Comparative Experimental Evaluation of Two Heat Recovery Modes under Steady-State and Transient Conditions in Packaged Air-Conditioning Units for Domestic Hot Water Production

Air conditioning units are the unsung heroes of modern comfort, working tirelessly to cool our homes and offices by moving heat from inside to the outside. However, this process is inherently wasteful; a significant portion of the energy used to run these machines ends up as hot gas that is simply dumped into the atmosphere. This rejected heat represents a lost opportunity. Instead of letting that thermal energy vanish, engineers have long explored ways to capture it and put it to work, such as using it to warm water for showers or sinks. The challenge lies in doing this without making the air conditioner itself less efficient or requiring complex, expensive modifications. The question is not just whether we can steal this waste heat, but how to do it in a way that actually helps the machine run better while providing a useful service.

A team of researchers from universities in Iran and China set out to answer this question by testing a specific, practical approach on a standard packaged air-conditioning unit. These are the self-contained systems often seen on rooftops or in commercial buildings, which are widely used but rarely studied for their potential to produce hot water. The researchers installed a compact heat exchanger—a device made of copper tubes inside a metal shell—directly into the path of the hot refrigerant gas leaving the compressor. This device acts as a bridge, allowing the scorching gas to transfer some of its heat to water flowing through the surrounding shell before the gas reaches the main outdoor condenser. They then put this modified system through its paces in two distinct ways to see how it performed under different real-world scenarios.

In the first scenario, the team simulated a steady flow of cold city water passing through the heat exchanger once, warming it up slightly before it went to a main water heater. This mimics a situation where the air conditioner is constantly running and the building needs a steady stream of pre-warmed water. They found that as they increased the speed of the water flowing through the device, the air conditioner actually became more efficient. The heat exchanger acted like a relief valve for the compressor; by cooling the hot gas early, it lowered the pressure the compressor had to work against. This reduction in pressure meant the compressor used less electricity to do the same amount of cooling. Specifically, when the water flow was high, the compressor's power consumption dropped by as much as 21 percent, and the overall efficiency of the system more than doubled because the recovered heat was counted as a useful output alongside the cooling.

The second scenario tested a different, more dynamic approach: heating a tank of water from scratch until it reached a usable temperature for domestic use. In this setup, the water was not flowing through once; instead, it was pumped in a closed loop, circling back and forth through the heat exchanger to gradually absorb heat. The researchers watched this process unfold over time, noting how the water temperature climbed from a cool 21 degrees Celsius to a hot 50 degrees Celsius within 120 minutes, with temperatures close to 60 degrees Celsius achieved after approximately 220 minutes of operation. Interestingly, during this heating phase, the air conditioner's compressor did not change its behavior much. The pressure and temperature of the gas leaving the compressor remained largely the same as they would have been without the heat exchanger. This happened because, as the water in the tank got hotter, it became harder for the heat exchanger to pull more heat out of the refrigerant, so the main outdoor condenser continued to do the heavy lifting. Despite this, the system still managed to heat the water effectively while maintaining its cooling performance, with the overall efficiency improving by more than 31 percent when the recovered heat was included in the calculation.

The study concludes that capturing waste heat from the compressor discharge line is a viable and effective strategy for packaged air-conditioning units. It offers a dual benefit: the system can produce hot water for daily use while simultaneously reducing the energy required to cool the building, or at least maintaining that cooling performance with less strain on the compressor. The researchers found that the best results for energy savings came when the system was used to preheat a steady stream of water, but the closed-loop method proved capable of heating water to high temperatures without disrupting the cooling cycle. These findings suggest that a relatively simple addition to existing air-conditioning hardware can turn a waste product into a valuable resource, making these common machines more sustainable and efficient without needing a complete redesign. Future work will need to see how these systems perform over long periods in real buildings, but the laboratory results provide a clear path forward for improving how we use energy in our built environment.