-capacity heaters. Fig. 2 shows the results of a charge and discharge test performed on a storage heater system according to the invention. Two storage medium temperatures T2 and T5 measured at different points of the core of the storage heater 1 are plotted in the diagram using a continuous and a dotted line. The electric input energy to the storage medium is plotted using a dashed line, and the heat energy output from the storage heater is plotted using a dot-dash line. The discharge and charge of the storage heater were started simultaneously, and the core temperature in the beginning of the test was slightly above 100 °C. The core was heated for 8 hours (480 min), during which time the core temperature was elevated slightly to above 500 °C, at which point the heating was stopped. In this test the store of heat was discharged so rapidly that the heat stored in the core was insufficient to cover a full 24-hours. Therefore, more electric energy was charged into the core when the test had lasted for 16 hours. This is visible in the energy input and temperature plots as small steps. As is evident from Fig. 2, the discharge energy plot remains linear until the core temperature falls below 100 °C. A corner point can be seen in the plot at this temperature. The core temperature and energy input rate to the core have thus no effect on the energy discharge rate via the condenser, whereby linear discharge of the heat store of the core becomes possible. In addition to that described above, the invention can have alternative embodiments. For example, a greater number of condensers may generally be necessary, e.g., one for hot water and one for the heating system of the premises. If several condensers are employed in the system, the heat load can vary rapidly, because much more energy than the average heat load imposed by the heating system is required for heating, e.g., hot water; yet even these requirements can be fulfilled by virtue of the invention. The system pressure is always determined by that one of the parallel-connected condenser operating at the lowest temperature. The control element of the system can be implemented using any apparatus similar to an expansion vessel that can provide a volume which varies according to the prevailing pressure difference. Such apparatuses are, e.g., spring, diaphragm or gas-loaded pressure accumulators employed in hydraulics as well as different types of bellows and flexible containers made of metal or other suitable materials. Although the pressure difference necessary for the function of the control element can be accomplished by means of, e.g., a spring or pressurized gas, the use of air as the external pressurizing means appears to be the simplest alternative. In all cases, the pressurizing area should be as large as possible to make the control element sensitive also to small changes in system pressure.

Claims: WHAT IS CLAIMED IS: 1. A control system for discharging the heat store of a massive-core storage heater (1) comprising at least one evaporation channel (2) by means of at least one condenser (4) connected to the evaporation channel (2), characterized by an expansion vessel (7) incorporating a water volume (8) of variable volume, a water pipe union (6) connecting the water volume (8) of the expansion vessel (7) to the evaporation channel (2), and a condensate pipe (5) adapted to the condenser (4) so as to connect the water volume (8), the union (6), the evaporation channel (2) and the condenser (4) into a closed circuit. 2. A control system as defined in claim 1, characterized in that one end of the condensate pipe (5) is connected to the water pipe union (6) in the section between the expansion vessel (7) and the evaporation channel (2). 3. A control system as defined in claim 1, characterized in that the closed circuit of the system contains liquid water and steam alone. 4. A control system as defined in claim 1. characterized in that ratio of water volume contained in the system to the charge acceptance of the storage heater is 1...4 liters to 100 kWh/d. 5. A control system as defined in claim 1, characterized by a normally-closed air-venting valve (12) adapted to the condenser (4).

Inventors: SALMELIN PENTTI, SALMELIN, PENTTI
Applicants: IMATRAN VOIMA OY
Application year: 1992
Main classification: F24D11
IPC codes: F24H7