Waste Heat Recovery from Refrigeration and AC Systems

High grade heat comes from ‘DE-superheating’ the refrigerant between the compressor and the condenser. This heat can be between 60ºC and 90ºC. A heat exchanger (the DE-superheater) is installed, with the refrigerant on one side and the fluid to be heated on the other. Not only does this DE-superheat the refrigerant, it reduces the cooling water or air needed by the condenser.

Heat Recovery is a very useful energy saving technique where the heat that is removed by the refrigeration systems or other processes can be utilized 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 your refrigeration plant or any other process present on site.

Depending on the specific temperature requirements of other cooling and heating loads throughout your 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.

Air conditioning / refrigeration systems are designed to remove heat from interior spaces or products and reject it to the ambient (outside) air. Heat rejection may occur directly to the air, as in the case of most conventional air source units, or to water circulating from a cooling tower. The circulating water eventually rejects the heat to the ambient air in the cooling tower. While this heat is of a "low grade variety," it still represents wasted energy. From an energy conservation standpoint, it would be desirable to reclaim this heat in a usable form. The best and most obvious form of heat recovery is for heating water.

Cooling generates considerable quantities of heat. If not utilized, this energy simply becomes waste heat. Siemens has developed modulating valves to control the direct utilization of waste heat. They provide for exact, demand-controlled heat recovery. The utilization of waste heat is profitable wherever heating and refrigeration are required at the same time, or where waste heat can be stored:

• In air conditioning systems to reheat dehumidified air
• In industrial processes
• In butcheries, dairies, hotels, etc., where, on the one hand, cold storage rooms are operated and where, on the other, there is always a great demand for domestic hot water
• In cold storage facilities, for heating and domestic hot water
• In shops, where in addition to cooling foodstuff, heat demand also occurs, e.g. mall heating

Energy recovery is accomplished in two major ways with the innovative Carnot Refrigeration system. 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 utilized 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. 

Synthetic chemical agents are replaced with less toxic and more natural solutions. CO2 is the preferred natural cooling agent in the Carnot Refrigeration energy recovery system. CO2 enables overall energy consumption to use half the kilowatt energy of traditional equipment, and reuses much of the derived heat from the cooling process. Cooling is conducted with the CO2 natural refrigerant in a safer, less hazardous method than with traditional agents such as ammonia. Ammonia refrigerants have significant risks associated with their use.

Heat recovery from discharge gas and oil cooling

Optimum heat recovery is achieved by installing:

• A common discharge gas DE-superheating heat exchanger, either on the low-stage or high-stage discharge
• A secondary oil cooler in series with the existing oil cooler on each high-stage compressor: the original oil cooler should be retained so that the critical function of oil cooling is not compromised and is independent of hot water demand.

Heat recovery from chillers and chiller-heat pump units

Chillers conventionally are used to generate chilled water or a chilled mixture of glycol and water. They reject the heat generated by condensation to the environment via air-cooled condensers or cooling towers. Modern chillers, in particular those using ammonia (R717) or carbon dioxide (R744) refrigerant, offer significant potential to recover otherwise wasted heat at useful temperature levels, i.e. greater than 50°C. This recovered heat can be used to offset the consumption of other operations, thus reducing overall site energy usage.

How much heat is available?
Before deciding whether heat recovery makes sense for an application, it is useful to know just how much recoverable heat is available. The total heat available is the heat removed from the space plus the heat of compression.  There are four areas in refrigerant systems where heat can be recovered

1. The condenser
2. Superheat in the discharge gas
3. Compressor jacket or oil coolers &
4. Totally enclosed water-cooled motors.

The discharge gas coming from the compressor is in a superheated state and some heat can be recovered from this gas by desuperheating it before it enters the condenser. The discharge temperatures in most refrigeration systems are quite high (in the range of 70°C to 100°C). The superheat can be used to heat the water to about 60°C. The amount of heat recovered would be of the order 10 to 15% of the total heat rejected by the condenser. The discharge temperatures of various types of compressors are generally as follows:

Compressor	Discharge Temperature
Screw Compressor (Indirect cooled)	70 ®  80 °C
Screw Compressor (Injection cooled)	50 ®  60°C
Reciprocating Compressors	85 ® 110°C
Boosters (Rotaries & Reciprocating)	75 ®  85°C

Waste Heat recovery with desuperheater

The estimates provided represent conservative rules of thumb. However, the amount of available heat that can be turned into useful heat in the form of hot water is limited. In a typical application the refrigerant line leaving the compressor will be connected to a heat exchanger unit Desuperheater. A return line from the Desuperheater will then be attached to the condensing unit. In this way, the hot refrigerant gases will flow from the compressor, through the Desuperheater, and then to the condenser. The Desuperheater has water circulating through it that is heated by the hot refrigerant gas. The hotter fluid transfers the heat to the colder water circulating through the Desuperheater.

Low grade heat recovery

Low grade heat comes from the refrigerant being condensed. This gives a temperature of between 20ºC and 40ºC. At higher temperatures, heat is easier to recover and use, but it’s only possible to recover between 5% and 10% of the total heat being rejected by a system. If plant uses air-cooled condensing, the air from the condenser can be ducted to where it’s needed. The warm water produced by water-cooled condensers can be recovered too, with a heat pump to raise the temperature to a more useful level if required

Control

The heat recovery condenser must be controlled in such a way that it reduces the load on the supply air reheater but does not give rise to the unnecessary start up of the cooler. The control sequences of the respective devices must be matched. Preheating of the outside air must operate in a stable manner; other-wise it will have a negative effect on the subsequent control loops. This requires a precise modulating control valve.

Recoverable Super Heat from Ammonia Refrigeration Systems :

Condensing Temperature °C	Estimated Discharge Temperature °C	Recoverable Heat Kcal/ hr/ kW
43	114	1990
41	104	1800
38	99	1730
35	91	1525
32	86	1450

              

Recoverable Super Heat From R-22 Systems :

Condensing Temperature °C	Estimated Discharge Temperature °C	Recoverable Heat Kcal/ hr/ kW
46	82	1875
43	77	1700
31	71	1650
35	66	1500

              

Advantages of Waste heat recovery system in AC & Refrigeration

Waste Heat Recovery Systems in industrial Air Conditioning and Refrigeration systems have number of advantages besides the recovery of heat.

1.  The operating “condenser” duty is reduced when a Heat Reclaim System is “retrofitted”. This reduces the condensing pressure, which in turn reduces the BKW of the compression equipment, and/or reducing the condenser fan horsepower. For new installation it is not recommended that the condenser size be reduced unless the  “heat reclaim” requirement coincides with the maximum plant load conditions.

2.  Where a Desuperheater is installed, the refrigerant gas temperature inlet to the condensers is reduced, thus reducing the “fouling’ tendency of the condenser & thus reducing water treatment requirement.

3.   Where a Desuperheater is installed, most of the oil that is normally in Vapor State will condense & a good oil separator can remove it. By removing the oil before it gets to the evaporators, the system efficiency is increased with the possibility that the evaporator pressure can be increased.

As a “thumb rule” a 1.8°C increases in evaporator temperature decreases energy requirements by approximately 6%. A 3°C increase will decrease the energy requirement by about 11%.

Desirable Properties of Desuperheaters:

1. Pressure drop on gas side across inlet and outlet of Desuperheater should not be more than 14KPa.

2. The Desuperheater should be “Mechanically cleanable” without interrupting the operation of the refrigeration system.

3. If hot water produced is used as potable water or as preheated water for the boilers, the design of Desuperheater must ensure that no cross contamination of water and refrigerant gas takes place in the event of tube leakage.

4. All heat recovery units should be provided with bypass valves that allow the unit to be isolated from the system in case of leaks or maintenance requirements.

Applications

Heat recovery from air-cooled condensers is most often used to supplement space heating in neighbouring buildings. Heat recovered from a water-cooled system or

a DE-superheater has wider applications, including:

process plant where demand for hot water is high, such as in steam boiler water pre-heating

food processing plants where, for example large amounts of hot water are needed for washing down

buildings that need air conditioning and hot water, such as swimming pools and hospitals.

Heat recovery can be applied to most sizes of plant, from small ‘split’ units for a room to units of more than 1,000kW used in food storage.