Absorption heat pump

Absorption heat pump configuration (type 1 refrigeration)

Absorption heat pump temperature (type 1); Q2-driving high temperature flow (desorber); Q0-low temperature flow (evaporator); Q1-intermediate heat flow (condenser).

Classified by temperature, AHPs can be divided into two categories. In type 1 AHP, the condenser temperature is higher than evaporator temperature[7] (also referred to as heat amplifier[8] and refrigeration[3]). Driven by a high-temperature heat source, the first type absorption heat pump extracts the heat of waste heat (waste heat) and outputs a medium-temperature heat medium that is 30–60 degrees Celsius higher than the waste heat.[9] This type is more common and could be an alternative to traditional compression machines. The coefficient of performance of the first type absorption heat pump is greater than 1, generally 1.5 to 2.5.[4]

The heat pump is composed of the main components such as generators, condenser, evaporator, absorber and heat exchanger, as well as the suction device, shielding pump (solution pump and refrigerant pump), and other auxiliary parts. The air extraction device removes the non-condensable gas in the heat pump and keeps the heat pump always in a high vacuum state.[4]

Absorption heat pump process scheme (type 2)

In type 2 AHP, condenser temperature is lower than evaporator temperature[7] (also referred to as heat transformer[10]). The type 2 absorption heat pump uses the heat of the medium-temperature waste heat intelligently, output high-temperature heat medium (hot water steam) 25–50 degrees Celsius higher than medium temperature waste heat.[9] The type 2 absorption heat pump could be driven by low-grade waste heat in the production process or in nature, which can achieve energy saving and emission reduction and reduce production costs, and it has practical application in petrochemical and coal chemical industries.[9] The coefficient of performance of the second type absorption heat pump is always less than 1, generally 0.4 to 0.5.[4]

Absorption heat pump temperature (type 2); Q1-intermediate driven heat flow (evaporator); Q2-high temperature revalued flow (absorber); Q0-low temperature rejected flow (condenser).

Ammonia and water absorption heat pump

Water is the refrigerant and LiBr the absorption medium.[1] Water and LiBr systems have bigger capacities and are applied in a broad range in the industry, the sizes vary from tens of kW to several MW.[3] The first type of lithium bromide absorption heat pump unit is a high-temperature heat source (steam, high-temperature hot water, fuel oil, gas) as the driving heat source, lithium bromide solution as the absorbent, and water as the refrigerant, and the low-temperature heat source (such as waste hot water) is recycled and used.

Ammonia is the refrigerant and water the absorption medium.[1] In the absorber and generator, the absorption or effect of the ammonia aqueous solution is used to radiate heat or absorb heat. In the evaporator and condenser, the phase change of pure ammonia is used to complete the external absorption or heat release.[4] Like a traditional heat pump, the refrigerant (ammonia) is condensed in the condenser, and heat is then released; the pressure is dropped after the expansion unit and the refrigerant is evaporated to absorb heat.

The ammonia/water heat pumps are essentially limited to residential applications because they are only commercially limited to small sizes (a few KW).[3][11] If the system absorbs heat from the residential building, it works as a refrigeration machine; if it releases heat to the interior of a residential building, it heats the house.[12]

The key component of heat pumps using ammonia and water on the market today is the generator absorber heat exchanger (GAX), which improves the thermal efficiency of the equipment by recovering the heat released when ammonia is absorbed into the water.[11] Other innovations applied to this type of heat pump include efficient steam separation, variable ammonia flow, and variable capacity, and low-emission capacity-variable gas combustion.[11]

Single, double, or triple iterative absorption cooling cycles are used in different solar-thermal-cooling system designs. The more cycles, the more efficient they are.

In the late 19th century, the most common phase change refrigerant material for absorption cooling was a solution of ammonia and water. Today, the combination of lithium bromide and water is also in common use. One end of the system of expansion/condensation pipes is heated, and the other end gets cold enough to make ice. Originally, natural gas was used as a heat source in the late 19th century. Today, propane is used in recreational vehicle absorption refrigerators. Innovative hot water solar thermal energy collectors can also be used as the modern "free energy" heat source.

Efficient absorption refrigerators require water of at least 88 °C (190 °F). Common, inexpensive flat-plate solar thermal collectors only produce about 70 °C (160 °F) water, but several successful commercial projects in the US, Asia and Europe have shown that flat plate solar collectors specially developed for temperatures over 93 °C (200 °F) (featuring double glazing, increased backside insulation, etc.) can be effective and cost-efficient.[13] Evacuated-tube solar panels can be used as well. Concentrating solar collectors required for absorption refrigerators are less effective in hot humid, cloudy environments, especially where the overnight low temperature and relative humidity are uncomfortably high. Where water can be heated well above 88+ °C (190 °F), it can be stored and used when the sun is not shining.

For more than 150 years, absorption refrigerators have been used to make ice.[14] This ice can be stored and used as an "ice battery" for cooling when the sun is not shining, as it was in the 1995 Hotel New Otani Tokyo in Japan.[15] Mathematical models are available in the public domain for ice-based thermal energy storage performance calculations.[12]

The earth as a huge and stable thermal storage resource, its shallow ground temperature and groundwater also have wide application prospects in energy use, especially for building energy saving is of great significance. Using absorption heat pump (refrigeration) technology, 65–90°C geothermal water can be used to produce 7–9°C refrigerant water for summer air conditioning. Reasonable use of corresponding heat pump technology can achieve the efficient and comprehensive utilization of geothermal resources at different temperature levels, greatly reducing energy consumption for heating and cooling of residential and commercial buildings.[4] The use of 65°C and above geothermal water (or waste/waste heat) to drive the absorption heat pump for cooling, and the corresponding heat pump type (heating/heating) for heating, can achieve good energy-saving and economic benefits.[4] For low-temperature heat sources of 15–25 °C, driven by a small amount of high-temperature heat sources (such as high-temperature steam or direct combustion), cold water at a temperature of 7–15 °C and hot water at a temperature above 47 °C can be prepared. 1.2, >1.5 when heating.[4]

Natural gas is a common-used heat source, therefore, absorption heat pumps are sometimes called gas-fired heat pumps.[11] Also, when other heat sources heat pumps (waste heat for example) are running at the heating mode, they can meet the overload heating requirements of very cold periods in an efficient manner through additional gas boilers.[6]

Illustratively, the waste heat drive system may cover cooling and heating loads by operating in a cooler and heat converter mode. It is possible that only one device can provide resources to the urban area in a resource-efficient manner throughout most of the year driven by waste heat.[6]