Fundamentals of solar-assisted ground source heat pumps
Temitayo Oketola | December 30, 2023As the world moves toward sustainable energy solutions, innovations in heating and cooling technologies are continuously improving. Among these, ground source heat pumps (GSHPs) have emerged as a compelling option, offering both eco-friendliness and operational efficiency.
Historically, GSHPs have been lauded for their ability to tap into the Earth’s underground temperatures, offering dual functionalities: heating in the winter and cooling in the summer. However, despite their merits, GSHPs face operational challenges. These challenges affect their overall performance and efficiency, particularly during extreme external temperatures.
Figure 1. Solar-assisted ground source heat pumps are helping to mitigate some of the limitations of conventional heat pumps. Source: costazzurra/Adobe Stock
To counteract these limitations, one innovative approach integrates solar energy into the GSHP, resulting in the solar-assisted ground source heat pump (SAGSHP). This hybrid system not only provides enhanced consistency in performance and potential cost savings but also makes significant strides toward a reduced carbon footprint.
Conventional ground source heat pumps and some of its challenges
The Earth’s temperature, several feet below its surface, is relatively constant throughout the year, irrespective of seasonal changes. A GSHP takes advantage of this consistency. It consists of a series of buried pipes (ground loops) filled with a heat transfer fluid (typically a mixture of water and antifreeze). During the winter, this fluid circulates through the loop, absorbing the heat from the ground and transferring this thermal energy to the building to meet the heating needs. Conversely, during summer, the system reverses by extracting the heat from the building and dissipating it into the ground, providing a cooling effect to the building.
While GSHPs are efficient, they can sometimes face challenges when the external temperatures are extreme or when there is a significant difference between the desired indoor temperature and ground temperature. For instance, when the external (or ambient temperature) is extremely cold during the winter, it could lead to a large temperature differential between the building’s indoor temperature and ground temperature. This means the GSHP has to work harder to elevate the ground-sourced heat to a level suitable for indoor heating. In contrast, during extremely hot weather, the GSHP has to work harder to dissipate the indoor heat into the already warm ground. In these situations, the GSHP’s performance can be compromised, leading to increased energy consumption and reduced performance.
A SAGSHP mitigates these challenges by harnessing the sun’s power.
Working principle of SAGSHPs
SAGSHPs integrate collectors (usually solar thermal panels) with the traditional GSHP system. On sunny days, these panels capture solar energy and convert it into heat. This thermal energy is primarily used for preheating the heat transfer fluid before it enters the ground loops, making the heat extraction process efficient. Moreover, the thermal energy can directly contribute to the heating needs of the building, reducing the load on the GSHP.
This dual mode of operation allows the system to operate at peak efficiency even during challenging periods when the ground temperature might not be ideal for heat extraction. Additionally, any excess solar energy generated during peak sunlight hours can be stored for use during periods of low sunlight, ensuring efficient and consistent operation around the clock.
[Learn more about solar energy technologies on GlobalSpec]
Some considerations for installing SAGSHPs
Some of the considerations for installing SAGSHP include:
- Site evaluation: Both geothermal and solar components require careful site evaluation. With SAGSHP systems, engineers not only ensure that the location has suitable ground temperatures for the heat pump loads, but they also ensure the location has adequate sun exposure for the solar panels.
- Initial costs: As expected, the upfront cost of a SAGSHP system is higher than installing conventional GSHP systems. However, the return on investment through reduced energy bills can be realized within a few years.
- Maintenance: Although solar panels and GSHPs have low maintenance needs, the integration of these two technologies requires navigating the intricacies of both. As such, regular checks on solar panels, heat pump systems and inverters will ensure smooth and reliable operation.
Conclusion
The integration of solar energy into conventional GSHPs can help to achieve a reliable, efficient and consistent operation of heat pump systems. However, given the intricacies associated with merging two technologies, there is a potential for challenges in terms of system design, control and optimization. As such, it is recommended to reach out to heat pump suppliers to discuss application requirements.
The article discusses gaining heat during the winter, when the heat pump is in a mode of extracting heat from the ground. I can see this working, although there is a need for additional controls, protection from freeze damage in the collectors (such as with a suitable fluid). This all increases the maintenance costs.
The article does not discuss summer cooling, when the collectors are gaining heat which is unwanted in the ground loop. I suggest that it is possible for the collectors to be used to radiate unwanted heat into the night-time sky. I have read about this as a separate application, although the practical amount of cooling is going to be less than the claimed ideals.
The cost/payback analysis for a proposed system will be interesting to see, since it would include actual data about seasonal ground temperature variations, insolation, operational energy costs, etc.
--JMM
Can't quite see the point of pre-heating the fluid before it is pumped underground to extract heat from the ground. or have I mis-read.
In reply to #2
It takes more work to pump thermal energy out of very cold water than merely cool water. One possible arrangement is to use the solar collectors to pre-heat the fluid going directly to the heat pump when needed, but if the collectors provide more than enough heat in the fall and early winter seasons, divert the excess into heating the groundwater and aquifer material. In the late winter and spring, stop storing heat because it is desirable to chill the water for the air conditioning season. Mid-spring would be the time to use the panels at night to begin radiating heat out of the groundwater as well as providing heat for the building in the daytime.