REFRIGERATION AS AIR-SIDE HEAT RECOVERY
Posted by Duncan Giel on
Advancements in energy recovery technology have allowed for the drastic reduction in the amount of new heating energy required for ventilation air. Many forms of “passive” air-side energy recovery exist such as heat-pipes, enthalpy wheels, or air-to-air heat exchangers. While total effectiveness of these systems has increased to nearly 90%, their applications still remain limited by factors such as:
- Contamination in the exhaust air which can damage the heat exchanger
- Air leakage that can potentially recirculate hazardous air back into the supply air stream
- Frost control that drastically reduces efficiency
These concerns have left behind a significant potential for further energy savings. Additionally, growing concerns about fossil fuels have put pressure on everyone to reduce natural gas use for heating, yet the cost for electric heat in most regions remains unreasonable.
The solution to these problems is found in the refrigeration cycle. Although the system is driven by an electric compressor, the efficiency gain far outweighs the additional electrical costs. A typical refrigeration system can achieve between 5 and 10 (COP) units of heat energy per unit of electrical energy. This means the operating cost is lower even when the price of gas is 5 to 10 times that of electricity.
As Figure 1 shows, the compressor extracts heat from the evaporator (blue coil) in heating mode. The evaporator cools the air in the exhaust air stream and condenses moisture, allowing for both sensible and latent heat recovery. The compressor upgrades the heat to the required temperature and rejects it into the condenser (red coil). This achieves an “active” form of energy recovery that is able to meet the supply air temperature set-point without the need for additional sources of new energy.
With the design conditions shown, the outdoor air temperature is -4°F and the heating COP achieved is 5. What makes this heating system unique is the efficiency increases at part-load. This concept is similar to the “seasonal” efficiency achieved by conventional air-conditioners. When the outdoor air is 30 °F, the COP is nearly 8. By reducing the load, the suction pressure in the evaporator can be increased, reducing the lift on the compressor. Since the lift and load are reduced simultaneously, the situation also becomes ideal for using a variable speed compressor.
In the summer, the refrigeration cycle can be reversed such that it cools the supply air while rejecting heat to the return air. SInce the return air from the building is typically cooler than the design ambient temperature, the efficiency in cooling mode is also improved.
This type of system doesn’t come without challenges. The potential for the outdoor air temperature to fluctuate makes things difficult for the refrigeration system. In cold climates, frost control on the evaporator is also a concern. The successful implementation of this concept depends on many factors:
- Having a robust compressor that can unload to nearly any condition
- Using various devices to ensure proper operation throughout the refrigeration cycle
- Implementing controls that can adjust operation to actively maintain head pressure and suction pressure
Figure 2: FreshAIR, manufactured by Advance Industrial Refrigeration.
You can get detailed product information from the ventilation section of our website.