Waste Heat Recovery from Refrigeration Systems
- Dr S S VERMA, Department of Physics, SLIET, Sangrur, Punjab

Energy conservation is a major focus point towards sustainable development and waste heat recovery is a very useful energy saving technique when a major part of energy is being wasted as heat in maximum household and industrial energy consuming units. Refrigeration has become a way of life in modern lifestyle and from small scale household units to industrial scale units, there is a large scope of waste heat recovery potential. To meet the demand of sustainable development with energy conservation and less environmental pollution there is a need to develop new technologies to recover and utilise waste heat from refrigeration systems (small or large). This article briefly highlight the need of waste heat recovery, potential of waste heat recovery in refrigeration units and introduce the latest technological developments towards waste heat recovery and utilisation from refrigeration systems.

Waste Heat from Refrigeration
Refrigeration from small scale household units to industrial scale units has a large scope of waste heat and scientists and engineers, to meet the demand of sustainable development with energy conservation and less environmental pollution, are developing new technologies to recover and utilise waste heat from refrigeration systems (small or large). The process of refrigeration has traditionally involved a substantial use of electric energy. Energy recovery has become incredibly important in all aspects of the commercial refrigeration industry in recent years. Supermarkets in particular comprise a significant proportion of energy use as well as heat waste disposal. Supermarkets, ice rinks, and warehouse food storage facilities have each drawn the attention of concerned consumers and environmental protection agencies. Many companies have addressed these concerns with the state of art development of their energy recovery systems for refrigeration.

Waste Heat Recovery
Energy conservation is a major focus point towards sustainable development and waste heat recovery is a very useful energy saving technique where the heat that is removed by the refrigeration systems or other processes can be utilised for various applications as opposed to this heat just being wasted, as is normally the case. Considerable energy savings can be achieved by re-using heat from the refrigeration plant or any other process present on a site. Depending on the specific temperature requirements of other cooling and heating loads throughout the facility, recaptured heat can be re-used directly, boosted through a heat pump to a higher temperature, used in an absorption chiller to satisfy another cooling requirement or stored for later use. Through extensive expertise with refrigeration and other thermal systems, one can identify and assess the viability of a waste heat recovery system, design a practical and sustainable solution and manage the project through the implementation. Energy recovery is accomplished when traditionally discarded heat is rerouted for other uses in the production process. Energy recovery ventilation systems are widely popular among owners and operators of existing refrigeration equipment. Virtually any existing equipment can be modified or adapted to accommodate green standards of energy recovery. Energy recovery ventilation systems are extremely adaptable, and can be customised to suit nearly any existing equipment. These energy recovery systems are extremely useful and efficient for reducing the hazardous impacts of heat waste upon the environment. They also preserve the use of expelled energy and conserve by efficiently distributing derived resources with those that are implemented.

Although compressor-driven refrigeration is far more dominant in today’s market, absorption chillers are slowly gaining more attraction in a variety of applications. Using heat instead of electricity to drive the cooling system, the absorption refrigeration cycle provides a way to make use of waste heat when another cooling load is present. Having much less moving parts then conventional compressor-driven chillers, an absorption chiller can be seen as a static system that runs noise free and has low maintenance and service costs. Absorption chillers operate on two fluids, an absorbent and a refrigerant. Commercially available absorption chillers either run on the LiBr-Water or Water-Ammonia working fluid pair. Each project needs to be specifically assessed to determine project feasibility. Further, introducing the use of compound refrigeration systems with integrated heat pump function has further improved its energy efficiency. The compound refrigeration systems use the entire waste heat from a commercial refrigeration installation to heat the store in winter. Up to an outside temperature of zero degrees Celsius no additional room heating is required. The compound refrigeration system is fitted with two additional compressors to ensure that this principle even functions on cold winter days when outside temperatures plunge below zero. These compressors work in winter as heat pumps and in summer function as refrigeration systems.

Technological Developments
The heat generated from a compressor’s activity during refrigeration process has long been emitted into the atmosphere. A simple change in technology, however, can make these compressors more powerful and efficient, and therefore able to divert that once-wasted heat for additional uses. Refrigeration technology really had not changed much in 150 years, with the result that no process had existed to convert heat released by compressors into other heating or cooling uses. Most refrigeration compressors use a traditional male-female rotor alignment to compress gas. While this ensures a smooth operation and seamless processing of gases, there is a limit as to how much pressure such a system can handle. Refrigerants such as ammonia are one option for companies to consider, both for its efficiency and the fact it does not contribute to ozone depletion or greenhouse gas emissions. But, for demanding industrial heating and cooling applications, plant managers had shunned ammonia because no compression pumps could handle this refrigerant at high pressure levels. The solution was a single screw compressor system. The mechanism looks simple: it is simply one main rotor that intermeshes with two gate rotors on both sides. The compressor’s design actually allowed gases to be processed at very high pressures – and higher differential pressure ranges – while offering low life-cycle costs. Because of the single screw’s compressor-balanced design, a factory also benefits from lower maintenance and operational costs because of their decreased strain on the compressor’s bearings.

Energy recovery is one of the most prominent concerns for consumers and corporations, particularly in the commercial refrigeration industry. Energy recovery equipment has been made available by several companies in recent decades. Industry leaders have accomplished with single units what companies earlier have tried to revolutionise in years past. Energy recovery is accomplished in two major ways. Heat is absorbed during the process of cooling, and released at high pressures through stainless steel pipes. This effectually reduces the impacts of harmful waste, and utilised the absorbed heat as a form of energy. Energy derived from the absorption process can be derived to other components as heat recovery. Energy recovery is accomplished, and energy recovery units are efficient in reducing cost and harmful output. This allows the entire facility to function on far less energy than traditional with systems. Energy efficiency is a key issue for every facility where new refrigeration equipment is being installed. Recent developments in heat recovery systems offer the potential for reducing energy bills. Energy recovery units in refrigeration require the knowledge of experts from the initial model to the implementation process. Maintenance, modification and adaption procedures for virtually any part of the energy recovery process are offered by the innovative engineers. The model with the latest technology and system management has brought a new frontier to the refrigeration industry, particularly for commercial endeavors. Supermarkets, ice rinks, and storage warehouses are now extended accessibility to the benefits of energy recovery units for commercial refrigeration. Energy recovery ventilation systems bring a new aspect of environmentally friendly processing to the refrigeration industry.

GEA Refrigeration Netherlands N.V. won the Dutch Refrigeration Prize last month for technology that combines a refrigerating plant and a heat pump. The Energy Enhancer, the name given by GEA to this refrigeration-heat solution, convinced the contest jury of its energy-savings potential. The Energy Enhancer raises exhausted heat — which is often dissipated into the environment without being used in conventional refrigeration units — to a higher temperature, where it becomes useful energy. This can eliminate the need to produce heat by gas-fired, hot-water heaters or steam generators. The solution uses an ammonia heat pump to raise the temperature of the heat exhausted from refrigeration compressors (originally emitted at about 350C) to a level of 800C, which can be effectively used as a heat source for processes throughout the plant. This enables efficient use of heat in, for example, milk pasteurisation, French fries production (blanching) and meat processing (cleaning machines). The heat can also be used anywhere else it’s needed, such as in heating the plant’s environment. This enables not only significant reduction in operating costs, but also leads to a large reduction in CO2 emissions.

Waste Heat Recovery System for Domestic Refrigerator
An attempt has been made to utilise waste heat from condenser of refrigerator. This heat can be used for number of domestic and industrial purposes. The study has shown that such a system is technically feasible and economically viable. Refrigerator which is been previously made exert a lot of amount heat through condenser to overcome this wastage of heat. This heat is utilised for heating water. A standard topology shows the unit connected to an existing hot water tank (pump, controls & special fittings are not shown). The unit uses the recovered heat to keep the water temperature above the tank’s thermostat set-point so that the tank heating element or burner does not need to operate. Hot refrigerant gas from the compressor enters and flows in the opposite direction as the water flow. The water picks up the heat from the refrigerant gas, cooling the gas and heating the water. The excess heat that previously would have been thrown away by the condenser is recycled. The remote unit mounts outdoors (or indoors if desired) and will usually be installed on an exterior wall somewhere near the compressor. There are two pairs of inlet/outlet connections on the unit. One inlet/outlet pair is for the compressor side and the other pair is for the hot water side. Inside the unit there is a small pump, sensor controls and a double helix all-copper vented double-wall heat exchanger that transfers the heat from the refrigerant (compressor side) to the hot water side. The system connects to the compressor through standard refrigeration lines, and connects to the hot water tank through standard insulated plumbing pipe, PEX etc. The pump circulates water from the tank, through the heat exchanger, and then back to the tank. The heat exchanger efficiently transfers the compressors high temperature waste heat to the water circuit of the unit. Dual-circuit units operate the same way, except they can connect to two adjacent compressors at the same time while connected to a single hot water tank.

Waste Heat Recovery with Industrial Heat Pumps
Industrial refrigeration systems reject a significant quantity of waste heat to the atmosphere. Heat pumps can capture this waste heat efficiently and use it to reduce the fossil fuels consumed to heat water. Industrial heat pumps are environmentally friendly and economical, allowing end-users to make the most of their energy resources. Industrial ammonia heat pumps offer end-users a comprehensive sustainable solution by reducing energy consumption, water, waste water, CO2 emissions and operating costs. They are environmentally friendly as ammonia is a natural refrigerant with an Ozone Depletion Potential (ODP) of zero and a global Warming Potential (GWP) of zero. Industrial heat pumps provide energy conservation, by converting the heat energy removed by ammonia refrigeration systems and transforming it into beneficial heat for use in satisfying plant hot water requirements. This source of energy is renewable as the heat is naturally occurring within food products and is made available through food preservation by the process of refrigeration, a reusable energy source. Ammonia heat pumps reduce operating costs. Ammonia refrigeration systems absorb heat from products, processes, equipment, people, building heat gains and infiltration, and then reject it as waste heat to the atmosphere through, typically, evaporative condensers. The majority of the heat delivered by heat pumps comes from the heat extracted from these cooling loads. An incremental amount of heat delivered by heat pumps comes from the electric energy consumed in converting the low grade waste heat into high temperature useable heat. An additional benefit of industrial heat pumps, applied as retrofits to existing systems, is that they add condensing capacity to systems. Industrial heat pumps divert load away from evaporative condensers, allowing existing system compressors to operate more efficiently at lower condensing pressures.

Conclusion
Heat recovery is the collection and re-use of heat arising from any process that would otherwise be lost. The process might be inherent to a building, such as space heating, ventilation and so on, or could be something carried out as part of business activity, such as the use of ovens, furnaces and the like. Heat recovery can help to reduce the overall energy consumption of the process itself, or provide useful heat for other purposes. Waste heat recovery has emerged as an effective way of increasing energy efficiency at factories where refrigeration is crucial. Any energy savings is important because even though commonly used refrigerants such as hydrofluorocarbons (HFCs) do not deplete the ozone level as chlorofluorocarbons (CFCs) did a generation ago, HFCs still pose GWP.