The present invention relates to a method and a system for heating. In particular, the present invention relates to such a method and to such a system for heating using a liquid-to- 5 liquid heat pump in combination with a carbon dioxide heat pump, such as an air-to-liquid carbon dioxide heat pump.

Heat pumps are well-known as such. A heat pump is an apparatus using an internal heat carrier loop to transfer thermal energy from a cold-side heat exchanger to a hot-side heat w exchanger. Normally, such a heat pump comprises a compressor, an expansion valve, a condenser and an evaporator. For instance, heat pumps are used for refrigeration in refrigerators and freezers, and for heating of indoors air and hot tap water.

Internal heat carriers in heat pumps can vary depending on prerequisites. Many of previze ously suggested heat carriers in the art that offer advantageous thermodynamic properties have proven to be harmful for the environment, poisonous, highly flammable and so forth. Harmless internal heat carriers have been suggested, such as carbon dioxide, but generally offer inferior thermodynamic properties. For instance, carbon dioxide has a critical point (CP), i.e. a maximum temperature at which the gas phase can condense, of about 31°C mak-0 ing it difficult to reach high heating energy efficiency. Such efficiency is normally measured in "COP" (Coefficient Of Performance, the amount of output heat produced divided by the electric energy supplied to produce that output heat). For a heat pump using above-CP carbon dioxide as the internal heat carrier, a typical COP value may be about 1.5 or even lower, to be compared to considerably higher COP values, such as 3 or 4, for heat pumps using5 alternative internal heat carriers.

A heat pump can be a liquid-to-liquid heat pump, transferring thermal energy from a coldside external liquid medium to a hot-side external liquid medium. For instance, a liquid-to- liquid heat pump can be a geothermal heat pump, harvesting thermal energy from an ex-0 ternal medium in the form of liquid circulating into the ground and in turn heating an external medium in the form of liquid circulating inside a house to heat indoors air in the house. A heat pump can also be an air-to-liquid heat pump, transferring thermal energy from air at said cold side to a hot-side external liquid medium, such as the mentioned liquid circulating inside a house.

For geothermal heat pumps, the maximum output heat power or energy is determined by, among other things, the size of the volume of ground from which the thermal energy is harvested and the temperature of that volume. In the example of geothermal wells (bored holes into the ground, such as into bedrock, where a liquid heat carrier is circulated in tubes), a total length of the well(s) can be selected with a maximum output heat power or energy in mind. Once the well(s) has or have been properly dimensioned, the heat pump can usually deliver highly reliable heating in a way that is not season-dependent. On the other hand, after prolonged use the surrounding ground can be cooled, in turn deteriorating the maximum output power or energy available.

Air-to-liquid heat pumps typically don't suffer from such deteriorating efficiency over time, but on the other hand experience higher COP values when the air used for heating is warmer. This means that at times when the need for indoors heating is the largest (i.e. during the winter), the available efficiency is at its lowest.

Many times, the requirements for heating power or energy vary over time, resulting in a requirement for peak heating in turn resulting in an over-dimensioned system. In particular, this may be the case during construction or renovation of properties, as total heating power or energy requirements are often much higher during such construction or renovation as compared to normal operation of the finished or renovated building.

The present invention solves one or several of the above described problems.

Hence, the invention relates to a system for heating a structure, wherein the system com- prises a first heat pump and a first heat exchanger, the first heat pump being a liquid-to- liquid heat pump, the first heat exchanger being arranged to deliver heat, in a first heating process, from a hot side of the first heat pump to a first hot-side external liquid, the first heat pump being arranged to receive heat from a cold-side external liquid circulating in the ground or a water body; and a second heat pump and a second heat exchanger, the second heat exchanger being arranged to deliver heat, in a second heating process, from a hot side 5 of the second heat pump to a second hot-side external liquid, the first and second hot-side external liquids possibly being the same and possibly circulating in a common circuit; wherein the system is arranged to heat the structure via heat exchange with at least the first hot-side external liquid, the system being characterised in that the second heat pump comprises an internal loop in which carbon dioxide is circulated as an internal-loop heat w medium, and in that the system comprises a third heat exchanger being arranged to, in said second heating process, cool the internal-loop heat medium to below a critical point (CP) of the internal-loop heat medium by heat exchanging the internal-loop heat medium to the cold-side external liquid, the cold-side external liquid as a result being heated. internal-loop heat medium is Moreover, the invention relates to a method for heating a structure, wherein the method comprises a first heating process, in which a first heat pump, being a liquid-to-liquid heat pump, is used to deliver heat, via a first heat exchange, from a hot side of the first heat pump heat to a first hot-side external liquid, the first heat pump receiving heat from a coldside external liquid circulating in the ground or a water body; and a second heating process,0 in which a second heat pump is used to deliver heat, via a second heat exchange, from a hot side of the second heat pump to a second hot-side external liquid, the first and second hot-side external liquids possibly being the same and possibly circulating in a common circuit; and heating the structure via heat exchange with at least the first hot-side external liquid, the method being characterised in that the second heat pump uses carbon dioxide5 as an internal-loop heat medium, and in that said second heating process comprises, in a third heat exchange, cooling internal-loop heat medium to below a critical point (CP) of the internal-loop heat medium by heat exchanging the internal-loop heat medium to the coldside external liquid, the cold-side external liquid as a result being heated. internal-loop heat medium 0 In the following, the invention will be described in detail, with reference to exemplifying embodiments of the invention and to the enclosed drawings, wherein: Figure 1 is an overview diagram of an at least partly conventional system for heating a structure;

Figure 2a is an overview diagram of a first system for heating a structure according to the s present invention;

Figure 2b is an overview diagram of a second system for heating a structure according to the present invention;

Figure 2c is an overview diagram of a third system for heating a structure according to the present invention; w Figure 3 is a flow chart illustrating a first method according to the present invention;

Figure 4 is a flow chart illustrating a second method according to the present invention;

Figure 5 is a flow chart illustrating a third method according to the present invention; and Figure 6 is a chart illustrating the operation of a carbon dioxide air-to-liquid air pump for use in a system according to the present invention.

15

The Figures share reference numerals for same or corresponding parts.

Figure 1 illustrates a system 100 that is at least partly conventional. The system 100 is for heating a structure 10, such as a building. The structure 10 has indoors air 11 and may also0 have a tank 13 for heated tap water, such as for use in the structure 10. The structure 10 is heated by the system 100 heating said indoors air 11 and/or said tap water 13.

The system 100 furthermore comprises a heat pump 110 of type liquid-to-liquid. 5 As used herein, and as already discussed above, a heat pump of type "liquid-to-liquid" is a heat pump arranged to transfer heat (thermal energy) from a first liquid to a second liquid or vice versa. In contrast, a heat pump of type "air-to-liquid" is a heat pump arranged to transfer heat from air to a liquid, or vice versa. It is realized that all heat pumps of type "liquid-to-liquid" as well as all heat pumps of type "air-to-liquid" are arranged with an inner0 heat pump loop in which an inner heat medium is circulated, past a circulation pump, a compressor (that can also in itself serve as the circulation pump), an expansion valve, an evaporator and a condenser, as is well-known as such and will not be described in any detail herein. The inner heat pump loop is typically a closed loop. It is also noted that there are also known heat pumps of type "air-to-air", such as conventional air conditioners.

5 heat pump is arranged with a cold side and a hot side, whereby the heat pump is arranged to transfer heat from the cold side to the hot side. This heat transfer requires electric energy to operate the pump/compressor. There are reversible heat pumps, in which the cold side and hot side can switch sides depending on a currently used mode of operation. In general, it is preferred that the heat pumps described herein are not reversible, so that the cold side w is always the cold side and correspondingly for the hot side.

In the example shown in Figure 1, the heat pump 110 is arranged to transfer heat, via a heat exchanger 112, from a cold-side 114 liquid, such as water, circulated in a closed loop 112b down into a geothermal well 160 in the ground 20 and/or into a water body 21, and to is provide heat thus harvested from the ground 20 and/or the water body 21, via heat exchanger 111, to a hot-side 113 liquid circulated in a closed loop 140, 141 in turn passing by a heat exchanger 12 arranged to heat the indoors air 11 and/or the tap water 13.

As used herein, a "geothermal well" is a hole, such as a drilled hole, in the ground 20, having0 a circulation pipe (such as circuit 112b) running down into, and up from, the hole. Such a hole can be drilled into bedrock, and it can be at least 30 meters, such as at least 100 meters, such as at least 200 meters, of depth. Several geothermal wells 160 can be provided, and then connected in parallel and/or in series with respect to the flow of the liquid in the circuit 112b. A geothermal well can also be in the form of a corresponding length of ground tubes5 arranged horizontally and spread underground at a lower depth across a certain ground area.

Hence, an internal heat medium circulation loop 115 of heat exchanger 110 is operated so as to transfer heat, via cold-side heat exchanger 112, from liquid circulated in the circuit0 112b, and to provide that heat, via hot-side heat exchanger 111, to liquid circulated in the circuit 140, 141. The liquid in circuit 112b is in turn heated by heat exchange with the ground 20, via geothermal well 160 and/or the water body 21. The liquid in the circuit 140, 141 is cooled by heat exchange with said indoors air 11 and/or tap water 13.

As is illustrated in Figure 1, being part of circuit 140, 141 an equalisation tank 140 accepts s liquid from the hot-side heat exchanger 111 into a larger pool of heated liquid kept in said tank 140. Then, the tank 140 provides such hot liquid to heat exchanger(s) 12 for heating of indoors air 11 and/or tap water 13.

A control unit 150 is in contact with a temperature sensor 152 for providing a reading of the w temperature of outdoors air 30 in the vicinity of the structure 10, and is arranged to use such temperature reading to control a valve system 151 to in turn control the flow of liquid in the various circuits 112b, 140, 141 depending on current heating requirements. The control unit 150 may also be arranged to operate the heat pumps 110, 120, such as to switch them on and off.

15

Furthermore, the system 100 comprises one or several pumps to circulate the various liquids in said circuits. These pumps are not shown in the drawings for reasons of clarity.

The system 100 also comprises an electric heater 120, arranged to accept liquid from the0 tank 140, via a circuit 142, and to heat this liquid before returning it the tank 140. It is realized that the liquid being circulated in circuits 141 and 142 is mixed in the tank 140, and the mixed liquid is then provided to the structure 10 for heating the latter.

Hence, the structure 10 is heated by the heat pump 110, exploiting thermal energy in the5 ground 20 and/or water body 21. The heat pump 110 is limited in terms of maximum heating power or energy, by the dimensions of the heat pump 110 itself; by the total depth of the geothermal well(s) 160; by the total length of the circuit 112b exposed to the water body 21; and so forth. The peak heating power or energy requirements, such as during cold weather, may be higher than the maximum heating power or energy that the heat pump0 110 can provide. The electric heater 120 is provided to handle such peak requirements, and can then be operated in parallel with the heat pump 110 as required. Of course, the electric heater 120 may be provided as an integrated part of the tank 140.

A problem with such a setup is that the electric heater 120 is a relatively inefficient heater s of the liquid in the tank 140. Instead of using the electric heater 120, it would be possible to use an additional heat pump. However, this is not only a much more expensive installation than the electric heater, it also involves providing an internal-loop heating medium that typically is both harmful for the environment and may present a fire hazard. w Figures 2a-2c illustrate respective systems 200 according to the present invention, arranged to perform a method according to the present invention.

Many parts of the system 200 are similar to the system 100, and what has been said in relation to system 100 above applies correspondingly, as applicable, to system 200. The last is two digits of reference numerals with respect to Figure 1 correspond to the last two digits of reference numerals with respect to Figures 2a-2c.

Figures 2a-2c are similar in many respects. What is said below in relation to parts of Figure 2a that are common to Figures 2a-2c is hence also applicable to the systems 200 according0 to Figures 2b and 2c.

Hence, the system 200, like system 100, is arranged for heating a structure 10, the structure 10 corresponding to the structure 10 shown in Figure 1. The system 200 comprises a first heat pump 210, of type liquid-to-liquid. The system 200 comprises a first heat exchanger5 211, in the example Figure 2a being the hot-side 213 heat exchanger 211 of the heat pump

210.

The first heat exchanger 211 is arranged to deliver heat, in a process herein denoted "first heating process" in which heat is transferred by the heat pump 210 from the ground 200 and/or water body 21 to the structure 10, from the hot side 213 of the first heat pump 210 to a first hot-side external liquid. In Figure 2a, this first hot-side external liquid is the liquid circulated in circuit 240, 241, corresponding to circuit 140, 141 above.

Hence, the system 200 may comprise an equalisation tank 240, arranged to receive said first s hot-side external liquid from said first heat exchanger 211 and a second hot-side external liquid provided from the second heat pump 220, such as from a second heat exchanger 221 of the second heat pump 220 (see below). The equalisation tank 240 may be arranged to provide a mixture of said first and second hot-side external liquids to said structure 10. w In other words, the first hot-side external liquid may be provided to the equalization tank 240 where it may be mixed with other heated liquid. From tank 240, the heated liquid may be provided to the structure 10. Alternatively, the tank 240 may be arranged as a part of the structure 10; the tank 240 may be arranged as an integrated part of either heat pump 210, 220; or the tank 240 may not be used at all and the first hot-side external liquid may is be provided directly to the structure 10. As will be described below, there may also be several separate equalization tanks, for instance each with a separate temperature hot-side external heat medium.

The first heat pump 210 is further arranged to receive heat from a cold-side 214 external0 liquid circulating in the ground 20 and/or the water body 21. In particular, this cold-side external liquid may be circulated in one or several geothermal wells 260 of the type described above (including any horizontally arranged ground-heat tubes arranged at lower depth) and/or in tubes submersed into a water body 21 such as a lake or a river. Normally, the cold-side external liquid is circulated in a closed circuit. The cold-side external liquid may5 be, for instance, water with or without any anti-freeze agent.

The system 200 furthermore comprises the second heat pump 220 and a second heat exchanger 221. The second heat exchanger 221 is arranged to deliver heat, in a process herein denoted "second heating process", from a hot side 223 of the second heat pump 220 to a0 second hot-side external liquid. The second heat pump 220 may be an air-to-liquid heat pump, in which case said heat is received by the second heat pump 220 from air, such as outdoors air 30, at a cold side 224 of the second heat pump 220, possibly via a cold-side 224 air heat exchanger 222. The second heat pump 220 may alternatively be a liquid-to-liquid heat pump, receiving heat from s a cold-side 224 liquid-to-liquid heat exchanger (not shown in the Figures). Such a cold-side 224 liquid-to-liquid heat exchanger may be arranged to transfer heat from a cold-side external liquid in turn arranged to absorb thermal energy from air and to deliver such thermal energy to the cold side 224 of the second heat pump 220; and/or the second heat pump 220 may be arranged to receive thermal energy from a different cold-side 224 liquid heat w source.

Hence, the second heat pump 220 may be arranged to receive, directly or indirectly, heat from air, such as outdoors air 30. is The air may be outdoors air (30), but the air may alternatively or additionally be some available indoors air having an available thermal energy that can be tapped.

Said second hot-side external liquid, which may be the same hot-side liquid as the one being heated in heat exchanger 211 (the first hot-side external liquid), may be circulated, such in0 circuit 240, 241, to heat the structure 10 in the way described above. As will be described below, however, in other embodiments the second hot-side external liquid may not be circulated to heat the structure 10 but instead only be circulated to heat the cold-side external liquid. This may apply for all or only some modes of operation of the second heat pump 220. 5 Hence, the system 200 is arranged to heat the structure 10 via heat exchange with the first, and possibly also the second, of said hot-side external liquids. Possibly, the first and second hot-side external liquids may be the same, and possibly they may both circulate in a common circuit 240, 241. 0 According to the present invention, the second heat pump 220 is a carbon dioxide heat pump. In other words, it comprises an internal heat pump loop 225, preferably being a closed loop, in which carbon dioxide is circulated as an internal-loop heat medium. Preferably, the internal-loop heat medium is pure carbon dioxide, but in other embodiments the internal-loop heat medium may comprise at least 50% carbon dioxide, such as at least 90% carbon dioxide.

5

During the circulation of the internal-loop heat medium, it passes an evaporating step and a condensation step, and therefore exists in the loop 225, during operation of heat pump 220, both in gaseous and liquid form. In particular the carbon dioxide exists in both gaseous and liquid form at different locations along the loop 225. w

On important finding of the present invention is that, by transferring thermal energy from the internal loop medium in the second heat pump 220 to the cold-side external liquid, the efficiency of the second heat pump 220 may be drastically increased while at the same time more heating power is supplied to the first heat pump 210, or alternatively the geothermal 15 well 260 can be replenished with added thermal energy.

Hence, further according to the present invention the system 200 comprises a third heat exchanger 230 being arranged to, in said second heating process, cool the internal-loop heat medium to below a critical point (CP) of the internal-loop heat medium, by heat exchanging0 the internal loop medium to the cold-side external liquid. As a result of this heat exchange, the cold-side external liquid is heated.

As mentioned above, the critical point herein refers to a maximum temperature at which the gas phase can condense. Hence, the critical point can alternatively be expressed as a5 critical temperature of the internal-loop heat medium. More precisely, the critical point refers to the liquid-vapor critical point, in other words the end point of the pressure-temperature curve for the internal-loop heat medium that designates conditions under which it can coexist in liquid and vapour form. At higher temperatures, the gas cannot be liquefied by pressure alone. 0 For carbon dioxide, the critical temperature (critical point CP) is 31°C, or more particularly 31.04°C.

In some embodiments, such as is exemplified in Figure 2a, the second hot-side external liq- s uid may be used to cool the internal-loop heat medium, via heat exchange. Then, the second hot-side external liquid may first be cooled by heat exchange with the cold-side external liquid. The third heat exchanger 230 may then be arranged to perform this cooling of the second hot-side external liquid after it has been heat exchanged to the structure 10 but before it reaches the second heat pump 220, such as before it reaches the second heat w exchanger 221. In other words, the second hot-side external liquid may then be circulated from the second heat exchanger 221, in which it is heated (via the second heat pump 220) using heat from for instance outdoors air, to the structure 10, where it is cooled by heating the structure 10, past the heat exchanger 230, where it is further cooled by heating the cold-side external liquid circulated in circuit 212b, and then back to the second heat ex- 75 changer 221.

In these and other embodiments, the cold-side external liquid may be heated using heat exchanger 230 after it leaves the ground 20 and/or the water body 21, as the case may be, and before it reaches the heat exchanger 212. In other words, the temperature of the cold-0 side external liquid entering the cold-side heat exchanger 212 is in this case higher than in a hypothetical case in which the heat exchanger 230 is not used.

It is realised that, to achieve said heat exchange, the temperature of the cold-side external liquid entering heat exchanger 230 is cooler than the other heat-exchanged liquid entering5 the heat exchanger 230, such as (in the present case) the second hot-side external liquid entering heat exchanger 230, in turn being the return liquid returning from the structure 10.

The third heat exchanger 230 may be arranged to cool the second hot-side external liquid0 to a temperature of less than 31°C, such as less than 20°C, such as less than 10°C, such as less than 5°C, before it reaches second heat exchanger 221. Each of the heat exchangers 211, 212, 230, 221, 222 may be of any suitable, per se known, type, such as a counter-flow heat exchanger. In particular heat exchanger 221 (or heat exchanger 221c, see below) may be arranged to cool the internal-loop heat medium of the s second heat pump 220, and in particular said carbon dioxide, circulated in circuit 225 to a temperature below 31°C, such as below 25°C, such as below 15°C, such as below 10°C, and at any rate below the critical temperature of the carbon dioxide.

The second heat exchanger 221 may cause the carbon dioxide in the internal loop 225 to be w condensed by being cooled to said temperature being below said critical temperature.

It is realised that the cooling of the second hot-side external liquid, and/or additional second hot-side external liquids (see below), via heat exchange to the structure 10 may be a direct cooling, such as the second hot-side external liquid being conveyed to a heat exchanger, is such as 12 and/or 13, arranged at or inside the structure 10. Alternatively, the hot-side external liquid in question may be circulated to equalisation tank 240, where it may be mixed with the first hot-side external liquid and from which the resulting mixed liquid is conveyed to said heat exchangers 12 and/or 13. Hence, the heating of the structure 10 may be indirect, via the tank 240 and/or mixing with the first hot-side external liquid. 0

The second heat pump 220 may be used to efficiently provide peak power or energy to the system 200. The first heat pump 210, as well as the geothermal well(s) 260 and/or water body 21 circuit, can be dimensioned for a normal or base case thermal load. The second heat pump 220 can be used in parallel to the heat pump 210 to temporarily increase the5 total system 200 power during peak load conditions. Since the second heat pump 220 uses carbon dioxide as the internal-loop heat medium, it is environmentally friendly and does not constitute a fire hazard. Using the heat exchange in heat exchanger 230, the working temperature of the internal loop 225 internal-loop heat medium can be low enough to keep the carbon dioxide below its critical temperature when condensed, resulting in high COP,0 also in cases where the temperature of the outdoor air 30 is sub-zero (when heating requirements are typically relatively high). This way, the total COP of the system 200 can be high even at high output powers.

Furthermore, such a system 200 allows selecting as the first heat pump 210 a I iq u id-to-liquid heat pump having a higher capacity for a given total geothermal well 260 depth or length and/or a total water body 21 heat exchange piping, since the cold-side external liquid will be heated in the heat exchanger 230 before arriving at the first heat pump 210 during operation of the system 200. This also provides a higher peak heating power or energy capacity.

As mentioned, the system 200 may comprise one or several geothermal energy wells 260 down into which the cold-side external liquid is arranged to circulate and be heated therein. Preferably, a total length or depth of these geothermal energy wells 260 may be dimensioned for a normal, non-peak, power load of the structure 10 when the structure 10 is finally built and is operated for continuous use. For instance, a total depth of the geothermal wells 260 may be at least 30 meters, such as at least 100 meters, such as at least 200 meter. The corresponding may be true for installed tubes arranged to heat the cold-side external liquid, via heat exchange, with a water body 21. In a way corresponding to what has been described above in relation to Figure 1, the system 200 may further comprise a control unit 250 and a valve system 251. The valve system 251 may comprise individual valves (as exemplified in Figures 2a-2c) controlling flow to heat exchangers 12, 13, 211, 212, 221, 221a, 221b, 221c, 222 and/or 230. In particular, the control unit 250 may be arranged to control the valve system 251 and/or the heat pumps 210, 220 so as to selectively perform either or both of said first heating process and said second heating process.

The system 200 may further comprise a temperature sensor 252 for measuring outdoors air 30 temperature, and the control unit 250 may be arranged to control the valve system 251 to selectively direct the various internal/external liquids to different heat exchangers, and in particular to selectively perform either or both of said first heating process and said second heating process, depending on a reading of the temperature sensor 252.

In some embodiments, the second heat pump 220 may be arranged in a standard-sized freight container 270. This way, the second heat pump 220 can easily be provided in a flexible manner when there is a need for higher peak heating power or energy, such as during construction, restructuring or renovation of the structure 10. Providing the second heat pump 220 may then only involve moving the container 270 to the site of the structure 10 and connecting lines for the second hot-side external liquid. The control unit 250, the tem- perature sensor 252 and/or at least some parts of the valve system 251 may also be arranged in the container 270, so that the control functionality for operating the added peakusage power or energy is included in the container 270.

Such a container 270 may comprise a quick-connect interface, in turn comprising connec- tions to piping for the second hot-side external liquid and/or any additional hot-side external liquids (see below), and also any communication interfaces required for the operation of the control unit 250. In some embodiments, the heat exchanger 230 may also be arranged inside the container 270. In that case, the container's interface may comprise connections to piping for the cold-side external liquid to and from the heat exchanger 230.

In some embodiments, the first heat pump 210 could also be arranged in said container 270. In some embodiments, at least part of the common circuit 240 may also be arranged in said container 270. In particular during construction of the structure 11, a container 270 containing at least the heat pumps 210 and 220, as well as the heat exchanger 230 and possibly at least part of the common circuit 240, may be arranged to provide heat to the structure. In some embodiments, more than one such container, each containing said parts, may be operated in parallel to accomplish a desired total power or energy. As also mentioned above, the structure 10 may be a building, and the heating of the structure 10 may comprise at least one of heating of indoors air 11 in the structure 10 and heating of tap water 13 in the structure 10. s As is illustrated in Figure 2a, and also in Figures 2b and 2c, the third heat exchanger 230 may be arranged to heat cold side external liquid that has left the ground 20, and in particular the well 260 and/or the water body 21, but before it again reaches the first heat pump 210, and in particular before it again reaches heat exchanger 212. w In the example illustrated in Figure 2a, the third heat exchanger 230 is arranged to, in said second heating process, cool the second hot side external liquid by heat exchange with the cold side external liquid, such as after it has been heat exchanged to the structure 10 but before it reaches the second heat pump 220, for instance before it reaches heat exchanger 221.

15

Turning now to Figure 2b, an alternative embodiment is illustrated, wherein the third heat exchanger 230 is arranged to transfer heat from the internal-loop heat medium of the second heat pump 220 to the second hot-side external liquid, but wherein the system 200 is arranged to heat exchange the second hot-side external liquid to the cold-side external liq-0 uid but not to the structure 10. Hence, in this case the second hot-side external liquid is circulated in its own closed loop 242c past heat exchanger 221c (in which it picks up heat from the internal-loop heat medium of the second heat pump 220) and heat exchanger 230 (in which it delivers this heat to the cold-side external liquid on its way to the first heat pump 210). In a way, the combination of heat exchangers 221c and 230 may be seen as one5 heat-exchanging arrangement for transferring heat from the internal-loop heat medium of the second heat pump 220 to the cold-side external liquid.

In Figure 2c, yet an alternative embodiment is illustrated, wherein the third heat exchanger 230 is instead arranged to selectively transfer heat directly from the internal-loop heat me-0 dium of the second heat pump 220 to the cold-side external liquid. This is performed via a part-loop 212c forming a branched part of the circuit 212b, so that the cold-side external liquid can be selectively diverted past heat exchanger 230, on its way to heat exchanger 212, by the control unit 250 controlling corresponding valves 251.

Common to Figures 2b and 2c is that the cold-side external liquid is thermally isolated from any hot-side external liquid being active to transfer heat from the second heat pump 220 to the structure 10. In contrast, Figure 2a shows the cold-side external liquid being heated by such hot-side external liquid.

To be clear, in Figure 2a the second hot-side external liquid is circulated past the heat ex- changer 230, the heat exchanger 221 and the structure 10; while in Figure 2b the second hot-side external liquid is instead circulated in a separate closed loop 242c not passing the structure 10, while other hot-side external liquids are circulated past the structure 10; and while in Figure 2c there is no second hot-side external liquid but only other hot-side external liquids of the type described in the following.

Hence, as illustrated in Figures 2b and 2c, the second heat pump 220 may further comprise one or several additional hot-side 223 heat exchangers 221a, 221b, in turn being separate from the third heat exchanger 221c/230 (or, more generally, separate from any heat exchanger active in transferring heat from the internal-loop heat medium to the cold-side ex- ternal liquid). Moreover, said one or several additional hot-side 223 heat exchangers 221a, 221b may be arranged to transfer heat from the internal-loop heat medium to a hot-side external liquid in turn being arranged to deliver heat, via heat exchange, to the structure 10. In particular, the second heat pump 220 may comprise two additional hot-side 223 heat exchangers 221a, 221b, at least one of which 221a being arranged to transfer heat from the internal-loop heat medium of the second heat pump 220 to a hot-side external liquid, in turn circulated in a closed loop 242a passing by a suitable structure 10 heat exchanger to thereby transfer heat to indoors air 11 of the structure 10. At least one other of said additional hot-side 223 heat exchangers 221, 221b may be one 221b being arranged to transfer heat from the internal-loop heat medium of the second heat pump 220 to a separate hot-side external liquid, in turn circulated in a closed loop 242b passing by a suitable structure 10 heat exchanger to thereby transfer heat to hot tap water 13 of the structure 10.

Hence, the second heat pump 220 may comprise three different heat exchangers 221a, 221b, 221c, each arranged to cool the internal-loop heat medium of the second heat pump 220 by heat transfer to a respective hot-side 223 external liquid. At least one of said external liquids may be heated to a different temperature by the heat exchange in question to the internal-loop heat medium, as compared to the other external liquid(s).

For instance, a temperature of at least one hot-side 223 external liquid leaving a hot-side 223 heat exchanger of the second heat pump 220 may be at least 60°C, such as at least 70°C. This is particularly the case for a hot-side 223 external liquid being used, for instance exclusively used, to heat tap water 13 of the structure 10. In Figures 2b and 2c, such external liquid is circulated in loop 242a.

Moreover, a temperature of at least one hot-side 223 external liquid leaving a hot-side 223 heat exchanger of the second heat pump 220 may have a temperature of between 40°C and 70°C, and/or be at least 10°C, or even at least 20°C, cooler than said temperature of said external liquid used for heating the tap water 13. For instance, such cooler external liquid may be used to heat indoors air 11 of the structure 10. In Figures 2b and 2c, such external liquid is circulated in loop 242b.

Furthermore, any hot-side 223 external liquid that is heat-exchanged to the cold-side external liquid may, after heat exchange to a hot-side 223 heat exchanger 221c may have a temperature of between 0°C and 50°C, and/or be at least 10°C, or even at least 20°C, cooler than any of the two external liquids discussed above (circulated in loops 242a and(/or 242b). In Figure 2b, such external liquid is circulated in loop 242c. In preferred embodiments, in case there are more than one such separate external liquid loop 242a, 242b, 242c having their own respective hot-side 223 second heat pump 220 heat exchanger 221a, 221b, 221c, the internal-loop heat medium of the second heat pump 220 is circulated, in said internal loop 225, past said heat exchangers 221a, 221b, 221c in order s of decreasing output temperature after heat exchange of the corresponding hot-side 223 external liquid. In other words, the internal-loop heat medium is successively cooled to lower temperatures by passing said heat exchangers 221a, 221b, 221c in order. In some embodiments, it is only in the most downstream such heat exchanger 221c (and/orthe heat exchanger 221c operative to heat the cold-side external liquid) that the internal-loop heat w medium passes below the critical temperature of the internal-loop heat medium. In other words, with respect to more upstream-arranged heat exchangers 221a, 221b the second heat pump 220 then functions as a gas heater of the hot-side 223 external liquid in question, while functioning as a heat pump with respect to said downstream heat exchanger 221c (that may be the one heating the cold-side external liquid).

15

In the case of Figure 2c, the heat exchanger 230 has the corresponding role, in this context as the heat exchanger 221c of Figure 2b.

It is noted that the external liquid being circulated in loop 242b is, by way of example, con-0 veyed to tank 240 for mixing with the first external liquid in a way corresponding to the one described in relation to Figure 2a.

It is also noted that the loops 242a and 242b may be selectively operated to allow respective hot-side 223 external liquid to circulated therein, depending on a current need for indoors5 air 11 and/or tap water heating in the structure 10.

Also, it is noted that the heat exchange between the internal-loop heat medium of the second heat pump 220 and the cold-side external liquid, effectively heating the cold-side external liquid as a result of this heat exchange, may take place directly via heat exchanger0 230 (as is the case in Figure 2c), or take place indirectly via heat exchanger 230 and further additional heat exchanger 221 (Figure 2a) or 221c (Figure 2b). Moreover, in Figure 2c a parallel-connection conduit 212c' is shown, allowing the control unit 250 to selectively direct the cold-side external liquid in a parallel loop past heat exchangers 230 and 212 without passing the ground 20 or water body 21, said parallel loop s being parallel to the cold-side external liquid loop passing past heat exchangers 230, 212 as well as the ground 20 and/or water body 21. Hence, the cold-side external liquid may be circulated in said parallel-connected loops so that a larger volume flow of cold-side external liquid is circulated past heat exchanger 230 than what is passed through the ground 20 or water body 21, while the cold-side external liquid mixed in said parallel-connected loops. w This allows for a larger heat power to be transferred to the cold-side external liquid from the second heat pump 220 even when the capacity of the loop passing the ground 20 and/or water body 21 is limited. The corresponding mechanism may also be applied in the example embodiments shown in Figures 2a and 2b. The flow in the conduit 212c', as in any conduit shown in the drawings, as the case may be, may be controlled using a separate pump (not is shown).

Figure 3 illustrates a method according to the present invention, for heating the structure 10 using the system 200. 0 In a first step, the method starts.

In a subsequent step, said first heating process is performed, in which the first heat pump 210 is used to deliver heat, via a first heat exchange performed using said first heat exchanger 211, from the hot side 213 of the first heat pump 210 to the first hot-side external5 liquid. As mentioned, as a part of the first heating process the first heat pump 210 receives heat from the cold-side external liquid, in turn circulating in the ground 20 and/or the water body 21.

In another step, performed after the method has started and before, during or after the0 first heating process, the method comprises performing said second heating process in either of the various ways described above. Hence, in the second heating process, the second heat pump 220 is used to deliver heat, via a second heat exchange using the second heat exchanger 221, 221c, from the hot side 223 of the second heat pump 220 to the second hot- side external liquid. In the second heating process, the second heat pump 220 may receive heat from the outdoors air 30.

5

The method further comprises heating the structure 10 via heat exchange with the first, and possibly also the second (and/or additional), hot-side external liquids. Also, the first and any one of the hot-side external liquids may possibly be the same and possibly circulating in the common circuit 240, 241. w

Moreover, said second heating process comprises, in a third heat exchange using the third heat exchanger 230, cooling the internal-loop heat medium by heat exchange to the coldside external liquid as described above. This may then take place by cooling the second hot- side external liquid by heat exchange with the cold-side external liquid as described above. 15 In particular, this cooling of the second hot-side external liquid takes place after it has been heat exchanged (directly or indirectly as described above) to the structure 10 but before it reaches the second heat pump 220 (such as before it reaches heat exchanger 221). As described above, the third heat exchange results in that the internal-loop heat medium is cooled to a temperature of less than said critical temperature or critical point (CP) of carbon0 dioxide, in other words to a temperature below 31°C.

The method may then iterate, shifting between operating according to the first heating process and/or the second heating process, in parallel or sequentially, as the heating requirements vary over time. It is noted that the third heat exchange may in some embodiments5 be performed only under condition that the first heating process is ongoing, so that the cold-side external liquid is heated before arriving at the heat exchanger 212. However, the cold-side external liquid may also be heated in case the first heating process is not ongoing, in other words when the first heat pump 210 is not operable for heating. This will be exemplified below. 0 It is further noted that the first, second and/or third heat exchanges can take place simultaneously, as the case may be, in a process that is at least temporarily continuous.

In a subsequent step, the method ends.

As mentioned above, the second heat pump 220 uses carbon dioxide as internal-loop heat medium.

In preferred embodiments, the third heat exchange results in that the internal-loop heat medium is cooled from an initial temperature of at least 31°C, such as from an initial temperature of at least 40°C, to a cooled temperature of less than 31°C.

On the other hand, in preferred embodiments a temperature of the internal-loop heat medium just upstream of the second heat exchanger 221, 221c, 230 (as the case may be) has a temperature of at least 50°C, such as at least 60°C, such as at least 70°C. In other words, the carbon dioxide circulated in the internal loop 225 may be cooled from such a high temperature of above 60°C or even above 70°C, to a low temperature being below the critical temperature of the carbon dioxide, as a result of the heat exchange in the heat exchanger 221, 221c, 230 in question, achieving condensation of the carbon dioxide.

In an exemplifying embodiment, the system 200 is operated using the following parameters at a point in time when the outdoors air 30 temperature is -20°C:

Electric power (EP) provided to air-to-liquid (second) heat pump: 23 kW Temperature (TEMP) of second external liquid arriving at third heat exchanger: 35°C

TEMP of second external liquid leaving third heat exchanger: 3°C

TEMP of second external liquid arriving at second heat exchanger: 3°C

TEMP of second external liquid leaving second heat exchanger: 80°C

Thermal power (TP) provided by air-to-liquid heat pump to structure: 50 kW TP provided by air-to-liquid heat pump to cold-side external liquid: 17 kW

TEMP of cold-side external liquid arriving at geothermal wells: -3°C TEMP of cold-side external liquid leaving geothermal wells: 0°C

TP provided by geothermal wells to cold-side external liquid: 68 kW

TP provided by cold-side external liquid to liquid-to-liquid heat pump: 85 kW

EP provided to liquid-to-liquid (first) heat pump: 38 kW s TP provided by liquid-to-liquid heat pump to first hot-side external liquid: 124 kW

TEMP of first hot-side external liquid arriving at first heat exchanger: 42°C

TEMP of first hot-side external liquid leaving first heat exchanger: 50°C

TEMP of hot-side external liquid mixture arriving at structure: 50°C

TEMP of hot-side external liquid mixture leaving structure: 30°C w TP provided by hot-side external liquids to structure: 174 kW

Total system COP: 2.8

In general, the temperature of the first hot-side external liquid leaving the first heat exchanger 211 may be lower, such as at least 10°C or oven at least 20°C lower, than the tem- 15 perature of a warmest one of the one or several hot-side external liquids leaving its respective heat exchanger 221, 221a, 221b, 221c in question.

Further generally, the temperature of the second hot-side external liquid may be cooled, in the third heat exchanger 230, at least 10°C, such as at least 20°C. 0

Figure 4 illustrates a method according to the above, wherein the selection as to which one or ones of the first heating process and the second heating process is or are performed depends on a detected temperature of the outdoors air 30. 5 In a first step, the method starts.

In a subsequent step, the outdoors air 30 temperature is measured, using sensor 252.

Then, in case the outdoor air 30 temperature is detected to be below a first threshold tem-0 perature, or whenever this is the case, the first heating process and the second heating process are performed in parallel. In other words, both the cold-side external liquid, the first hot-side external liquid and the second hot-side external liquid are circulated in their respective circuits (or, in the case of Figure 2c, the cold-side external liquid is circulated in part-loop 212c past heat exchanger 230); the first heat pump 210 is operated to transfer heat from the cold-side external liquid to the first hot-side external liquid (or directly to the s internal-loop heat medium); and the second heat pump 220 is operated to transfer heat to the second and any additionally used hot-side external liquids, such as from outdoors air 30. Also, the third heat exchanger 230 is operated to transfer heat from the internal-loop heat medium to the cold-side external liquid as described above. w In contrast, in case the outdoor air 30 temperature is detected to be equal to or above said first threshold temperature, or whenever this is the case, the first heating process is performed but not the second heating process. In other words, the cold-side external liquid and the first hot-side external liquid are circulated in their respective circuits but not the second hot-side external liquid (or the circulation in part-loop 212c is stopped, in the case is of Figure 2c); the first heat pump 210 is operated to transfer heat from the cold-side external liquid to the first hot-side external liquid; but the second heat pump 220 is not operated to transfer heat, neither to the second hot-side external liquid. Also, the third heat exchanger 230 is not operated to transfer heat from the internal-loop heat medium to the cold-side external liquid. 0

The structure 10 is heated using the first hot-side 223 external liquid (or other/additional hot-side 223 external liquids, as described), depending on what process(es) is/are active; what loops 240, 241, 242, 242a, 242b, 242c; and if the various liquids in question are mixed or not; all as described above. 5

In a subsequent step, performed after any iteration, the method ends.

Using such a method, the second heat pump 210 can be used as a true peak power provider during cold outdoors temperatures, such cold outdoors temperatures implying high heating0 power or energy requirements in the structure 10. Since the carbon dioxide second heat pump 220 is capable of offering high COP values even at relatively low outdoor air 30 temperatures, the total system 200 COP value will be acceptable while the first heat pump 210 (and also for instance the wells 260) can be dimensioned for a less-than-peak energy requirement. The first threshold temperature may be at least -10°C, such as at least -5°C, such as at least 0°C. Moreover, the first threshold temperature may be at the most 15°C, such as at the most 10°C.

At an outdoor air 30 temperature of 0°C, being above the first threshold value, in an exem- plifying embodiment the system 200 may then be operated using the following parameters:

Electric power (EP) provided to air-to-liquid (second) heat pump: 0 kW

Temperature (TEMP) of second external liquid arriving at third heat exchanger: - TEMP of second external liquid leaving third heat exchanger: TEMP of second external liquid arriving at second heat exchanger:

TEMP of second external liquid leaving second heat exchanger:

Thermal power (TP) provided by air-to-liquid heat pump to structure: 0 kW

TP provided by air-to-liquid heat pump to cold-side external liquid: 0 kW

TEMP of cold-side external liquid arriving at geothermal wells: -3°C TEMP of cold-side external liquid leaving geothermal wells: 0°C

TP provided by geothermal wells to cold-side external liquid: 68 kW

TP provided by cold-side external liquid to liquid-to-liquid heat pump: 68 kW

EP provided to liquid-to-liquid (first) heat pump: 24 kW

TP provided by liquid-to-liquid heat pump to first hot-side external liquid: 95 kW TEMP of first hot-side external liquid arriving at first heat exchanger: 32°C

TEMP of first hot-side external liquid leaving first heat exchanger: 40°C

TEMP of hot-side external liquid mixture arriving at structure: 40°C

TEMP of hot-side external liquid mixture leaving structure: 28°C

TP provided by hot-side external liquids to structure: 95 kW Total system COP: 3.9 Figure 5 illustrates another method according to the present invention, wherein the selection as to which one of the first heating process and the second heating process is performed also depends on a detected temperature of the outdoors air 30. The method illustrated in Figure 5 may advantageously be combined with the method illustrated in Figure 4.

In a first step, the method starts.

In a subsequent step, the outdoors air 30 temperature is measured, using sensor 252. Then, in case the outdoor air 30 temperature is detected to be above a second threshold temperature, or whenever this is the case, the second heating process is performed but not the first heating process. In other words, the second hot-side external liquid is circulated in its circuit (or the circulation in part-loop 212c is activated, in the case of Figure 2c) but not the first hot-side external liquid; whereas the second heat pump 220 is operated to transfer heat to the second hot-side external liquid (such as from outdoors air 30) but the first heat pump 210 is not operated to transfer heat from the cold-side external liquid to the first hot- side external liquid.

However, the third heat exchanger 230 is in this case still operated to transfer heat from the internal-loop heat medium to the cold-side external liquid as described above. To this end, the cold-side external medium is circulated into the ground 20 and/or the water body 21 as described above, but without operating the first heat pump 210. This means that the cold-side external liquid is circulated in its closed-loop circuit 212b, being heated via the third heat exchanger 230 and transferring this heat into the ground 20 and/or water body 21. It is preferred, in this case, that the cold-side external liquid is circulated into one or several geothermal wells 260, whereby the ground 20 surrounding the well(s) 260 is heated as a result. Preferably, no or substantially no heat transfer takes place in heat exchanger 212. This will recharge the ground 20 with thermal energy that will then be available for later use, such as when the outdoors air 30 temperature is lower or when the heating power or energy requirements are for any other reasons higher. s In case the outdoor air 30 temperature is measured to be equal to or below the second threshold temperature, the control unit 250 may control the system 200 to perform both the second heating process and the first heating process, including operating the third heat exchanger 230 as described above. Another option is then to have the control unit 250 control the system 200 to behave in accordance with the method illustrated in Figure 4, in other w words to select between only the first heating process or a combination of the air and first heating processes as a function of the outdoor air 30 temperature in relation to the first threshold temperature.

In a subsequent step, the method ends.

15

This way, the system 200 may be operated to replenish the ground 20 with thermal energy when heating power or energy requirements are relatively low, using only the second heat pump 220 and not the first heat pump 210. 0 In some embodiments, the method according to Figure 5 may be used even if there is no comparison to said second threshold value, but instead for instance whenever the instantaneous total power requirements are detected to be below a certain power (or a certain total energy usage across a particular stretch of time) threshold value for any other reason. 5 The second threshold temperature may be higher than said first threshold temperature, such as at least 5°C or even at least 10°C higher than the first threshold temperature. In some embodiments, the method according to Figure 5 can be performed even in case the first threshold value and the method according to Figure 4 is not implemented or used by the system 200. In this case, the second threshold value may be at least 0°C, such as at least0 5°C, such as at least 10°C; and it may also be at the most 20°C, such as at the most 15°C. It is understood that, when the outdoor air 30 temperature is detected to be less than or equal to the second threshold temperature, both the second heating process and the first heating process may be used in parallel, such as in the way illustrated in Figure 3. When the outdoor air 30 temperature is measured to be above the first threshold temperature but s below the second threshold temperature, the method according to Figure 4 may instead be used.

At an outdoor air 30 temperature of 5°C, being above the second threshold value, in an exemplifying embodiment the system 200 may then be operated using the following pa- w ra meters:

Electric power (EP) provided to air-to-liquid (second) heat pump: 11 kW

Temperature (TEMP) of second external liquid arriving at third heat exchanger: 28°C

TEMP of second external liquid leaving third heat exchanger: 3°C is TEMP of second external liquid arriving at second heat exchanger: 3°C

TEMP of second external liquid leaving second heat exchanger: 70°C

Thermal power (TP) provided by air-to-liquid heat pump to structure: 50 kW

TP provided by air-to-liquid heat pump to cold-side external liquid: 18 kW

TEMP of cold-side external liquid arriving at geothermal wells: 3°C0 TEMP of cold-side external liquid leaving geothermal wells: 0°C

TP provided by geothermal wells to cold-side external liquid: -18 kW

TP provided by cold-side external liquid to liquid-to-liquid heat pump: 0 kW

EP provided to liquid-to-liquid (first) heat pump: 0 kW

TP provided by liquid-to-liquid heat pump to first hot-side external liquid: 0 kW5 TEMP of first hot-side external liquid arriving at first heat exchanger:

TEMP of first hot-side external liquid leaving first heat exchanger:

TEMP of hot-side external liquids arriving at structure: 35°C

TEMP of hot-side external liquids leaving structure: 25°C

TP provided by hot-side external liquids to structure: 50 kW0 Total system COP: 4.4 The present invention may be particularly advantageously applied during and after construction of the structure 10. In such cases, methods according to the present invention may further comprise a structure construction process preceding a construction operation process. A structure construction process may, for instance, be a process during which a build- ing (the structure 10) is built, re-built or renovated. Correspondingly, a structure operation process may involve operating the built building for its intended use, such as in the form of a factory, office space or apartments.

In general, such a structure construction process is associated with a higher time-averaged heating power or energy requirement than a time-averaged heating power or energy requirement associated with a corresponding structure operation process. For instance, during the construction process the structure 10 may be more open, such as lacking windows, doors or other construction parts. One particular example is when the construction process involves drying or setting of building material, such as concrete, requiring active heating to speed up or otherwise control the drying or setting process.

Then, during at least part of said structure construction process, both the first heating process and the second heating process may be performed in parallel, whereas, during at least part of said structure operation process, only the first heating process is performed and not the second heating process.

For instance, one or several containers 270 of the above-discussed type may be temporarily located at the construction site, exploiting for heating of the under-construction structure 10 thermal power from the same geothermal wells 260 that are to be used for heating of the finalised structure 10 also during the structure operation process, but providing top-up thermal heating power using the second heat pump 220 as described above. Once the construction of the structure 10 is finalised, some or all of the containers 270 may be removed. In some cases, one or several of the containers 270 may be kept to provide an increased peak heating capacity also during the structure operation process. In yet other embodiments, the second heat pump 220 may be permanently installed at the structure 10 for use both during the structure construction process and also subsequently during the structure operation process. This may imply that a smaller number of meters of geothermal well 260 need to be drilled, since the available thermal power or energy of the s geothermal well(s) 260 per meter is increased by the second heat pump 220.

In particular, methods according to the present invention may comprise a first heat pump 210 installation step, in turn comprising installing the cold-side external liquid circulation loop 212b being dimensioned for providing said (lower) time-averaged heating power or w energy associated with said structure operation process and not for providing said (higher) time-averaged heating power or energy associated with said structure construction process. This may also imply installing geothermal wells 260 having a total depth or length so that they are dimensioned for said lower time-averaged heating power or energy and not for said higher time-averaged heating power or energy.

15

Figure 6 illustrates a log pH-diagram (log vertical axis is pressure p, horizontal axis is enthalpy H) showing the operating heat pump cycle of an exemplifying second heat pump 220 when operated for heating the second hot-side external liquid as described above. As is especially noted, the condensation of the carbon dioxide takes place below its critical tem-0 perature.

At point 2, the compressor has compressed the heat medium.

At point 3, a certain pressure loss has occurred.

At point 4, the heat medium has condensed. 5 At point 5, the enthalpy of the heat medium has decreased further by internal heat exchange leading in turn to a lowering of the heat medium temperature.

At point 6, the pressure has dropped across the expansion valve.

At point 7, the enthalpy of the heat medium has increased by evaporation in the evaporator.

At point 8, a certain pressure loss has occurred. 0 At point 1, said internal heat exchange has led to a temperature increase, resulting in increased enthalpy. Figure 6 also shows the phase transition lines.

Above, preferred embodiments have been described. However, it is apparent to the skilled s person that many modifications can be made to the disclosed embodiments without departing from the basic idea of the invention.

For instance, the system 200 may comprise additional equipment, such as sensors, control units and regulators for indoors air climate in the structure; additional heat pumps; solar w panels; and so forth.

Several liquid-to-liquid heat pumps 210 may be connected in parallel; and/or several air-to- liquid heat pumps 220 may be connected in parallel. is Everything that has been said in relation to the present system is equally applicable to the present method, and vice versa. Furthermore, the various exemplifying embodiments described herein are generally combinable, as applicable.

Hence, the invention is not limited to the described embodiments, but can be varied within0 the scope of the enclosed claims.