Combining geothermal with hydronic systems
Leveraging existing hydronic and geothermal technology allows installers and homeowners alike to enjoy more eco-friendly, energy-efficient solutions when it comes to indoor pool design, and reap the benefits of more comfortable and better controlled living environments.
The loops (geothermal ground source heat exchangers) carry water or an environmentally friendly antifreeze (hydronics) into the indoor heat pump system to transfer the earth’s natural heating and cooling properties. In the winter, the loops act as a source for heat; in the summer they serve as a place to reject heat.
A geothermal loop is an ideal way to reject excess heat. According to the second law of thermodynamics, heat transfer always occurs from a higher-temperature region to a cooler-temperature region; therefore, heat exchange efficiency is a function of the difference in temperature between the two mediums (i.e. the movement of heat from one place to another). From an efficiency standpoint, rejecting heat to a geothermal system buried in the ground, where the average temperature is normally 10 C (50 F), will be more effective than an outdoor air-cooled condenser with a big blower that is rejecting heat to the hot summer air.
Using the process heat pump, as noted above, leverages the flexibility of energy-efficient hydronic systems and heat pump technology in indoor pool installations, which can also be implemented in a chill pool system.
A chill pool is a concept that is enjoying a revival as an addition to a home spa and/or pool environment. They fit perfectly into an energy-efficient design when used as a source to withdraw heat for other processes. By using the chilling process as a heat source for other systems, such as pool/spa water, energy is again transferred, resulting in potentially significant savings.
Using a geothermal loop system in tandem with a pool dehumidification system offers considerable options on the heating side as well. Using a heat pump in conjunction with the ground loop generates returns on the loop investment and delivers added value all year round. A properly designed and installed geothermal heating and cooling system can completely replace other heating alternatives, as it can efficiently generate heat up to 43 to 46 C (110 to 115 F).
Geothermal and hydronic system design parameters
The two design factors critical to maximizing energy efficiency are load temperature and source temperature.
The load temperature is the temperature required in the distribution system for services such as floor warming, space or pool water heating. The load supply water temperature (LSWT) and load leaving water temperature (LLWT) are measured as the load liquid enters and leaves the heat pump on the load side.
The source temperature is the temperature of the liquid flowing through the ground source side. The source entry water temperature (SEWT) and source leaving water temperature (SLWT) are measured as they enter and leave the heat pump on the source or groundside.
The amount of energy delivered can be calculated by considering the load and source temperature differences, together with the flow of the liquids measured in litres/gallons per minute (lpm/gpm).
In heating mode, the coefficient of performance (COP) is highest when the geothermal system has to generate a low LSWT, and the SEWT from the ground loop is high. For example, if a floor-warming system requires 26 C (80 F) LSWT to heat a pool deck and the liquid from the ground is 10 C (50 F) SEWT, this heat pump system will out perform a unit requiring 49 C (120 F) LSWT for a low cubic feet per minute (cfm) fan coil when the liquid from the ground is –1 C (30 F) SEWT.
In fact, the most efficient way to heat an occupied space is by floor warming utilizing a well-designed geothermal heating source because of the low approach temperatures and large surface area used to deliver heating energy to where it is needed most.
|HYDRONIC AND GEOTHERMAL CONCEPTS IN ACTION|
|The following factors were considered to provide an elegant ‘green’ solution for a residential indoor swimming pool installation in Halton Hills, Ont.:
• Converting the residential heating system from propane to geothermal heating/cooling;
• A closed-circuit indoor swimming pool dehumidifier and pool water heater;
• A heat recovery ventilator designed for indoor swimming pools;
• A geothermal heat pump, to provide water heating/cooling;
• A process heat pump, to provide simultaneous water heating/cooling;
• A high-efficiency modulating heating boiler, to provide back-up heating;
• Buffer tanks for heating and cooling water;
• An indirect hot water tank for domestic use in the indoor pool addition; and
• Distribution systems designed to utilize low-grade (low load supply water temperature [LSWT]) and ‘free’ heating sources, such as pool room floor warming, under-pool water heating, large, air heating/cooling coils for the pool room, a domestic hot water preheat tank for the residence and a hot water (excess energy recovery) preheat tank.The integrated project design used the concepts described herein to minimize energy use in an environment that included high humidity and extreme solar loads, large exterior glass surfaces and a variety of heating and cooling demands.
For more on this project, see article on page 6.
Steve Hamoen is the founder and acting president of Zonelife Inc., a contracting and design firm specializing in custom hydronic and geothermal systems for residential and commercial applications in Cambridge, Ont. Hamoen can be reached through his blog at www.geothermalinstallers.ca, or by e-mail at email@example.com.