How much heat is available
The heat rejected at the condenser is roughly the cooling delivered plus the electrical energy the compressor uses. A plant delivering 10 kW of cooling with a compressor drawing 4 kW rejects around 14 kW of heat. That heat is available whenever the refrigeration is running, which for cold rooms and freezers is most of the day.
Not all of it is at a useful temperature. A small part, the superheat in the hot discharge gas, is at a high temperature. Most of it is released as the refrigerant condenses, at a temperature only somewhat above the room or outdoor air.
Common heat recovery methods
| Method | How it works | Suits |
|---|---|---|
| Desuperheater | Heat exchanger on the discharge line captures high-temperature superheat | Preheating hot water in hotels and kitchens |
| Full condensing recovery | Water-cooled condenser heats water at condensing temperature | Low-temperature water uses, preheating |
| Air heat recovery | Condenser air ducted to warm spaces | Cooler climates, warehouses, drying |
| CO2 system recovery | High discharge temperatures of CO2 heat water | Supermarkets and hotels with CO2 plant |
Preheating rather than heating
In most cases, recovered heat is used to preheat water that is then brought to its final temperature by the existing geyser, boiler or heat pump. Even raising the incoming water by a few tens of degrees cuts a large share of the fuel or electricity used for water heating. Kitchens and hotels that wash dishes, linen and floors throughout the day are the natural candidates.
Pitfalls to avoid
- Do not raise condensing pressure just to get hotter water; that makes the refrigeration less efficient and can cost more than it saves.
- Match timing: heat is available when refrigeration runs, so a storage tank is usually needed to bridge to when hot water is used.
- Plan for the case when the tank is already hot; the system must still be able to reject heat normally.
- Use double-walled or properly designed heat exchangers so refrigerant and potable water cannot mix.
- Follow hot-water hygiene good practice for storage temperatures and circulation.
What about small plug-in cabinets?
Self-contained cabinets release their condenser heat into the room. In an air-conditioned shop or café, that heat then has to be removed by the air conditioner, so every kWh the cabinets use counts twice. Recovering that heat is rarely practical for individual cabinets. Instead, reduce it at source with efficient cabinets, or, in larger stores, consider remote or water-cooled systems that move condenser heat outside the conditioned space or into useful work.
Is it worth it for your site?
- Do you run refrigeration of meaningful size for many hours a day, such as cold rooms or central plant?
- Do you use a lot of hot water at the same site, ideally spread across the day?
- Is there space for a storage tank and pipework near the refrigeration plant?
- Can your refrigeration contractor and plumber design it together?
- What does your current water heating cost per year?
Getting started
Begin by measuring: how many hours a day your refrigeration plant runs, its approximate capacity, and how much hot water you use and when. A competent refrigeration engineer can then estimate the recoverable heat and the effect on refrigeration efficiency. Heat recovery is easiest to add when a plant is being designed or replaced, so raise it at that stage rather than as an afterthought.
Frequently asked questions
- Can heat recovery replace my water heater?
- Usually not entirely. Recovered heat typically preheats water, and the existing heater tops it up to the final temperature. The saving comes from the heater doing much less work.
- Is heat recovery possible with a few plug-in cabinets?
- Rarely in a practical way. Plug-in cabinets are small and spread out. The better route is to reduce the heat they put into the room through efficient cabinets, or to plan a central or remote system with heat recovery when refitting a larger store.
- Does heat recovery make the refrigeration less efficient?
- A well-designed desuperheater can improve or barely affect refrigeration efficiency. Raising condensing temperature to get hotter water reduces efficiency, so design must balance both.
