Two thermally-based renewable energy technologies that grab many headlines these days are solar water heating and geothermal heat pumps. Both of these technologies are ways of capturing solar energy. The solar thermal system does it in real time, whereas the geothermal heat pump system, operating from a horizontal earth loop, extracts solar heat driven into the soil several months earlier. This time shift between when the energy is available and when it is needed, allows combinations of solar thermal hardware and geothermal heat pumps to be complementary.
Take a look at the system schematic in Figure 1. It combines both solar thermal and geothermal heat pump subsystems into an overall system for space heating, space cooling and domestic water heating.
The primary heat source for space heating is the water-to-water geothermal heat pump. During the heating season, it extracts low temperature heat from the earth loop, converts it to higher temperature heat and parks that heat in a well-insulated buffer tank.
When the heat pump is gathering heat from a horizontal earth loop, the fluid in the earth loop is at a relatively low temperature, especially during mid to late winter. In a northern climate, this fluid may even be less than 32F at times. In such systems, the earth loop fluid is typically a 15 to 20 per cent solution of propylene glycol or other antifreeze.
A low temperature distribution system delivers that heat when and where it is needed. A variable-speed, pressure-regulated circulator operates in response to the differential pressure across the headers. When a zone valve opens, the differential pressure across the headers attempts to decrease. The circulator senses this electronically and immediately increases its speed to restore the original (design) differential pressure.
The heat pump responds only to the temperature of the buffer tank, as monitored by an outdoor reset controller. The role of the heat pump, based on this control scenario, is to keep the buffer tank temperature within a certain range of a target temperature whenever space heating may be required. The latter is calculated based on the settings of the reset controller and the current outdoor temperature. This approach minimizes the temperature of the buffer tank based on the prevailing conditions. In doing so, it improves both the heating capacity and the heat pump coefficient of performance (COP). Figure 2 shows an example of how the reset line of such a controller could be set for a low temperature distribution system.
A similar operating mode is used for chilled water cooling. The heat pump chills the buffer tank and dissipates the absorbed heat to the earth loop. Chilled water flow is controlled by a second variable speed, pressure-regulated circulator in response to zone valves on each chilled water air handler. The temperature of the buffer tank is now likely to be controlled by a setpoint device that keeps the water in the range of 45F to 60F whenever the cooling mode is active.
This configuration works well provided that the building does not require heating and cooling within a short time of each other. It is obviously not very efficient to heat the buffer tank to supply heat in the morning and then chill it down to supply cooling that afternoon.
There are climates where heating is required in the morning, followed by a need for cooling in the afternoon. In such cases, one operating mode has to take precedence during swing seasons, or unusual weather conditions, until the system settles into a stable mode for the duration of the season. Another more costly and complex solution is to design the system with two buffer tanks – one for heated water and the other for chilled water.
The solar subsystem shown in Figure 1 allows for two operating modes:
A. The solar heat collected is delivered to the DHW storage tank through the tank’s internal coiled heat exchanger.
B. The solar heat is delivered to the earth loop through the brazed plate heat exchanger.
If the sun is out and the domestic water temperature is lower than some limit, say 140F, the diverter valve routes the fluid leaving the collector through the tank’s internal coiled heat exchanger.
If the tank reaches the upper limit and the sun is still shining, the diverter valve would reroute flow from the collector array to a brazed plate heat exchanger in the earth loop. Connecting the collectors to the earth loop forces them to operate at a relatively low temperature, perhaps just a few degrees above that of the earth loop fluid. Under this operating mode, the collector array is partially unloading the earth loop. Over time this will keep the soil around the earth loop slightly warmer than it would otherwise be without the solar assist. This operating mode is particularly appealing in late winter and early spring; when the fluid temperature supplied by horizontal earth loops is bottoming out and the solar gains are getting stronger.
The lower the operating temperature of the collectors, the higher their thermal efficiency. For example: A typical flat plate collector operating with an entering fluid temperature of 40F, at a time when the solar radiation intensity is 250 Btuh/ft2 and outside temperature is 30F, has a thermal efficiency of about 67 per cent. If the inlet temperature to this collector was raised to 120F under the same ambient conditions, its thermal efficiency would only be about 37 per cent. This implies that the collector operating at the lower temperature is gathering about 80 per cent more heat than the collector operating at the elevated temperature.
Because of this, you may be thinking; If the efficiency is so much higher, why not just connect the collector array to the earth loop and forget about operating it at a higher temperature for domestic hot water?” The answer is based on two considerations. First, if “auxiliary” water heating is provided by an electric element or tankless electric heater, the cost of that heat may by three or four times greater than the cost of heat produced by the heat pump (assuming the latter has an average COP of three or four). Thus, displacing heat produced by the electric heating element will always be more cost effective and should be the priority mode. Secondly, if the collector array is only connected to the earth loop, it serves no purpose during warmer weather when the heat pump is operating in cooling mode. In this mode, the earth loop should remain as cool as possible. During this time there is plenty of solar energy available to heat water, but no way to collect it.
When the heat pump is operating in cooling mode, the diverter valve directs the hot antifreeze solution returning from the collector array through the heat exchanger in the solar storage tank. In this mode, the solar subsystem is effectively isolated from the heat pump system.
The foremost Achilles heel of closed-loop, antifreeze-based solar thermal systems is what to do with excess solar heat in summer. Simply turning off the collector circulator if the storage tank reaches a high limit can cause rapid degradation of glycol-based antifreeze fluids within the collector. It can also cause steam flash in the collector array and the opening of the collector circuit pressure relief valve.
The system shown in Figure 1 allows the option of dumping excess solar heat gain to the earth loop. The
diverter valve directs fluid from the collector array through the brazed plate heat exchanger in the earth loop, while the earth loop circulator operates.
In heating-only systems, or systems with minimal cooling load, this heat dump mode is easy to implement. The possibility of overheating the earth loop due to occasional heat dumping is certainly less in northern climates, where earth loop temperatures, even during late summer, are in the range of 65F to 80F. However, this mode may or may not be viable in locations with long/hot summers and significant cooling loads. I see it as viable provided that the temperature of the earth loop fluid entering the heat pump does not rise above a point where heat pump cooling performance is significantly reduced. This could be detected by a setpoint controller, with the subsequent action of invoking another means of heat dumping. This also assumes that heat dumping is an occasional occurrence, rather than something that occurs every sunny summer day.
MORE TO COME
It is possible to combine solar subsystems and geothermal heat pumps in other ways. One uses the solar array to add heat to the same storage tank that is otherwise heated by the ground source heat pump. A coil heat exchanger suspended in the upper portion of this tank, or a brazed plate heat exchanger outside the tank, serves to preheat domestic water. I had such a system in for several years and I would likely still have it, had we not added building space that required more capacity than the heat pump could deliver.
I expect more research will be undertaken on how to best combine arrays of solar thermal collectors with geothermal heat pumps. The optimal configuration must address the relative size and timing of the space heating, space cooling and DHW loads. Look for this combination of subsystems to remain a popular topic in the future. <>
John Siegenthaler, P.E. is the author of Modern Hydronic Heating (the third edition of this book is now available). For reference information and software to assist in hydronic system design visit www.hydronicpros.com.