WO2008025850A2

WO2008025850A2 - An energy system with a heat pump

Info

Publication number: WO2008025850A2
Application number: PCT/EP2007/059145
Authority: WO
Inventors: Troels Pedersen
Original assignee: Colipu A/S
Priority date: 2006-08-31
Filing date: 2007-08-31
Publication date: 2008-03-06
Also published as: WO2008025849A3; WO2008025850A3; EP2061997A2; WO2008025849A2; US20100038441A1

Abstract

System for collection of thermal energy. The system comprises an energy collector (2), an energy reservoir (3), and an energy collection medium for circulation between the energy collector and the energy reservoir. Furthermore, the system comprises a heat pump (4) which is adapted to decrease the temperature of the energy collection medium outside the energy collector.

Description

AN ENERGY SYSTEM WITH A HEAT PUMP

Technical field

The present invention relates to a system for collection of thermal energy. The system comprises a heat pump, an energy collector, an energy reservoir, and an energy collection medium.

Background of the invention

Traditionally, energy collectors such as solar air collectors, solar water collectors, and combinations thereof have been connected to the domestic hot water system, since such energy collectors at the northern hemisphere are most efficient during summer time where the solar radiation and the outdoor temperature are high. At the same time the need for heating of the buildings is very low and sometimes non-existing. During periods with lower solar radiation and/or lower outdoor temperature, the efficiency of energy collectors are lower and their ability to collect an amount of energy which is significant compared to the amount of energy which is needed for production of domestic hot water and/or heating of the building is decreased.

Summary of the invention

It is an object of embodiments of the invention to provide an improved system for collection of thermal energy.

Thus, in a first aspect, the invention provides a system for collection of thermal energy, the system comprising an energy collector, an energy reservoir, and an energy collection medium for circulation between the energy collector and the energy reservoir, wherein the system further comprises a heat pump being adapted to decrease the temperature of the energy collection medium outside the energy collector. By decreasing the temperature of the energy collection medium outside the energy collector it may be possible to decrease the mean temperature inside the energy collector and thereby increase the temperature gradient over the energy collector and thus increase the efficiency of the system. As an example, this may be done by decreasing the temperature of the energy collection medium before it is circulated back to the energy collector.

The energy reservoir may be a water reservoir with domestic hot water, a reservoir with water for heating the building, or a combined reservoir from which a part of the water is used as domestic hot water and another part is used for heating the building. In the latter embodiment, the reservoir may be divided into two separate reservoirs in order to be able to circulate the water for heating without mixing it with the domestic hot water. The two separate reservoirs may thus be in thermal contact with each other, e.g. by mounting one of the reservoirs inside the other reservoir.

The energy collector may comprise an inlet and an outlet for the energy collection medium in order to be able to circulate the energy collection medium between the energy collector and the energy reservoir. The inlet and outlet may be connected to each other by a set of pipes so that the energy collector, the inlet and the outlet together with the pipes form a closed loop being in thermal communication with the energy reservoir. In an alternative embodiment, the energy reservoir may be part of the loop which in this case may not be a closed loop, since water may be tapped from the energy reservoir e.g. for heating a building.

In one embodiment, the diameter of the pipes may be chosen so that the pipes themselves form the energy reservoir. In this embodiment, circulation of the energy collection medium between the energy collector and the energy reservoir is constituted by circulation of the energy collection medium between the energy collector and the pipes. This embodiment may be particularly relevant if only a small energy reservoir is needed. Depending on the use of the thermal energy in the energy reservoir, the size of the energy reservoir may vary. As an example, a small energy reservoir may have a volume in the range of 10-20 litres, whereas a medium sized energy reservoir may have a volume in the range of 20-100 litres, and a large energy reservoir may have a volume of more than 100 litres. It should be understood, that the mentioned volumes may vary and that the volume of the energy reservoir may include the volume of the pipes connecting the energy reservoir to the energy collector. Furthermore, the pipes themselves may constitute the energy reservoir and may consequently have a volume of e.g. 10-20 litres.

The energy collector may be a solar air collector, a solar water collector, or a combination thereof. By a solar air collector is in this connection understood, an element in which air is heated by solar energy when air is circulated through the element. Likewise is a solar water collector an element in which water or another suitable liquid medium is heated by solar energy when the liquid medium is circulated there through. As an example, traditional solar collectors comprising plates between which water or air may be heated can be used in the present invention. Furthermore, vacuum solar collectors may be used. As a further example, a concave mirror may be used together with a set of pipes.

The energy collector may comprise an absorber in order to enhance the collection of solar energy. The absorber may comprise a photovoltaic panel which may produce electricity based on incident solar radiation on the panel. The photovoltaic panel may be coupled to the heat pump in order to supply electricity of the heat pump. In some embodiments, the photovoltaic panel may be of a size which is large enough to fully cover the need for electricity of the heat pump.

The energy collection medium may be air or a liquid medium such as water depending on the energy collector chosen. In alternative embodiments, a combination of air and a liquid medium may be used. If air is chosen as the energy collection medium this may in one embodiment be outdoor air being circulated through the energy collector and subsequently circulated to the energy reservoir, whereas the energy collection medium in another embodiment may be indoor air.

It should be understood, that the use of a liquid medium such as water as energy collection medium may require adding of an anti-freeze solution to the liquid medium in order to prevent freezing of the energy collection medium and thereby prevent damaging of the energy collector. Therefore, water as an energy collection medium may in this connection comprise water including an anti-freeze solution. An example of a suitable anti-freeze solution may be glycol. Furthermore, for the described embodiments comprising water as an energy collection medium, the energy collection medium may likewise be another liquid medium where water with or without an anti-freeze solution is meant as an example.

In case of an energy collector in the form of a solar water collector, water or another suitable liquid medium is used as an energy collection medium. When water is circulated between the energy reservoir and the energy collector and through the energy collector, the water may be heated due to incident solar radiation on the energy collector and due to the temperature difference between the energy collection medium, in this case water, and the energy collector. The heated water is circulated back to the energy reservoir leading to a temperature increase of the water in the energy reservoir.

Thus, the energy collector may be coupled to the energy reservoir so that thermal energy can be transported from the energy collector to the energy reservoir by circulation of the energy collection medium.

It may in one embodiment also be possible to combine air and water as energy collection medium, thus applying two energy collection media in the system. The energy collector may in this embodiment be a combined solar air and water collector. Alternatively, two separate energy collectors may be applied, one being a solar air collector and one being a solar water collector. It may in one embodiment be possible to switch between the two energy collection media and/or the two energy collectors e.g. dependent on the need of energy. In one embodiment, the water in the energy reservoir, e.g. water for heating a building, may be circulated through the energy collector, in which case the energy collection medium is a part of the water in the energy reservoir.

The heat pump may be adapted for transportation of thermal energy from a first heat exchanger to a second heat exchanger. By transportation of energy is in this connection understood, that the temperature of the first heat exchanger is decreased while the temperature of the second heat exchanger is increased.

At least one of the two heat exchangers of the heat pump may be positioned so that thermal energy can be exchanged between the heat pump and the energy reservoir. I.e. the at least one heat exchanger may be positioned so that it can collect thermal energy from the energy reservoir, thus the energy reservoir may be cooled. And/or the at least one heat exchanger may be positioned so that it can supply thermal energy to the energy reservoir, thus the energy reservoir may be heated.

In order to separate the energy collection medium from the water in the energy reservoir, the energy collector may be coupled to the energy reservoir via a third heat exchanger. Thus, the energy collection medium may be circulated in a closed loop without being mixed with the water for domestic hot water not with the water for heating of the building.

In one embodiment, the first heat exchanger may be positioned so that it can collect thermal energy from the energy reservoir, whereas the second heat exchanger may be positioned so that it can supply thermal energy to the energy reservoir. Thus, in this embodiment both the first and the second heat exchanger are in thermal contact with the energy reservoir. As a consequence, it may be possible to increase the temperature of the water at an upper level of the energy reservoir and decrease the temperature of water at a lower level of the energy reservoir.

Consequently, the first heat exchanger may be positioned substantially vertically below the second heat exchanger in the energy reservoir, thereby enlarging the natural temperature difference in an energy reservoir. By substantially vertical below should in this connection be understood that the first heat exchanger is positioned at a level which is situated below a level at which the second heat exchanger is situated.

Due to the layout of the system in this embodiment, the heat pump may be dimensioned to a low power level and at the same time being able to cope with sunny days with a high level of solar incident resulting in a large amount of thermal energy being transferred to the energy collection medium in the energy collector.

In another embodiment, the system may comprise an extra energy reservoir. The first heat exchanger may be positioned so that it can collect thermal energy from the energy reservoir, and the second heat exchanger may be positioned so that it can supply thermal energy to the extra reservoir. Thus, the heat pump may be positioned so that it can cool the energy reservoir while heating the extra energy reservoir. By cooling the energy reservoir, the energy collection medium may be cooled, thereby allowing for an increased performance of the energy collector, since the energy collection medium may be cooled before being circulated to the energy collector.

In yet another embodiment, the first heat exchanger may be positioned so that it can collect thermal energy from the energy collection medium before the energy collection medium is received by the energy collector, and the second heat exchanger may be positioned so that it can supply thermal energy to the energy reservoir. By collecting thermal energy from the energy collection medium, the first heat exchanger may cool this medium, and thus allow for an increased performance of the energy collector. Furthermore, the energy reservoir may be heated as the second heat exchanger may supply thermal energy hereto.

In the embodiments in which both the second and the third heat exchanger are positioned in the energy reservoir, the second heat exchanger may be positioned substantially above the third heat exchanger. This may enlarge the natural temperature difference in the energy reservoir.

In case the energy collection medium is too cold, condensation may occur in the energy collector, when the energy collection medium is circulated to the energy collector. In order to prevent this, the system may further comprise a heater being adapted to heat the energy collection medium before the energy collection medium reaches the energy collector. The heater may e.g. be a traditional radiator, or a solar air collector, through which or along which the energy collection medium is lead and thus heated. The heater may further comprise a ventilator in order to facilitate heating of the energy collection medium. Other heaters may also be used.

The system may further comprise a control system being adapted to control a first exchange of thermal energy between the energy collector and the energy reservoir and adapted to control a second exchange of thermal energy to and from the heat pump in such a way that the first and second exchange of thermal energy are controlled independently. This may allow for separate control strategies for the first and second exchange of thermal energy.

As an example, the control system may allow the energy collection medium to be circulated in periods in which the incident solar radiation and/or the outdoor temperature are sufficient to increase the temperature of the energy collection medium. Independently of this, the control system may limit the exchange of energy to and from the heat pump to periods in which the efficiency of such an exchange is above a pre-defined lower level. Additionally, the control system may prevent circulation of the energy collection medium when the outdoor temperature is below a pre-defined value. Other control strategies may also be applicable.

Two separate control strategies may furthermore allow the energy collection medium to collect thermal energy constantly, whereas the heat pump may only be switched on when it is reasonable. The heat pump may comprises a circuit for circulation of a refrigerant between the first and the second heat exchanger and cooling device for compression and expansion of the refrigerant during circulation in the circuit.

The heat pump may as an example include different types of cooling principles, such as Stirling or Scroll. Furthermore, the heat pump may comprise a gas, e.g. CO₂, Helium, Argon, or air.

Accordingly, the heat pump may provide heat transportation by compression of a gas and optionally by establishing a phase shift of the refrigerant during circulation in the circuit - such principles are well known in refrigeration industry. Typically, the refrigerant is circulated between a condenser in which a compressed medium cools down and thus condenses, an evaporator, in which the compressed medium is expended and evaporated and in which the refrigerant thus absorbs thermal energy, and a compressor which compresses the evaporated refrigerant. In this case, the condenser may constitute one of the heat exchangers and the evaporator may constitute the other heat exchanger.

Alternatively or additionally, the heat pump may comprise one or more Peltier elements. The use of Peltier elements may be particularly relevant if the temperature difference between the first and second heat exchanger during the majority of hours of operation is relatively low, since operation of a Peltier element at a high temperature difference may decrease the efficiency of the element. Running Peltier elements at maximum capacity may lower the efficiency, and it may therefore be an advantage to apply a plurality of Peltier elements, and thus operate them at around medium capacity.

It may even be an advantage to combine Peltier elements with a more traditional compressor based heat pump and to shift between the two principles based on the temperature difference across the heat exchanger so that the Peltier elements works at temperature differences which are low and the compressor based heat pump works when the temperature difference becomes above a specific level, e.g. above 10 degrees in difference between the hot and the cold site of the heat pump. In that case, the user may benefit from silent operation whenever the Peltier elements are active.

In order to minimize heat loss from the system, a lower mean temperature may be desired. This may be achieved by choosing an energy reservoir having a lower heat capacity. Accordingly, a smaller amount of energy may be contained in the energy reservoir, which furthermore has the advantage that the system reacts fast. In other connections, a larger heat capacity may be preferred. Therefore, the heat capacity of the energy reservoir may vary dependent on the use of the system. As an example, the heat capacity of the energy reservoir including the medium contained herein may in the range of 5-100 kJ/K. In another example, the heat capacity of the energy reservoir may be above 100 kJ/K.

Furthermore, the heat capacity of the energy collector and the first heat exchanger may vary. As an example, the heat capacity of the energy collector including the energy collection medium and the first heat exchanger may be above 10 kJ/K. In an alternative example, the heat capacity of the energy collector including the energy collection medium, the first heat exchanger, and a pipe connecting an inlet and an outlet of the energy collector may be above 10 kJ/K.

Brief description of the drawings

Embodiments of the invention will now be further described with reference to the drawings, in which :

Fig. 1-6 illustrate different embodiments of a system for collection of thermal energy according to the invention. Detailed description of the drawings

Fig. 1 illustrates an embodiment of a system 1 for collection of thermal energy. The system 1 comprises an energy collector 2, an energy reservoir 3, and an energy collection medium (not shown) for circulation between the energy collector 2 and the energy reservoir 3. Furthermore, the system 1 comprises a heat pump 4 being adapted to decrease the temperature of the energy collection medium outside the energy collector 2.

In the illustrated embodiment, the energy collection medium is a liquid medium in the form of water with an anti-freeze solution. Accordingly, the energy collector 2 is a solar water collector through which the energy collection medium is circulated and thus heated. However, it should be understood, that the illustrated embodiment is not limited to a liquid energy collection medium, air may also be used.

The heat pump 4 is adapted for transportation of thermal energy from a first heat exchanger 5 to a second heat exchanger 6, in the illustrated embodiment both being positioned so that thermal energy can be exchanged between the heat pump 4 and the energy reservoir 3. As a consequence, it is possible to increase the temperature of the water at an upper level of the energy reservoir 3 and decrease the temperature of water at a lower level of the energy reservoir 3, thereby enlarging the natural temperature difference in the energy reservoir 3.

The energy collector 2 is coupled to the energy reservoir 3 so that thermal energy can be transported from the energy collector 2 to the energy reservoir 3 by circulation of the energy collection medium. The energy collector 2 is coupled to the energy reservoir 3 via a third heat exchanger 7. Thus, thermal energy is transferred from the energy collection medium to the energy reservoir 3 via the third heat exchanger 7.

Since the first heat exchanger 5 is arranged so that it decreases the temperature of the water at a lower level of the energy reservoir 3, the energy collection medium being circulated back to the energy collector 2 is colder than it otherwise would have been. Circulation of the energy collection medium is facilitated by a pump 8.

A metal plate 9 is attached to the inner surface of the energy reservoir 3. The plate 9 has an aperture 10 in the centre allow for the water in the energy reservoir 3 to pass the plate 9. By attaching the plate 9 to the inner surface of the energy reservoir 3 mixing of the water in the energy reservoir 3 is limited.

In order to ensure a sufficient temperature of the water in the energy reservoir 3, an additional heater 11 is positioned at the top of the reservoir. The heater 11 may be connected e.g. to district heating or a central oil or gas boiler.

Water is tapped from the energy reservoir 3 through the outlet 12 in the top of the reservoir 3 where the temperature of the water is the highest, and returned to the energy reservoir 4 through the inlet 13. The water may be used for domestic hot water or for heating the building or for both.

In case the energy collection medium is too cold, condensation may occur in the energy collector 2, when the energy collection medium is circulated to the energy collector 2. Since the energy collector 2 may not be completely tight, water contained in the surrounding air may condensate on surfaces inside the energy collector 2. In order to prevent this, the system comprises a heater 14 being adapted to heat the energy collection medium before the energy collection medium reaches the energy collector 2. In the illustrated embodiment, the heater 14 is an air/liquid heat exchanger, through which the energy collection medium is lead and thus heated. The heater 14 comprises a ventilator in order to facilitate heating of the energy collection medium.

Finally, the illustrated system 1 further comprises a three-way valve 16. The three-way valve 16 may be used to close the energy collector 2 e.g. during maintenance, repair or dehumidification of the energy collector 2. By closing the energy collector 2 by the use of the three-way valve 16, the energy collection medium is by-passed the energy collector 2 instead of being lead through it. During normal operation the three-way valve 16 leads the energy collection medium through the energy collector 2.

Fig. 2 illustrates a second embodiment of a system 1' for collection of thermal energy. The system 1' comprises an energy collector 2, an energy reservoir 3', and an energy collection medium (not shown) for circulation between the energy collector 2 and the energy reservoir 3'. Furthermore, the system 1' comprises a heat pump 4' being adapted to decrease the temperature of the energy collection medium outside the energy collector 2.

The energy reservoir 3' is similar to the energy reservoir 3 illustrated in Fig. 1, but in this embodiment it further comprises a second energy reservoir 17. The energy reservoir 3' contains water for heating the building, whereas the second energy reservoir 17 contains domestic hot water. The domestic hot water is tapped through the outlet 18 and the re-circulated part hereof is returned through the inlet 19.

The heat pump 4' is adapted for transportation of thermal energy from a first heat exchanger 5' to a second heat exchanger 6. The first heat exchanger 5' is positioned so that it can collect thermal energy from the energy collection medium before the energy collection medium is received by the energy reservoir 3. The second heat exchanger 6 is positioned so that it can supply thermal energy to the energy reservoir 3'. By collecting thermal energy from the energy collection medium, the first heat exchanger 5' cools this medium, and thus allows for an increased performance of the energy collector 2, as the mean temperature inside the energy collector 2 is decreased. Furthermore, the energy reservoir 3' is heated as the second heat exchanger 6 supplies thermal energy hereto.

Depending on the temperature of the energy collection medium after having been cooled by the first heat exchanger 5', the energy collection medium may be cooled even further when it reaches the energy reservoir 3', as the energy collection medium is in thermal communication with the reservoir 3' via the third heat exchanger 7. The three-way valve 16 illustrated in Fig. 1 could also be applied in the system 1' illustrated in Fig. 2. The metal plate 9 can likewise be applied.

Fig. 3 illustrates a third embodiment of a system 1" for collection of thermal energy. The system 1" comprises an energy collector 2, an energy reservoir 3", and an energy collection medium (not shown) for circulation between the energy collector 2 and the energy reservoir 3". Furthermore, the system 1' comprises a heat pump 4" being adapted to decrease the temperature of the energy collection medium outside the energy collector 2.

The energy reservoir 3" is similar to the energy reservoir 3 illustrated in Fig. 1, but in this embodiment without the additional heater 11. Instead the system 1" comprises an extra energy reservoir 20 comprising an additional heater 11.

Two energy reservoirs 3", 20 being in thermal communication via a heat pump 4 allows for excessive extraction of energy from the first energy reservoir 3". The level of extraction may be so high that the content of the first energy reservoir 3" solidifies. The content may be a liquid medium such as water, a salt, wax, or another suitable medium for storage of energy.

In the illustrated embodiment, both energy reservoirs 3", 20 comprise an inlet 13, 13" and an outlet 12, 12". In an alternative embodiment, only the extra energy reservoir 20 comprise an inlet 13 and an outlet 12 as only the water of the extra energy reservoir may be used for heating the building and/or used to heat domestic water.

The heat pump 4" is adapted for transportation of thermal energy from a first heat exchanger 5 to a second heat exchanger 6. The first heat exchanger 5 is positioned so that it can collect thermal energy from the energy reservoir 3", and the second heat exchanger 6 is positioned so that it can supply thermal energy to the extra reservoir 20. Thus, the heat pump 4" is positioned so that it can cool the energy reservoir 3" while heating the extra energy reservoir 20. By cooling the energy reservoir 3", the energy collection medium is cooled, thereby allowing for an increased performance of the energy collector 2, since the energy collection medium is cooled before being circulated back to the energy collector 2.

It should be understood, that the three-way valve 16 and the metal plate 9 illustrated in Fig. 1 could also be applied in the system 1" illustrated in Fig. 3. Furthermore, the second energy reservoir 17 illustrated in Fig. 2 could be applied in connection with the second energy reservoir 20.

Fig. 4 illustrates a fourth embodiment of a system 1'" for collection of thermal energy. The system Y" comprises an energy collector 2'", an energy reservoir 3'", and an energy collection medium (not shown) for circulation between the energy collector 2'" and the energy reservoir 3'". Furthermore, the system Y" comprises a heat pump 4'" being adapted to decrease the temperature of the energy collection medium outside the energy collector 2'".

The energy reservoir 3'" is similar to the energy reservoir 3" illustrated in Fig. 3, but the transfer of thermal energy from the energy collection medium to the energy reservoir 3'" is carried out without the used of a heat exchanger, since the water in the energy reservoir 3"' is also used as the energy collection medium.

The energy collection medium is a liquid medium such as water possibly containing an anti-freeze solution such as glycol. The energy collection medium is heated in the energy collector 2'" being a solar air/liquid collector in which air is used to heat the liquid energy collection medium. Air is heated by solar radiation in the duct 21 in which a ventilator 22 is arranged to provide forced ventilation. A air/liquid heat exchanger 23 is arranged to provide heating of the energy collection medium in the energy collector 2'".

The heat pump 4'" is adapted for transportation of thermal energy from a first heat exchanger 5 to a second heat exchanger 6, and thus adapted for transportation of thermal energy from the energy reservoir 3'" to the extra energy reservoir 20. It should be understood, that the three-way valve 16 and the metal plate 9 illustrated in Fig. 1 could also be applied in the system Y" illustrated in Fig. 4. Furthermore, the second energy reservoir 17 illustrated in Fig. 2 could be applied in connection with the second energy reservoir 20.

Additionally, the energy collector 2'" could replace the energy collector 2 illustrated in Figs. 1-3 in alternative embodiments.

Fig. 5 illustrates a fifth embodiment of a system 1"" for collection of thermal energy. The system 1"" is similar to the system Y illustrated in Fig. 2. Though, is system 1"" does not comprise a second energy reservoir 17.

The heat pump 4"" is adapted for transportation of thermal energy from a first heat exchanger 5"" to a second heat exchanger 6. The first heat exchanger 5"" is positioned so that it can collect thermal energy from the energy collection medium, and the second heat exchanger 6 is positioned so that it can supply thermal energy to the energy reservoir 3.

Subsequently, the first heat exchanger 5"" cools the energy collection medium even further, since the first heat exchanger 5"" is positioned so that it can collect energy from the energy collection medium before the energy collection medium reaches the energy collector 2.

Fig. 6 illustrates a sixth embodiment of a system Y"" for collection of thermal energy. The system Y"" is identical to the system Y illustrated in Fig. 2, except that the system Y"" does not comprise a second energy reservoir 17. It should be understood, that the three-way valve 16 and the metal plate 9 illustrated in Fig. 1 could also be applied in the system 1"", 1'"" illustrated in Figs. 5 and 6. Furthermore, the second energy reservoir 17 illustrated in Fig. 2 could also be applied. Additionally, the energy collector 2'" could replace the energy collector 2 in order to illustrate alternative embodiments.

Claims

1. A system for collection of thermal energy, the system comprising an energy collector, an energy reservoir, and an energy collection medium for circulation between the energy collector and the energy reservoir, wherein the system further comprises a heat pump being adapted to decrease the temperature of the energy collection medium outside the energy collector.

2. A system according to claim 1, wherein the heat pump is adapted for transportation of thermal energy from a first heat exchanger to a second heat exchanger, at least one of these heat exchangers being positioned so that thermal energy can be exchanged between the heat pump and the energy reservoir, and wherein the energy collector is coupled to the energy reservoir so that thermal energy can be transported from the energy collector to the energy reservoir by circulation of the energy collection medium.

3. A system according to claim 2, wherein the energy collector is coupled to the energy reservoir via a third heat exchanger.

4. A system according to claim 2 or 3, wherein the first heat exchanger is positioned so that it can collect thermal energy from the energy reservoir, and wherein the second heat exchanger is positioned so that it can supply thermal energy to the energy reservoir.

5. A system according to claim 4, wherein the first heat exchanger is positioned substantially vertically below the second heat exchanger in the energy reservoir.

6. A system according to claim 2 or 3 further comprising an extra energy reservoir, wherein the first heat exchanger is positioned so that it can collect thermal energy from the energy reservoir, and the second heat exchanger is positioned so that it can supply thermal energy to the extra reservoir.

7. A system according to claim 2 or 3, wherein the first heat exchanger is positioned so that it can collect thermal energy from the energy collection medium before the energy collection medium is received by the energy collector, and the second heat exchanger is positioned so that it can supply thermal energy to the energy reservoir.

8. A system according to any of claims 3-7, wherein the second heat exchanger is positioned substantially above the third heat exchanger in the reservoir.

9. A system according to any of the preceding claims further comprising a heater being adapted to heat the energy collection medium before the energy collection medium reaches the energy collector.

10. A system according to any of the preceding claims further comprising a control system being adapted to control a first exchange of thermal energy between the energy collector and the energy reservoir and adapted to control a second exchange of thermal energy to and from the heat pump in such a way that the first and second exchange of thermal energy are controlled independently.

11. A system according to any of claims 2-10, wherein the heat pump comprises a circuit for circulation of a refrigerant between the first and the second heat exchanger and cooling device for compression and expansion of the refrigerant during circulation in the circuit.

12. A system according to claim 11, wherein the cooling device is adapted to establish a phase shift of the refrigerant during circulation in the circuit.

13. A system according to any of claims 1-10, wherein the heat pump comprises one or more Peltier elements.

14. A system according to any of the preceding claims, wherein the energy reservoir has a volume in the range of 10-20 litres.

15. A system according to any of the preceding claims, wherein the energy reservoir has a volume in the range of 20-100 litres.

16. A system according to any of the preceding claims, wherein the energy reservoir has a volume of more than 100 litres.

17. A system according to any of the preceding claims, wherein the heat capacity of the energy reservoir is in the range of 5-100 kJ/K.

18. A system according to any of the preceding claims, wherein the heat capacity of the energy reservoir is above 100 kJ/K.

19. A system according to any of the preceding claims, wherein the heat capacity of the energy collector and the first heat exchanger is above 10 kJ/K.

20. A system according to any of the preceding claims, further comprising a pipe connecting an inlet and an outlet of the energy collector, wherein the heat capacity of the energy collector, the first heat exchanger and the pipe is above 10 kJ/K.