How to Heat your Pool for free with the Heat Rejected by Air Conditioning Units or Geothermal Heat Pumps?
How to Heat your Pool for free with the Heat Rejected by Air Conditioning Units or Geothermal Heat Pumps?
Cooling in the summer of indoor spaces is exhausting heat to the outdoor or to the ground. This blog shows how to recycle exhausted heat and use it to heat swimming pool.
1.0 - Introduction
Cooling in the summer of indoor spaces is heat extraction. Heat is captured by the evaporator of an AC unit, pumped by its compressor toward the condenser where it's being rejected to the environment.
In the case of Air Cooled Condensers, Refrigerant Pressure is increased by the AC unit's compressor in order to increase refrigerant temperature above the outdoor dry bulb temperature and Heat is evacuated to the outdoor air by blowing air through the condenser.
When water cooled condensers are used, Refrigerant Pressure is increased by the AC unit's compressor (same as for air cooled ones), however heat is rejected by pumping either water or water and glycol mixture through a refrigerant to water heat exchanger. Pumped fluid is either sent to a cooling tower, dry cooler or to a ground loop heat exchanger.
In either cases, heat can be recovered and used to heat swimming pool's water. In this blog we will address two examples of heat reclaim: Air cooled air conditioning units and Liquid Source Heat Pumps (Geothermal Heat Pumps).
Heat reclaim is best described as the process of reclaiming heat that would normally be rejected by an outdoor air cooled condenser or water cololed condenser. Typically, the refrigerant is diverted to an Air Handler or a Fan Coil Unit in an area that requires cooling. There are many applications of heat reclaim in locations with a constant supply of heat removed from either space cooling or process cooling. Today there are many cost-effective processes of heat reclaim in refrigeration, air conditioning, dehumidification, and heat pump systems.
This blog will only focus on space cooling applications in single occupancy residential buildings.
2.0 - Air Cooled Air Conditioning Units
When standing in front of an operating Air Conditioning unit in the summer, we all feel the blown up hot air leaving its condenser. If we put our hand at the suction of its condenser's fan, we feel that succed air is much colder than blown air. The temperature difference between suction and supply sides of condenser's fan, represent the dumping of the extracted heat from the indoor space to the outdoor.
The above rejected heat can be reclaimed. Today more and more contractors accross the US and Canada are buying our Refrigerant to Water Heat exchangers for “heat reclaim” as a mean of providing a supplementary or even a primary heat source for their swimming pools. Heat reclaim is dumping the heat exracted from an indoor space (called heat source ) to a swimming pool (called heat sink) . This Process can significantly lower energy costs.
Essentially any application that requires hydronic heat (such as Swimming Pools) can recover the heat from a refrigeration or air conditioning system via the usage of Refrigerant to Water Heat Exchanger on the discharge line of the compressor.
2.1 - Application
Adding a Refrigerant to Water Heat Exchanger (made of Titanium), at the compressor's refrigerant discharge, to an existing AC units allows dumping of the evacuated heat from an indoor space to a swimming pool.
The figure on the right shows a typical piping schematics for the addition of another water cooled condenser for heat reclaim (A refrigerant to water shell and tube heat exchanger) combined with a refrigerant three way valve and a controller for switching between the two condensers (Existing air cooled condenser and the newly added water cooled one).
The parallel arrangement of the two condensers requires that reclaim condenser be sized to handle 100% of the rejected heat at the conditions and time at which the reclaim coil is being utilized. According to the second law of thermodynamics, heat rejected to the condensers is the sum of the heat extracted by the evaporator and the heat added by the compressor.
When the idle condenser is pumped down to suction pressure, it's recommended to add a small solenoid valve in order to pressurize the idle condenser prior to the 3-way valve shifting. This may reduce the potential for stress and fatigue failure of the refrigerant piping.
We recommend using a 3-Way Heat Reclaim Valves with 3-way pilot valves equiped with a “bleed” port.. The bleed port allows the refrigerant to be removed from the heat reclaim coil or heat exchanger when it is not being used.
There are two reasons why the refrigerant is removed from the heat reclaim coil:
- To maintain a proper balance of refrigerant in the system (refrigerant left in the reclaim coil could result in the remainder of the system operating short of charge).
- To eliminate the potential of having condensed refrigerant in an idle coil. When an idle reclaim coil has condensed or even subcooled liquid refrigerant sitting in the tubes there is a potential for a problem. When refrigerant liquid, either saturated or subcooled, is mixed with hot gas refrigerant, the reaction of the mixing can cause severe liquid hammer. Hot gas mixed with liquid can create thousands of pounds of force and has the potential of breaking refrigerant lines and valves.
NB A check valve should be installed in the heat reclaim pump out or bleed line whenever the reclaim heat exchanger is exposed to temperatures lower than the saturated suction temperature of the system (AC units equiped with Low Ambient Kits and running in cooling mode in Canadian Winter). This will prevent migration of refrigerant to the coldest location in the system.
3.0 - Geothermal Heat Pumps
Heat Reclaim with Geothermal Heat Pumps is less intrusive than in Air Cooled AC units, since it can be done at the outlet of the geothermal heat pump condenser. A Water to Water Tiatnium Heat exchanger can be installed at the gournd loop supply header.
In this type of applications, it's crucial to properly select the water/water titanium heat exchanger. Geothermal Heat reclaim is a low temperature hot water process that requires a larger heat exchange area. Rated capacities of this type of heat exchangers are calculated for 60°C (108°F) temperature difference between heat source and heat sink.
Heat transfer fluid leaves the condenser of Ground loop geothermal heat pumps at 95-120°F in the summer (Heat Source) while the temperature of water coming from the pool and entering the titanium heat excahnger (Heat Sink) ranges between 78-82°F. Actual temperature difference between heat source and heat sink is between 13-42°F (less than 60°C (108°F)). To compensate for the lower temperature difference, a larger heat exchange area is needed to transfer heat from the heat source to the heat sink.
4.0 - Conclusion
Adding Heat Reclaim Water Cooled heat exchanger are better done by a qualified professional espacially when adding the heat reclaim on an existing AC unit. Always check with AC unit's manufacturer since adding three way valve and a second water cooled condenser might affect equipement's warranty.