NO20230050A1 - A new more cost-effective reservoir-based groundwater solution of extracting heat or cold from the subsoil for use in for either a ground sourced heat pump (GSHP) or for direct use in for example ventilation/refrigeration - Google Patents

Description

Title

A cost-effective alternative method of collecting heat from the soil for use in ground sourced heat pump (GSHP) using a reservoir-based solution

Purpose of the invention

The purpose of the invention is to reduce the initial cost of installing a ground sourced heat pump. The largest cost related to the investment in a ground sourced heat pump is the excavation and work required to set up the collecting side (outdoor part) of the heat pump (heating mode). As of today, there exists two established solutions for how a GSHP can be set up to collect heat from the soil.

The first established solution is called a horizontal setup, where trenches are dug at depths of approximately 0.5 to 2 m below the surface and then usually high-density polyethylene pipes are placed into the trenches. As the name suggest these pipe/trenches are laid over a large horizontal field in a given pattern and by pumping circulation fluid through the pipes we will be able to absorb the heat from the ground which has a higher temperature than the air during winter (heating mode). This solution only requires an excavator, making it relatively easy to obtain the equipment and workforce to set up a horizontal solution. The problem with a horizontal solution is that the temperature although higher than the air during winter, still is very affected by the air temperature and does not give such a great advantage over an air sourced heat pump (ASHP) which collects its heat from the ambient air directly. This defeats the whole purpose of the GSHP which is to be able to collect heat from a source which has a temperature closer to the “required end temperature” (example in home this would be the indoor temperature) than the ambient air is. The advantage this problem create is that due to the shallow depth of the trenches the soil surrounding the pipes is able to regenerate the heat lost during the offseason (summer) when heating is not required as the air, sun and surface water is able to heat the soil directly. The reason regeneration of the soil is so important is that if the soil never is able to obtain the lost heat during heating seasons its average temperature will continue dropping until it is no longer usable (equal/lower temperature than the ambient air). Regeneration can also be done by reversing the heat pump, cooling the indoor air (if it is connected to a home) and inserting heat into the soil. But in many countries a cooling is not required due to relatively chill air even during summer seasons, making the heating season (extracted heat) a lot larger than cooling season (inserted heat). Another major problem with the horizontal setup is the horizontal space required. A common minimum requirement used is 700 m2 to be able to collect enough energy to heat a “normal” single-family home. Limiting this solution to rural communities with large gardens/fields to use. Cost of a horizontal setup with the GSHP unit (heating a single-family home) and everything required to get it operational was approximately €19,200 to €28,360 in Norway 2021.

The second established solution is the more common one. It is known as the vertical setup. The vertical setup is the complete opposite of the horizontal setup as the name suggest. In the vertical setup a boring rig will create boreholes, drilling minimum 50 m down typically. Often, multiple holes or an even longer single hole is necessary, it depends on the heating load. By pumping circulation fluid down through the borehole and back up again, it will absorb heat from the soil surrounding the casing/pipe of the borehole. This setup requires a boring rig, specialized equipment, and a lot of environmental planning. This makes the equipment hard to obtain especially in smaller cities as it is not sustainable for a company to invest in this equipment if they do not have access to a large number of customers within their vicinity. The environmental planning is due to the borehole penetrating through multiple confined layers of groundwater and exposing confined groundwater to unconfined groundwater as well as surface water. This is a heavily regulated subject due to confined water often being used as a water source for humans, animals, and farming. The advantage that makes it so common is the little required area needed of a garden/field. It only requires a couple square meters which is needed for the boring rig to stay on during drilling, after drilling all that is left is a pipe size opening which is connected directly to the GSHP unit. Another advantage is that as it is so deep it is not affected by the ambient air, further down into the soil one goes the higher the temperature typically will be and less affected by the ambient air. Therefore, vertical solutions can provide larger amount of heat and increased temperature of the circulating fluid than its horizontal counterpart. The problem this advantage will create is that due to its lack of connection with the ambient temperature it will not naturally regenerate its heat during the off-season (summer). The borehole if not artificially replenished will become colder and colder, losing its efficiency as time progresses and can reach a point where its advantage over using air directly in a ASHP is little to none. Therefore, running it in reversed mode (cooling indoor air) is required during the summer to insert some heat back into the soil to maintain the borehole to its original temperature. Cost of a vertical setup with the GSHP unit (heating a single-family home) and everything required to get it operational was approximately €26,300 minimum in Norway 2021.

By creating a hybrid between these solutions, we are able to find a method that can use each one’s strength to reduce their disadvantages. This hybrid solution is the invention proposed in this application and has been given the name “reservoir setup”. The reservoir setup is placed below groundwater level which for the Nordic countries typically is 0 to 10 m below the surface and utilize a reservoir to collect heat from the ground, the reservoir serving as a groundwater holding tank and is made up from gravel not a physical tank. This makes the reservoir solution able to be placed/excavated by a standard excavator and “basic” equipment can be used. Another important feature is that the reservoir tank itself is easy to place as it is not a physical tank and gravel can be dumped into the excavated volume to create the tank. It is also so shallow that it will remain only in the loose soil of the unconfined area, not penetrating into lower confined areas or bedrock. This will greatly reduce potential environmental impact and therefore avoid the regulations related to groundwater contamination. The depth the reservoir setup works at makes it have such a huge advantage. At this depth the ambient air affection on the temperature is negligible, holding a high temperature through the coldest winter months (as well as the warmest summer months in case of “cooling mode”). The temperature difference between the groundwater table level and 50 m down into a borehole is much smaller than the temperature difference between the trenches at 0.5 to 2 m and the groundwater table level. This makes the reservoir setup able to reach “efficiency” levels close to the vertical setup. But since the reservoir setup still is inside the unconfined area, we are allowed to use the surface water to regenerate the reservoir and surrounding soil during off-season (summer). Adding a gravel “highway” from the reservoir to close to the surface makes it possible for warm rain-/surface-water to quickly move to the reservoir during the off-season (summer) and reheat the surrounding soil without violating any environmental regulations as this all happens inside the unconfined layer. The reservoir setup only requires a couple of square meters during excavation as the vertical setup making it suitable for urban buildings as well as it provides a better efficiency than the horizontal setup making it a suitable option for rural buildings too. A better overview of the reservoir setup can be seen in figure 1 which is attached to the patent application.

Having a lower initial cost which is a biproduct of the simplicity of installation on such a setup would make it possible for more people to do the investment as well as it would make it easier for such a setup to return a positive net present value. The basic equipment needed also makes it easier for people outside larger cities to invest in GSHP (not needing a specialized boring rig etc.).

Background for the invention and its working in detail

First, we can look a little through what a heat pump is, why using a heat pump is advantageous as well as what affect the heat pump efficiency (also known as COP).

A heat pump is a general term used for products/solutions which use a refrigerant fluid to move heat from an evaporator to a condenser. The first classification that can be made for heat pump is to classify it by the need of a compressor. The heat pump type that does not utilize a compressor is known as an absorption heat pump, the type that utilize a compressor is known as a compression heat pump, this invention is classified in the latter. This products/solutions in this group typically all have four components in common which is the evaporator, condenser, compressor, and an expansion valve.

The heat pumps job is to move or “pump” heat, in heating mode the heat is moved from a place of low temperature to a place of high temperature typically from outdoor to indoor. It can also be used in cooling mode where heat is moved from a place of high temperature to a place of low temperature and is then usually referred to an “air conditioning unit” rather than a heat pump (AC is not able to heat, only cool while a heat pump is able to do both). The heat pump is able to move this heat by utilizing a refrigerant. The refrigerant is decided so that for its given operating condition the refrigerant will turn into gaseous form inside the evaporator and into liquid form inside the condenser for the given temperature it is in their surroundings. It is this phase change that is able to extract/insert so much heat and makes a heat pump so efficient.

The operation cycle of the heat pump in heating mode begins when the refrigerant in gaseous form is compressed to a higher pressure through the compressor. Exiting the compressor, the refrigerant which still is in its gaseous form has now been given a higher pressure and a higher temperature. The temperature increase is a product of the pressure increase, they are correlated through the gas laws (see “ideal gas law”). The next step in the cycle is for the refrigerant to move through the inside of the condenser where the refrigerant with its higher temperature will transport heat to the colder medium that is surrounding the outside of the condenser. It is during this cooling of the refrigerant that it will change into liquidous form. Exiting the condenser, the refrigerant is now in liquidous form, but the high pressure still remains. The next step in the cycle is for the refrigerant to move through an expansion valve where the refrigerant pressure drops from a high pressure to a low pressure. Exiting the expansion valve the refrigerant is still in liquidous form but has been given a lowered pressure and a lowered temperature (again a corelated relationship through the gas laws). The next step is in the cycle is for the refrigerant to move through the inside of the evaporator where the refrigerant with its low temperature will absorb heat from the warmer medium surrounding the outside of the evaporator. It is during this heating of the refrigerant that it will change into gaseous form. Now the operational cycle repeat. A heat pump running in cooling mode works exactly the same as a heat pump in heating mode only running the compressor and therefore the cycle in the opposite direction which in practice will make the condenser the new evaporator and evaporator the new condenser.

A heat pump using the cycle described in the paragraph above is able to extract/convert approximately 2 to 5 times more energy in the form of heat than the mechanical energy it uses to run the compressor. This efficiency is known as COP, which is a specific efficiency relating to heat pumps which is able to extract more energy in the form of heat than they use, meaning they have a system efficiency above 100%. For comparison a typical electrical heater also known as a resistance heater will be able to maximum extract/convert the same amount of heat energy as the electrical energy used by it, meaning a COP of 1. The smaller temperature difference between the heat source (typically outdoor from air, soil etc.) and heat sink (typically indoor from water-heater, air heating, waterborne heating etc.) the greater the efficiency. This is the primary reason why ground sourced heat pumps are so efficient (COP) at locations where the air can become very cold (heating mode) or very warm (cooling mode), creating a large temperature difference from the heat sink, the soil can act as the intermediary. By using the heat of the ground, we are able to reduce the temperature difference and increase the efficiency (COP), this COP gain can be very large for locations that experience large change in temperature from summer to winter (typically inland locations which is not kept constant by the ocean temperatures).

The reservoir setup uses a reservoir that is placed below the watertable line (meaning it will be filled with groundwater) to collect heat. The reservoir itself serves as a groundwater tank. The reservoir is not a physical tank from sheet metal but rather a volume filled with a porous material typically gravel. This porous material serves as a “highway” for the groundwater to flow quickly with little to no resistance just as it was a physical tank. It is inside the reservoir that the heat-exchanger which typically is a coil of HDPE pipe is placed. This fluid circulating the coil will absorb the heat from the reservoir (heating mode) and move this heat to a GSHP unit.

Inside the reservoir a coil of HDPE is placed (reservoir heat-exchanger). Inside the coil the circulating fluid, which in our case is water mixed with antifreeze as for example ethanol, is looped around to collect heat from the reservoir (antifreeze hindering solidification when fluid is cooled by the GSHP unit (heating mode) as well as it can have beneficial properties as reducing fouling inside the pipe). This circulation is done by the circulating pump which is to be placed at the surface by the GSHP unit in series. Ethanol is a good candidate to use as the antifreeze as it is non-toxic to the surrounding soil (in case of leakage), safe for personnel to work with and it has good properties both on the heat transport side (thermal properties) as well on the fluid transport side (fluid properties). For the reservoir to be able to maintain the heat transport required for a single house in for example a Nordic country, which typically is around ≈6-9 kW, it must employ convection (seepage) as conduction alone is unable to extract this heat from the surrounding soil. This is the main reason for having the reservoir below the water table. By having a pump inside the reservoir constantly pumping fluid from the reservoir to the surrounding soil (see Figure 1), we can retrieve heat from a higher volume/quantity of soil (come in contact with) that the reservoir walls are able to do alone. The cold groundwater pumped out of the reservoir will move back on its own toward the reservoir again from the head difference in the watertable, this will form a continuous loop that is able to sustain our heat requirements, constantly retrieving heat by letting the cooled groundwater seep through the soil absorbing its heat from huge amount of soil as the seepage move through multiple layers of soil on its way back into the reservoir. The amount of soil required can be chosen by both the amount of injection sites we have as well as the length from the reservoir to the injection site. These parameters can be tweaked as needed. The head difference occurs when the reservoir pump is active as it will create a low-pressure point at the pump location this will lower the water table at inlet point as well as the water table at the exit points will have a heighten water table due to being at the high-pressure points (groundwater accumulating). This will make the flow move back toward the reservoir as it does during normal operation.

Depictured in the figure is also an implementation to regenerate the heat in the soil. Depletion of heat in the soil is one of the major problems for GSHP solutions as a lowered soil temperature increases the temperature gap to the heat sink (meaning to the indoor in case of building installation) which is the whole purpose of investing in a GSHP. Especially vertical setup which tap into long term heat which has been stored for a long time have this issue.

By having a gravel “highway” from the reservoir to close the surface (reason for not having it all the way to the surface is to have a small soil layer between, isolating the highway from the air temperatures) we allow warm rainwater during the summer (non-heating season) to move down into the reservoir. This will heat the groundwater in the reservoir tank. To also heat the surrounding soil around the tank which we have seeped cold groundwater through the whole winter (heating season), we can run the reservoir pump and force warmed groundwater from the reservoir to seep through the surrounding soil regenerating it just as we do during “normal operation” during the summer (off-season). This is the regeneration mode of the reservoir solution invention. The power of running the reservoir pump is very small, typically less than 100 W for full flow, making the reservoir solution’s regeneration mode hugely less energy demanding than for example a vertical solution where one need to run the whole system (heat pump circulation pump) to regenerate the heat in the borehole. The regeneration also will not affect the consumers in any way as all this regeneration happens locally in the reservoir and an absorption of the indoor heat (cooling indoor as the heat pump cycle is reversed) will not be required as it is for a vertical setup’s heat regeneration.

As described in this section the invention is built on existing principles/inventions but combine them in a unique way to make it function as the product it is, creating a new method of collecting heat for use in a GSHP solution. The invention can also act as a cooler by running “cooling mode” which simply is done by just reversing the GSHP cycle.

Summary

A new more cost-effective reservoir-based groundwater solution of extracting heat or cold from the subsoil for use in for either a ground sourced heat pump (GSHP) or for direct use in for example ventilation/refrigeration. Its characteristics include the use of a reservoir (5) made from a porous material, for example gravel, to be placed below the groundwater table which serves as a groundwater storage tank collecting heat from the surrounding soil (4 & 8) by running the reservoir pump (7). The reservoir pump (7) is a pump placed inside the reservoir (5) that serves the purpose of removing cold “used” groundwater from the reservoir (5) and inject it to one or more injection sites (A & B) a distance from the reservoir (5), as cold groundwater seep through the surrounding soil (4 & 8) moving toward the reservoir (5) due to head difference in the water table it will be heated by the surrounding soil (4 & 8) and enter the reservoir (5) reheated, this loop continues as long as the reservoir pump (7) is running. Its characteristics also include the heat exchanger (6) placed inside the reservoir which is in series with the GSHP unit (3) and the circulation pump (1), absorbing heat from the “new” reheated groundwater in the reservoir (5). Also included in the characteristics of the reservoir solution is the regeneration mode for reheating the soil during off-season (summer), a necessity for all ground sourced heat solutions. This mode includes a vertical highway of porous material connected to the reservoir (5) and extend almost all the way toward the surface. This serves as a highway for warm surface-/rainwater to easily flow down into the reservoir (5). The mode also includes running the reservoir pump (7) to force the now warm groundwater from the reservoir (5) to the injection sites (A & B) for reheating of the surrounding soil (4 & 8) of the reservoir (5).

(Note, all characteristics in this summary have been written for heating mode meaning that the heat sink of the solution (consumer) is extracting heat from the reservoir. But the solution is also able to work without any modifications for cooling mode, injecting heat into the reservoir.)

(Also note, this patent is the result of the master thesis “Study and design of a new solution using a reservoir to extract heat from the subsoil for use in GSHP” by Andre N.K. Kansa, owner and inventor of this patent application. The thesis won’t be published before the patent application process is complete.)

Description of the drawing/figure with explanation of component role/purpose Figure 1 which is the only figure provided with this patent application shows the whole invention, the “reservoir solution”, and how it is to be connected to a GSHP unit as well as the circulation pump.

Following the numeration in the drawing we can explain each component and its purpose. Beginning with the circulation pump (1) which purpose it is to move the heat from the reservoir to the GSHP. It moves fluid (typically a water and antifreeze mixture) through the loop which will in turn absorb heat from the reservoir and inject it to the GSHP unit (heating mode). The evaporator (2) inside the GSHP unit (heating mode) which is the component that will absorb the heat from the circulating fluid. The GSHP unit (3) is located at the surface, depending on which GSHP it is it can be located outdoor or indoor. The soil between the injection site (A and/or B) and the reservoir (5) is drawn as a heat exchanger in figure 1, these heat-exchangers (4 and/or 8), is where the cooled groundwater from the reservoir will be heated as it moves/seep through the warm soil. The reservoir (5) is the “main” component of the invention. It is made from a porous material typically gravel and serves as a groundwater tank where groundwater can move “freely”. It is the reservoir tank that will “store” the heat from the surrounding soil and act as the heat source (heating mode). The heat exchanger (6), typically a HDPE pipe coil, placed inside the reservoir is the heat sink during heating mode. It is the heat exchanger that will absorb the heat from the reservoir. The reservoir pump (7) is also a key component in the reservoir setup invention. It moves the cooled water from the reservoir to injection sites which has been given symbols A and B in figure 1. The injection sites can be one, two, three, etc. and their distance can vary depending on heat load and space available. The reservoir pump is the component that makes the reservoir able to retrieve the heat from much larger quantity of soil that the walls of the component would be to do alone. Groundwater seepage coming in contact with a magnitude more soil than the walls are able to do directly.

Can identify two loops from the invention as seen in figure 1.

Circulation loop:

The circulation loop fluid moves between the reservoir (5) and the GSHP unit’s evaporator (2). In this loop we have the following components in series:

i. Circulation pump (1)

ii. GSHP unit’s evaporator (2) – for heating mode, cooling mode this will be the condenser.

iii. Reservoir heat exchanger (6)

Reservoir loop:

The reservoir loop moves cooled groundwater from the reservoir (5) to the injection sites (A and B) located a distance from the reservoir in the soil. By head difference in the water table, the groundwater moves/seep through the soil, absorbing the soils heat, toward the reservoir where the process will repeat.

i. Reservoir pump (7)

ii. Injection sites (A and B)

iii. Surrounding soil between reservoir and injection points (4 and 8)

Note that the drawing symbolic is made for heating mode, meaning heat is to be absorbed from the reservoir to the GSHP (heating the consumer).

Table 1 - Component list from drawing fig.1

Claims (1)

Patent demand

1. A new more cost-effective reservoir-based groundwater solution of extracting heat or cold from the subsoil for use in for either a ground sourced heat pump (GSHP) or for direct use in for example ventilation/refrigeration. Its characteristics include the use of a reservoir (5) made from a porous material, for example gravel, to be placed below the groundwater table which serves as a groundwater storage tank collecting heat from the surrounding soil (4 & 8) by running the reservoir pump (7). The reservoir pump (7) is a pump placed inside the reservoir (5) that serves the purpose of removing cold “used” groundwater from the reservoir (5) and inject it to one or more injection sites (A & B) a distance from the reservoir (5), as cold groundwater seep through the surrounding soil (4 & 8) moving toward the reservoir (5) due to head difference in the water table it will be heated by the surrounding soil (4 & 8) and enter the reservoir (5) reheated, this loop continues as long as the reservoir pump (7) is running. Its characteristics also include the heat exchanger (6) placed inside the reservoir which is in series with the GSHP unit (3) and the circulation pump (1), absorbing heat from the “new” reheated groundwater in the reservoir (5). Also included in the characteristics of the reservoir solution is the regeneration mode for reheating the soil during off-season (summer), a necessity for all ground sourced heat extraction/injection solutions. This mode includes a vertical highway of porous material connected to the reservoir (5) and extend almost all the way toward the surface. This serves as a highway for warm surface-/rainwater to easily flow down into the reservoir (5). The mode also includes running the reservoir pump (7) to force the now warmed groundwater from the reservoir (5) to the injection sites (A & B) for reheating of the surrounding soil (4 & 8) of the reservoir (5).