EP1866579A2

EP1866579A2 - A system for utilization of thermal energy

Info

Publication number: EP1866579A2
Application number: EP06716780A
Authority: EP
Inventors: Kjell Emil Eriksen
Original assignee: Individual
Current assignee: Thermia Varme AB
Priority date: 2005-03-23
Filing date: 2006-03-23
Publication date: 2007-12-19
Also published as: NO20051564D0; WO2006101404A3; WO2006101404A2; NO20051564L; EP1866579A4; NO327321B1

Abstract

System for utilization of thermal energy. The system comprises a heat pump and accompanying control circuit for optimization of the energy output. The heat pump comprises a first primary circuit collecting thermal energy from air and a second primary circuit collecting energy from another source, e.g. the ground. It also comprises a secondary circuit receiving energy from the primary circuits and distributing this to a building. The control circuit has a storage medium for a set of pre-set parameters and a comparator to compare these parameters with measurements in connection with the primary circuits. Based on this comparison the control circuit decides which of the primary circuits is to be used for energy outtake.

Description

A SYSTEM FOR UTALIZATK)N OF THERMS

The present invention concerns a system for utilizing thermal energy in accordance with the preamble of the following Claim 1.

In recent years, heat pumps, and in particular liquid-liquid heat pumps, have seen a great improvement in efficiency. It has also become more common to connect the heat pump to the hot water supply. In the case of the latter, reference is made to the following prior art:

DE 2815974 describes a system in which a heat pump is used both to provide cooling for a cooling unit and heat for a hot water tank. Here, the coolant flow to the heat pump passes through an evaporator in the cooling unit, and then through a condenser in the hot water tank. This will allow the heat absorbed by the coolant in the evaporator to be transferred to the hot water in the hot water tank.

This solution is conditional on a constant requirement for both cooling and heating. If one does not have a cooling plant that allows heat extraction, this solution will be completely useless. In addition, the valuable superheat will not be utilized, as all the heat from the heat pump is extracted in one place. Thus the temperature of the domestic hot water will be limited.

NO 313062, by the same applicant, describes a hot water tank connected with a heat exchanger. Water circulates from the lower part of the tank, through the heat exchanger, and returns to the tank a small distance above the outlet. This heats the water up to a temperature of about 40 ⁰C. This water is heated further by an electric heating coil, and circulates in the middle section of the tank, where there is provided a heat exchanger that extracts heat for heating the building. In the upper section of the tank there is provided an additional electric heating coil, which heats the water in this area to a temperature sufficient for water that is to be consumed.

This solution has proven not to be ideal. Although a higher efficiency is achieved, it has proven difficult to make the water in the hot water tank circulate in the allocated zones of the tank. Experiments have been conducted with an insulating plate intended to float at the boundary between the two temperature zones of the tank, but his has proven impossible to achieve.

JP 2003056905 describes a system for producing both hot domestic water and water for heating. The system makes use of a heat pump and a storage tank for hot water. This tank communicates with an auxiliary tank, supplying this auxiliary tank with hot water. From the auxiliary tank, hot water can circulate to an underfloor heating system.

Even though this system uses two separate tanks, the auxiliary tank supplies both hot domestic water and water for heating. As the temperature of domestic water is considerably higher than that of heating water, this is not an ideal solution.

Heat pumps depend on an accumulator or buffer tank to achieve smooth operation. To ensure an adequate lifetime for the compressor and the best possible efficiency, the temperature difference between the coolant/heating medium and the water in the tank should be as small as possible. The new CO₂ heat pumps make this even more important, as the temperature difference between incoming and outgoing water is of crucial importance to the operating results.

EP 1248055 describes the use of energy from three different sources; ambient air, terrestrial heat and solar heat. The system consists of several circuits. A first circuit is connected to an air collector and a solar absorber. The use of valves allows the user to decide how much heat to draw from each energy source. The decision is based on temperature measurements in the circuit. The first circuit has a heat exchange relationship with a second circuit. The second circuit includes a burner. The second circuit has heat exchange relationships with a secondary circuit, both directly via a heat exchanger and indirectly via a condenser circuit. The secondary circuit includes a hot water tank.

The choice of energy source in accordance with this publication is based solely on temperature measurements. DK 136497 describes a heat pump tied in to a boiler. Here, the superheated gas from the compressor is directed to a heat exchanger. The heat exchanger is a water-filled tank with a pipe coil. The gas then passes to a heat exchanger in the boiler, after which the gas is liquefied. The liquid is passed via various components to a heat exchanger in the heat pump.

Thus this makes use of the superheat to heat hot water to a temperature of about 9O⁰C.

NO 135444 illustrates a heat pump installation much like the preceding publication. Here, the superheated gas is also first cooled in a first condenser. The rest of the heat is liberated in a second condenser.

It is an object of the present invention to provide a system where it is possible to extract energy in an efficient manner at any time. The aim is to develop a system where the choice of energy source is not dependent only on measurements of the temperature differences between the air and the heat pump medium and between the inflowing and outflowing heat pump medium, respectively, such as in EP 1248055, but also depends on other important parameters.

According to the invention, this is achieved through the heat pump comprising a first primary circuit arranged to extract thermal energy from air, a second primary circuit arranged to extract thermal energy from at least one other source, and at least one secondary circuit arranged to receive thermal energy from the primary circuits, and that the control circuit includes a storage medium for a set of predetermined parameters, a comparing element for comparing the predetermined parameters with measurements tied to the first and second primary circuits, in order to determine which of the two primary circuits energy is to be extracted from at any time, based on the relationship between the measurements and the parameters.

Preferably the parameters are temperatures in the primary circuits, temperature differences between air and the second energy source, time of year, and expected consumption over a predetermined period. If, in the cold season, the criteria for selecting a primary circuit as that from which energy is to be extracted, include allowing the air temperature to fall below the temperature of the borehole/water by a certain value before the second primary circuit is selected in preference to the first primary circuit, the utilization of the available energy will be even more efficient.

According to a preferred embodiment of the invention it comprises a water heater, the heat pump being arranged to transfer heat and/or provide cooling for water in the water heater, that the water heater comprises at least two tanks for thermal storage of hot and/or cold water and/or ice, which tanks are thermally insulated from each other, a first tank having a first inlet for cold water and the second tank having an outlet for hot water, that the first tank has a heat exchange relationship with the heat pump via a first heat exchanger having a first temperature level and the second tank has a heat exchange relationship with the heat pump via a second heat exchanger having a second temperature level higher than the first temperature level. This provides a compact system for utilizing the energy to heat hot water and heat or cool a building.

Having the first tank in liquid communication with the second tank makes it possible to preheat the water in the first tank before it enters the second tank.

Arranging the heat exchanger of the first tank to receive hot or cold liquid from the heat pump and further connecting the heat exchanger to a heating/cooling system for heating or cooling of a building, provides an efficient system for distribution of heating/cooling.

Arranging the second tank to comprise a heat exchanger which is arranged to receive hot liquid from the heat pump, preferably the heat pump superheat circuit for heating the water in the second tank, provides an efficient system for provision of hot domestic water.

Placing the first and second tanks in a common jacket provides a compact unit that contains all central functions. In a further embodiment the system comprises an outlet between the first and the second tank, for drawing off water, especially for drawing off chilled water. This makes it possible to provide cold drinking water in an energy-saving manner.

Having a heat exchange relationship between the water heater and waste water, possibly via a third tank in liquid communication with one of the other tanks of the water heater, allows recovery of heat which would otherwise go to waste.

In a further embodiment the system comprises a heat exchanger for heat transfer from waste water to the external circuit of the heat pump. With this, heat which would otherwise go to waste can also be recovered and optionally accumulated.

Arranging the heat pump in a heat exchange relationship with a tank containing liquid such as water, and arranging it to provide cooling of the liquid for accumulation of ice and/or chilled liquid, makes it possible to accumulate ice or ice water to act as a cooling reservoir.

In a further preferred embodiment excess heat from the air may be stored for subsequent use, e.g. by sending it down into the ground via a borehole.

The invention will now be explained in greater detail with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates the principles of the system according to the present invention;

Figure 2 shows details of the heat pump in Figure 1 ;

Figure 3 shows a combined system for heating, cooling and heat recovery from waste water according to an alternative embodiment of the invention; and

Figure 4 shows yet another embodiment of the invention, in which heat from waste water is recovered via the primary circuit of the heat pump. Figure 5 shows a diagram of the air temperature and the borehole temperature in the course of a year; and

Figure 6 shows a Mollier diagram for a heat pump.

Figure 1 schematically illustrates a heat storage system according to the invention, comprising a multi centre 1 that includes a water heater and a heat pump 2. The multi centre has two tanks; and upper tank 3 and a lower tank 4. The tanks 3 and 4 are insulated against both the environment and each other.

Cold water is supplied to the lower part of the lower tank 4 through a cold water inlet 5. From the upper part of the lower tank 4, water may flow on to the lower part of the upper tank 3 via a transfer passage 6. From the passage 6 there extends a branch 7 equipped with a stop valve 8 for drawing off chilled drinking water, as will be explained in greater detail below. There is also a passage 22 connecting the upper tank 3 with the cold water supply.

There is a hot water outlet 9 from the upper part of the upper tank 3.

In each tank 3 and 4 there is provided a heat exchanger 10 and 11, respectively. The lower heat exchanger is connected to the heat pump 2 via an upper inlet 12 and to an underfloor heating pipe 14 via a lower outlet 13. So-called fan coils 21 may also be used for heating or cooling of the building, as an addition to or replacement for underfloor heating pipes 14. In this case the heat exchanger is an air-to-liquid heat exchanger. It can be configured to recover heat from exhaust air leaving the building. The upper heat exchanger 11 is connected to the heat pump 2 via an upper inlet 15 and a lower outlet 16.

The multi centre 1 is also provided with an electric heating coil 19, 20 in each tank. These may act as a supplemental energy supply or as back-up heating if the heat pump fails. Reference is now made to Figure 2, which shows the principles of a heat pump according to the present invention. The circuit that includes underfloor heating and fan coils has been omitted from this figure but may obviously be present.

The heat pump 2 includes secondary circuits. The first circuit is a superheat circuit comprising a unit 17 that extracts a quantity of energy from hot steam, lowering the temperature of this. From this unit, water (or optionally another suitable liquid that can transport thermal energy) circulates through the heat exchanger 11. The second circuit is a heating circuit that comprises a condenser 18, in which the steam condenses. From here, water circulates through the heat exchanger 10. The heat pump further comprises a compressor 22 that compresses hot steam. It further comprises an evaporator 23 that extracts thermal energy from either a borehole 35 or air (represented by arrow 24). Thus the heat pump comprises two primary circuits; a first primary circuit arranged to extract thermal energy from air and a second primary circuit arranged to extract thermal energy from the ground or possibly water. A control device is connected to the heat pump, which determines which of the two primary circuits is to supply energy to the secondary circuits, based on temperature measurements carried out in the ground/water and in the air. Otherwise, the construction of the heat pump in itself is well known to a person of skill, and as such will not be explained in any greater detail herein. However, an attempt has been made to reduce the fill-up level of heating/cooling medium (e.g. CO₂) in the heat pump as far as possible. Consequently, this is used only in the circuits of the actual central unit 2, while for transfer of heat/cold to the borehole, water heater, underfloor heating etc., use is made of a mixture of water and alcohol. This prevents any environmental impact in the case of a leak.

The operation of the system of Figures 1 and 2 will now be explained. As mentioned above, the multi centre 1 has a lower rustproof pressure vessel or pressure tank 4 with a rustproof heat exchanger 10, which either preheats the cold water or cools it down, depending on the setting of the heat pump 2. From the heat exchanger 10, the circuit continues to the underfloor heating pipes 14 (or convectors) for heating or cooling requirements. In the cooling position, the draw-off 8 also provides chilled drinking water. The upper pressure tank 3 with the reheating exchanger 11 from the super heat circuit 17 of the heat pump provides hot water at a temperature that is high enough to make electric reheating unnecessary.

As there are two completely separate and insulated tanks, both heat and hot domestic water or cooling and hot domestic water can be accumulated in the multi centre.

In a specific operating mode cold water at a temperature of approximately 10 ⁰C will flow in through the cold water inlet 5 from the water supply grid. Water at a temperature of about 48 ⁰C from the heat pump condenser 18 is circulated through the heat exchanger 10, heating the cold water in the lower tank 4 to a temperature of approximately 45 ⁰C. The water from the heat pump 2 is circulated further out of the heat exchanger 10 to an underfloor heating pipe 14. The heating medium now holds a temperature of between 25 and 35 ⁰C₅ which is an ideal temperature range for underfloor heating. Advantageously there is a conventional temperature controller and a thermostat connected to the underfloor heating pipe, allowing the temperature of the building to be adjusted and set to the desired temperature.

The heating medium returns to the heat pump and is reheated to approximately 48 ⁰C.

As hot water is drawn off from the upper tank 3, the water in the lower tank 4 will flow as preheated cold water through the transfer passage 6 and into the upper tank 3. Here, the water will be reheated by the heat exchanger 11. The heat exchanger is supplied with water at a temperature of approximately 90 °C from the superheat circuit 17 of the heat pump 2. Finally, hot domestic water can be drawn off through the hot water outlet 9 at a temperature of between 60 and 80 ⁰C. The overall temperature difference of between 60 and 80 ⁰C is highly beneficial to the efficiency of, among other things, a CO₂ heat pump.

If there is a requirement for cooling (especially in the summer but also for larger buildings with a high total heat production) the heat pump can be reversed, so as to allow cold liquid to circulate through the heat exchanger 10 and on into the underfloor heating system 14 for cooling of the rooms. This will cool the cold water in the lower tank 4. This water may be drawn off as chilled drinking water via branch 7. The thermal energy recovered from the water in the cooled lower part of the tank 4 may be utilized in the heat pump, for heating the water supplied to the heat exchanger 11. Thus the heat exchanger 11 will still receive hot water from the superheat circuit 17 of the heat pump 2, as in the above case, and the hot water in the upper tank 3 is heated about as much as in the first case above.

Moreover it is possible (especially in countries with high cooling requirements) to use the lower tank 4 for accumulation of ice. In this case, it would be expedient to supply cold water to the upper tank 3 directly from the water mains via passage 22, rather than from the lower tank 4. In areas where the price of electricity is lower in the night time, it will then be possible to accumulate ice in the lower tank 4 during the night and draw a cooling effect from this during the day. A quantity of 100-150 litres of ice will correspond to a power consumption of 70-80 kWh, which should be sufficient to cool a building for 12-14 hours. Furthermore it will be easier to get rid of excess heat due to the lower outdoor temperature at night, with the heat conduction taking place via the evaporator 23. In hot areas the cooling requirement will be great enough to ensure excess heat regardless, thus providing plenty of hot water in the tank 3.

Figure 6 shows a simple Mollier diagram for a heat pump, where the numbers in the circles correspond to corresponding numbers in Figures 1 and 2. The number 1 represents the energy extracted from the ground or the air. The number 3 is represents extraction of high-grade superheat. The number 2 is heat extracted from the condenser 18. The number 4 is the sum of the heat from the superheat circuit 17 and the condenser 18. The curve 45 is the vapour pressure curve for the heating medium. Thermal energy is added to the heating medium along line 40, causing an increase in temperature and, with that, energy content. At the same time, the heating medium evaporates at a constant pressure. Then the gas is compressed along line 41. This increases the pressure, while also giving a slight increase in temperature. The gas is now superheated. For the first part of line 42, up to where the line 42 meets the vapour pressure line 45, superheat is extracted from the gas in the superheat circuit 17. After the line 42 has passed the curve 45, the condenser 18 extracts most of the heat. To the left of the curve 45, the heating medium will consist of sub cooled liquid which then expands at near constant pressure along line 43 until the liquid starts to evaporate. Alternating the energy extraction to the heat pump, in a controlled manner, between the air and the ground/water makes it possible at all times to extract energy from the most appropriate location. Figure 5 shows a diagram of the daily mean temperature of air (curve L) in the course of a typical one year cycle somewhere in Norway. The figure also includes a curve showing the temperature in a borehole (curve B) used for energy extraction for a heat pump. As can be seen from this, the temperature of the borehole falls rapidly throughout the autumn, and around New Year it falls below zero. At this point, the quantity of energy extracted from the borehole is reduced substantially. This coincides with the coldest time of year. The temperature of the borehole does not rise above zero again until early summer.

Curve B' shows the temperature of the borehole when this is connected to a heat pump that alternates between extracting energy from air and from the borehole. When the air temperature is higher than a predetermined value all the energy extracted will come from the air. The air temperature will fall throughout the autumn but will still remain largely above this value. Thus no significant quantities of energy will be extracted from the ground. When the air temperature falls below the predetermined value (in Figure 5, this happens in December) energy will be extracted from the borehole instead. The temperature of the borehole will then begin to fall. Throughout the winter, the heat exchanger will alternate between extracting energy from the air and from the borehole, depending on the temperature. By doing so, the temperature of the borehole will remain above the temperature of the borehole for case B, where there is extraction from the borehole only, and will fall below zero for short periods only. This will increase the efficiency of the heat pump considerably compared with heat pumps that rely solely on one source of energy.

The predetermined value at which the energy extraction switches from air to ground or vice versa will, in a simple embodiment of the invention, be an inconstant value. More appropriately the value depends on the temperature difference between the air and the borehole, also taking into consideration the time of year, i.e. date and local conditions. It is also possible to use a model that calculates the remaining energy expected to be consumed before the air is again at a stable, sufficiently high temperature. The choice of which energy source energy is to be extracted from can then be made dependent on the extent of the expected consumption for the rest of the winter. An advanced model may also take into account the weather forecast for the coming winter. It can also take into account any other sources of energy used for heating the building, e.g. wood firing.

Thus the control circuit will act as an expert system calculating how the energy extraction from the at least two sources of energy should be in order to achieve the highest possible efficiency during the winter or the year in general. This means that the instantaneous efficiency may be lower than the optimum, but that it will result in a higher overall efficiency.

The system of at least two sources of energy will make it possible to store excess heat produced e.g. during the summer, for instance in the ground surrounding a borehole. This will bring the temperature of the borehole up to a higher level, providing more energy for extraction during the winter.

The diurnal variations both in the temperatures of the air and the ground and in the heating (and thus energy) requirements may also be utilized in a similar manner to the principles described above. By way of example, a high temperature in the middle of the day, while the house is empty, may be used to store heat in the ground.

An example of simple criteria for determining which energy source to use is as follows: If the air temperature is higher than that of the borehole, the air will be used as a source of energy. If the air temperature is lower than that of the borehole, the control system will check the date (e.g. the month) and look up a table that shows how far below the borehole temperature the air temperature is allowed to be. This table is adjusted for local conditions but in principle it is set up as follows: When the cold season approaches one will seek to save the energy in the borehole. Therefore, energy will be extracted from the air even though the air temperature is slightly lower than that of the borehole. Thus the table states how large the temperature difference must be before changing to energy extraction from the borehole. After passing a given date in the cold season, energy will start to be extracted from the borehole while the temperature of this is higher than that of the air. One then assumes that the energy of the borehole will be sufficient to last out the cold season. In this way, one avoids lowering the temperature of the borehole to a temperature which will give a low recovery of energy during the cold season.

In the case of unusually high energy requirements it should be possible to override the control system manually. This may be relevant if the house needs to be heated up quickly after a period of non-occupancy.

Figure 3 shows a further embodiment of the invention. Here, the water heater 1 includes three tanks 3, 4 and 30 which are thermally insulated from each other. The tanks are interconnected via transfer passages 6, 31. The lowermost tank 30 is supplied with cold water from the cold water mains via a cold water inlet 33, and is provided with a heat exchanger 32 which is supplied with waste water (so-called grey water) at a temperature which will vary, naturally, but which will always be higher than the temperature of the cold water. Through this, the cold water is preheated in a first step.

The two other tanks of the water heater work the same way as in the embodiment of Figure 1, and so do not require further explanation.

Figure 4 shows a further embodiment of the invention. This comprises a water heater 1 and a heat pump 2. These units work more or less the same way as explained in connection with figure 1, and so do not require any further explanation. In addition, the system includes a heat exchanger 33 for waste water. The heat exchanger is located in a buffer tank 34 for the cooling/heating medium of the heat pump 2. The cooling/heating medium is passed through the buffer tank 34 and down into a borehole 35. In summer, this system allows heat to be extracted from the waste water and stored in the borehole 35. Thus at the start of the cold season, the ground around the borehole 35 will be at a higher temperature than that which would otherwise be the case, and the heat may then be recovered by means of the heat pump. In the winter, the waste water will also make a positive contribution of heat.

Although assembling the tanks into one unit provides certain advantages, as shown in the figures, it would obviously also be possible to realize the invention through tanks arranged in separate units but operationally connected. However, in terms of level the tanks should be placed so as to ensure that the tank for the hottest water is located at a higher level than the tank for the coldest water.

Claims

T/NO2006/0001H14CLAIMS

1.

A system for extraction of thermal energy, comprising a heat pump with an associated control circuit for optimization of the energy output, char act eri z ed in that the heat pump comprises a first primary circuit arranged to extract thermal energy from air, a second primary circuit arranged to extract thermal energy from at least one other source, and at least one secondary circuit arranged to receive thermal energy from the primary circuits, and that the control circuit includes a storage medium for a set of predetermined parameters, a comparing element for comparing the predetermined parameters with measurements connected with the first and secondary primary circuits, in order to determine, on the basis of the relationship between the measurements and the parameters, which of the two primary circuits energy should be extracted from at any time

2.

A system in accordance with Claim 1, characterized in that the parameters are temperatures in the primary circuits, temperature differences between air and the second source of energy, time of year, and the expected consumption during a predetermined period.

3.

A system in accordance with Claim Ior2, characterized in that the criteria for selecting the primary circuit from which to extract energy, include allowing the air temperature to fall below the temperature of the second source of energy by a predetermined value before choosing the second primary circuit over the first primary circuit.

4. A system in accordance with Claim 3, characterized in that air is preferred over the second source of energy for the first part of the cold season, even when the second energy source gives a higher efficiency.

5.

A system in accordance with one of the preceding claims, characterized in that it comprises a water heater (I)₅ the heat pump being arranged to transfer heat to and/or provide cooling of water in the water heater (1), that the water heater comprises at least two tanks (3, 4) for thermal storage of hot water and/or cold water and/or ice, which tanks (3, 4) are thermally insulated from each other, that the first tank (4) has a heat exchange relationship with the heat pump (2) via a first heat exchanger (10) which has a first temperature level, and the second tank (3) has a heat exchange relationship with the heat pump (2) via a second heat exchanger (11) which has a second temperature level higher than the first temperature level.

6.

A system in accordance with Claim 5, characterized in that the first tank (4) is in fluid communication with the second tank (3).

7.

A system in accordance with Claim 5 or 6, characterized in that the heat exchanger (10) of the first tank (4) is arranged to receive hot or cold liquid from the heat pump (2), and that the heat exchanger (10) is further connected to a heating/cooling system (14, 21) for heating or cooling of a building.

8.

A system in accordance with Claim 5, 6 or 7, characterized in that the second tank (3) comprises a heat exchanger (11) arranged to receive hot liquid from the heat pump (2), preferably the superheat circuit (17) of the heat pump (2), for heating of the water in the second tank (3).

9. A system in accordance with Claim 5, 6, 7 or 8, characterized in that the first and second tanks (3, 4) are placed within a common jacket.

10.

A system in accordance with Claim 5 , 6, 7, 8 or 9, characterized in that it comprises an outlet (7) between the first and the second tank (3, 4) for drawing off water, especially for drawing off chilled water.

11.

A system in accordance with Claim 5,6, 7, 8, 9 or 10, characterized in that the water heater (1) has a heat exchange relationship with waste water, optionally via a third tank (30) that is in liquid communication with one of the other tanks (4) of the water heater.

12.

A system in accordance with one of the preceding claims, characterized in that it comprises a heat exchanger (33) for heat transfer from waste water to the external circuit of the heat pump.

13.

A system in accordance with one of the preceding claims, characterized in that the heat pump (2) has a heat exchange relationship with a tank (4) that contains liquid such as water, and that it is arranged to provide cooling of the liquid for accumulation of ice and/or chilled liquid.

14. A system in accordance with one of the preceding claims, char acterized in that heat extracted from one of the two primary circuits can be transferred to storage, for instance in a borehole.

15. A system in accordance with one of the preceding claims, characterized in that the second source of energy is the ground surrounding a borehole.

16. A system in accordance with one of Claims 1-14, characterized in that the second source of energy is a type of water.