Vapor absorption cycle

In the early years of the century, the vapor absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapor compression cycle, it lost much of its importance due to its low coefficient of performance (about one-fifth that of the vapor compression cycle). Today, the vapor absorption cycle is used only when heat is easier to come by than electricity, such as waste heat provided by solar collectors, or off-grid cooling in recreational vehicles.

The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber that dissolves the refrigerant in a suitable liquid, a liquid that increases the pressure of the pump and a heat generator that also starts the refrigerant vapor from the high-pressure liquid. Some work is required by the liquid pump but, for a given amount of refrigerant, it is much less than that needed by the compressor in the vapor compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent) and water (refrigerant) and lithium bromide (absorbent).

gas cycle

When the working fluid is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a gas cycle. Air is most often this working fluid. Since there is no condensation and evaporation in a gas cycle, components corresponding to the condenser and evaporator of a vapor compression cycle are the hot and cold gas-gas heat exchangers.

For extreme temperatures, a gas cycle may be less efficient than a vapor compression cycle because the gas works in the inverse Brayton cycle rather than the inverse Rankine cycle. Therefore, the working fluid does not receive or reject heat at constant temperature. In the gas cycle, the cooling effect is equal to the product of the specific heat of the gas and the rise in gas temperature on the low temperature side. Therefore, for the same cooling load, gas refrigeration cycle machines require a higher mass flow rate, which in turn increases their size.

Due to its lower performance and larger bulk, the evaporator cycle is not often applied in terrestrial refrigeration. The air cycle machine is very common, however, in gas turbine jet aircraft; since compressed air is readily available in compressor sections of engines. Aircraft cooling and ventilation units also serve the purpose of heating and pressurizing the aircraft cabin.

The Stirling Engine

The Stirling cycle heat engine can be driven in reverse, using an input mechanical energy to transfer heat from the unit in a reverse direction (i.e., a heat pump or refrigerator). There are several design configurations for these types of devices that can be built. Several of these configurations require rotating or sliding joints, which can introduce difficult trade-offs between friction losses and coolant losses.

Comparison with combined heat and power (CHP)

A heat pump can be compared to a combined heat and power (CHP), for a steam condensing plant, as switches to produce heat, electrical energy is lost or not available, just as the power used in a heat pump is not available. Normally for every unit of energy lost, then 6 units of heat will be available at about 90°C. Thus CHP has an effective coefficient of performance (COP) compared to a heat pump of 6% on average. Because losses are proportional to the square of the current, during peak seasons losses are much greater than this and it is likely that widespread i.e. widespread city use of heat pumps would cause overloading of transmission and distribution networks unless they are considerably strengthened.[2][7]​.

Inverted Carnot cycle

Since the Carnot cycle is a reversible cycle, the four processes that make it up are two isothermal and two isentropic processes, everything can be reversed like this. When this happens, it is called a Carnot cycle reversal. A refrigerator or heat pump that acts on the reverse Carnot cycle is called a Carnot refrigerator and Carnot heat pump respectively. In the first stage of this cycle (process 1 - 2), the refrigerant absorbs heat isothermally from a low temperature source, TL, in the amount QL. Then, the refrigerant is isentropically compressed (process 2-3) and the temperature is raised to the high temperature source, TH Then at this high temperature, the refrigerant rejects heat isothermally in the amount QH (process 3-4). Also during this stage, the refrigerant changes from a saturated vapor to a saturated liquid in the condenser. Finally, the coolant expands isentropically where the temperature drops back to the low temperature source, TL (process 4 - 1).[2]​.

Vapor absorption cycle

In the early years of the century, the vapor absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapor compression cycle, it lost much of its importance due to its low coefficient of performance (about one-fifth that of the vapor compression cycle). Today, the vapor absorption cycle is used only when heat is easier to come by than electricity, such as waste heat provided by solar collectors, or off-grid cooling in recreational vehicles.

The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber that dissolves the refrigerant in a suitable liquid, a liquid that increases the pressure of the pump and a heat generator that also starts the refrigerant vapor from the high-pressure liquid. Some work is required by the liquid pump but, for a given amount of refrigerant, it is much less than that needed by the compressor in the vapor compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent) and water (refrigerant) and lithium bromide (absorbent).

gas cycle

When the working fluid is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a gas cycle. Air is most often this working fluid. Since there is no condensation and evaporation in a gas cycle, components corresponding to the condenser and evaporator of a vapor compression cycle are the hot and cold gas-gas heat exchangers.

For extreme temperatures, a gas cycle may be less efficient than a vapor compression cycle because the gas works in the inverse Brayton cycle rather than the inverse Rankine cycle. Therefore, the working fluid does not receive or reject heat at constant temperature. In the gas cycle, the cooling effect is equal to the product of the specific heat of the gas and the rise in gas temperature on the low temperature side. Therefore, for the same cooling load, gas refrigeration cycle machines require a higher mass flow rate, which in turn increases their size.

Due to its lower performance and larger bulk, the evaporator cycle is not often applied in terrestrial refrigeration. The air cycle machine is very common, however, in gas turbine jet aircraft; since compressed air is readily available in compressor sections of engines. Aircraft cooling and ventilation units also serve the purpose of heating and pressurizing the aircraft cabin.

The Stirling Engine

The Stirling cycle heat engine can be driven in reverse, using an input mechanical energy to transfer heat from the unit in a reverse direction (i.e., a heat pump or refrigerator). There are several design configurations for these types of devices that can be built. Several of these configurations require rotating or sliding joints, which can introduce difficult trade-offs between friction losses and coolant losses.

Comparison with combined heat and power (CHP)

A heat pump can be compared to a combined heat and power (CHP), for a steam condensing plant, as switches to produce heat, electrical energy is lost or not available, just as the power used in a heat pump is not available. Normally for every unit of energy lost, then 6 units of heat will be available at about 90°C. Thus CHP has an effective coefficient of performance (COP) compared to a heat pump of 6% on average. Because losses are proportional to the square of the current, during peak seasons losses are much greater than this and it is likely that widespread i.e. widespread city use of heat pumps would cause overloading of transmission and distribution networks unless they are considerably strengthened.[2][7]​.

Inverted Carnot cycle

Since the Carnot cycle is a reversible cycle, the four processes that make it up are two isothermal and two isentropic processes, everything can be reversed like this. When this happens, it is called a Carnot cycle reversal. A refrigerator or heat pump that acts on the reverse Carnot cycle is called a Carnot refrigerator and Carnot heat pump respectively. In the first stage of this cycle (process 1 - 2), the refrigerant absorbs heat isothermally from a low temperature source, TL, in the amount QL. Then, the refrigerant is isentropically compressed (process 2-3) and the temperature is raised to the high temperature source, TH Then at this high temperature, the refrigerant rejects heat isothermally in the amount QH (process 3-4). Also during this stage, the refrigerant changes from a saturated vapor to a saturated liquid in the condenser. Finally, the coolant expands isentropically where the temperature drops back to the low temperature source, TL (process 4 - 1).[2]​.