Pointed out by Chinese famous heating engineering expert Zhonghua Wu, “Energy Cascade Use with matched temperature” and the temperature of the heating source is in the range of low temperature, it is the heating pump that is used in raising the grade to building heating level.

(1) Heating Pump
Heating pump is the installation used in raising low energy grade to high grade. Like workers on the first floor can not finish the work on the tenth floor, the potential energy will be raised by sending the workers to tenth floor by elevator. In this way, workers on the tenth floor will be able to finish the same work as counterparts on the first floor. However, the elevator can only raise the potential energy and have nothing to do with the work.
The low grade heat such as heat of air, soil and waters are everywhere. However, the energy is not appropriate to be used as buildings heating source because of low temperature. The energy only can be used after heated.
The heating pump which uses air as the low grade heating energy source is called air-sourced heating pump. The pump which uses waters as source is called water source heating pump, pump which uses the soil as source is called the earth-source heating pump and the pump which uses shallow geothermal energy as source is called the geothermal energy source heating pump.

(2) The Operational Principle of Heating Pump
The refrigerator works according to the operational principle of heating pump. Moving out the heat inside is the only way to cool the refrigerator. Therefore, the cooler heats up when the freezer works. This whole process is completed by heating pump.
The heating pump is constituted with evaporator, condenser, compressor and expansion valve. The operational principles refers to chart4

When heating pump works, the heat energy from low grade source is absorbed by the low-pressure liquid medium in order to be evaporated into gas working medium. The gas is condensed by the compressor to high-temperature and high –pressure gas working medium.  And high- temperature and high-pressure gas gives off heat to become the high –pressure liquid working medium. This liquid working medium is decompressed by the expansion valve to become the low-pressure liquid medium which absorbs the heating energy of low grade source through the evaporator …The endless will meet building heating need.
It can be seen from the above operational principle; heating pump absorbs the low grade energy and gives out high grade energy.  In general, the compressor absorbs 3 unit of low grade heat and 1 release 4 units of high grade heat by consuming 1 unit of electricity power. The efficiency of the heating pump is always called Energy Consumption Rate (COP)= Released Heating Power/Consumed Electricity of Compressor.
In sum, high grade heating energy provided by heating pump is efficient, energy-saving and green if stable low grade heating energy can be collected.