How Water Source Heat Pumps Work

Figure 1: Water source heat pump unit.

A heat pump is a device that transfers thermal energy from a low-grade heat source to a high-grade heat source. Heat pumps, driven by electricity, usually first obtain low-grade heat energy from natural air, water or soil, and then provide high-grade heat energy.

Heat pumps are divided into different types according to different heat sources: air source heat pumps, water source heat pumps, ground source heat pumps, dual fuel heat pumps, etc. Water source heat pump is currently the cooling and heating method with the highest energy efficiency ratio in the air conditioning system.

1. What is a Water Source Heat Pump

The water source heat pump realizes the transfer of heat energy from low temperature to high temperature by inputting a small amount of high-grade energy (such as electric energy). Water source heat pumps refer to all heat pumps that use water as a source of heat and cold. Water sources include groundwater, rivers, lakes and oceans.

Figure 2: Water source heat pump system.

1.1 Components of Water Source Heat Pump System

The water source central air conditioning system is composed of three parts: the terminal (indoor air treatment terminal, etc.) system, the water source central air conditioning main unit (also known as the water source heat pump) system and the water source water system.

1. The terminal system (indoor terminal, etc.) is composed of user-side water pipe system, circulating water pump, water filter, electrostatic water treatment instrument, various air treatment terminal equipment, and related valve accessories.
2. The water source central air conditioner main unit is composed of compressor, evaporator, condenser, expansion valve, various refrigeration pipe fittings and electrical control system.
3. Water source water system is composed of water source water intake device, water intake pump, water treatment equipment, water delivery pipe network and valve accessories.

2. Working Principle of Water Source Heat Pump

The working principle of the water source heat pump unit is to transfer the heat in the building to the water source in summer; in winter, it extracts heat energy from the water source at a relatively constant temperature, uses the heat pump principle to raise the temperature of air or water as heat transfer medium, and then sends it to the building.

Its specific working principle is as follows (water as heat transfer/cooling medium, while air as heat/cooling medium in the figures given below):
In cooling mode, the high-temperature and high-pressure refrigerant gas comes out of the compressor and enters the condenser, and the refrigerant releases heat to the groundwater to be a high-temperature and high-pressure liquid, which increases the temperature of the cooling water.
The refrigerant becomes a low-temperature and low-pressure liquid after the throttling function of the expansion valve, enters the evaporator to absorb the heat in the building cooling water, evaporates into low-pressure refrigerant vapor, and lowers the temperature of the chilled water.
The low-pressure refrigerant vapor enters the compressor and is compressed into a high-temperature and high-pressure gas, which circulates in the above way, and frozen water can be obtained as a cooling medium in the evaporator continually.

Figure 3: Cooling mode of water source heat pump.

In the heating mode, the high-temperature and high-pressure refrigerant gas comes out of the compressor and enters the condenser. The refrigerant releases heat into the building’s heating water and cools into a high-pressure refrigerant liquid, which raises the temperature of the hot water.
The refrigerant become a low-temperature and low-pressure liquid after the throttling function of the expansion valve, enters the evaporator to absorb the heat in the groundwater, evaporates into low-pressure refrigerant vapor, and lowers the temperature of the low-temperature heat source water.
The low-pressure refrigerant vapor enters the compressor and is compressed into a high-temperature and high-pressure gas, which circulates in the above way, and hot water can be obtained as heat transfer medium in the condenser continually.

Figure 4: Heating mode of water source heat pump.

3. Pros and Cons of Water Source Heat Pump

3.1 Advantages

(1) Water source heat pump is a renewable energy utilization technology.
The water source heat pump is a heating and air conditioning system that uses the solar energy stored in the earth's water as a cold and heat source to convert energy. In fact, after the groundwater passes through the heat pump unit, it only exchanges heat, and the water quality does not change. So it will not cause pollution to the original water source after the water is recharged into the stratum or back into the surface water system.

(2) The water source heat pump has high operating efficiency, low cost and energy saving.
The water temperature available to the water source heat pump unit is 12~22°C in winter, which is higher than the outdoor air temperature in winter, so the evaporation temperature of the heat pump cycle is increased, and the energy efficiency ratio is also improved. Compared with electric heating, a well-designed water source heat pump unit can reduce power consumption by more than 70%.

(3) The water source heat pump operates stably and reliably.
The temperature of the water body is relatively stable throughout the year, and especially the groundwater, whose temperature fluctuation range is much smaller than that of the air, is a good cold and heat source for heat pumps. Therefore, the operation of the heat pump unit is reliable and stable, and there are no difficult problems like winter defrosting of the air source heat pump.

(4) Water source heat pumps have remarkable environmental benefits.
The operation of the water source heat pump unit has no pollution (no combustion, no smoke exhaust, and no waste, etc.) and it can be built in residential areas.

(5) Water source heat pump is multi-purpose and has a wide range of applications.
The water source heat pump system can provide heating, cooling, and domestic hot water. One machine has multiple functions, and one system can replace two sets: boilers and air conditioners. Especially for buildings with both heating and cooling requirements, water source heat pumps have obvious advantages, not only saving a lot of energy, but also reducing the initial investment of equipment.

Figure 5: Working principle diagram of water source heat pump.

3.2 Disadvantages

(1) The issue of available water sources.
Water source heat pumps can use all water resources in theory, but in actual engineering, the cost of using different water resources varies quite a lot. In the water source heat pump utilization mode, the closed system generally has a higher cost. For an open system, whether a suitable water source can be found is the limiting condition for using a water source heat pump. For open systems, the water source must meet certain temperature, water volume and cleanliness.

(2) The geographical structure of the water layer.
For the use of water intake and recharge, the geological structure of the location must be considered to ensure that a suitable water source can be found by drilling a well under economical and reasonable conditions, and at the same time, the water recharge can be realized.

(3) Groundwater also contains different ions, molecules, compounds and gases, which make the groundwater have chemical properties such as pH, hardness, and corrosion. If it enters the unit without treatment, it will have a certain impact on the material of the unit. Besides, the unit system is easily blocked by mud and sand in groundwater.

(4) The water source heat pump has the function of performing automatic protection. When the water flow rate is too low, and the pressure of refrigerant is too low or too high, the unit will start automatic protection, which will affect its normal use.

(5) In different regions and countries, affected by the energy policies, the fuel prices, and the different basic conditions of the water sources, the one-time investment and operating costs will vary with different users.

(6) Well water recharging is difficult to implement, recharging wells are often blocked, and 100% recharging cannot be achieved, resulting in waste of groundwater resources and land subsidence probably.

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