Central Solar Systems - How A Pool Heat Pump Works

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Heat Pump Pool and Spa Heaters - How Do They Work?

Renewable Energy

Heat pumps offer the most energy efficient way to provide heating and cooling in many applications. This is because they can use renewable heat sources in our surroundings, such as air and water. Even when we think that it is cold, air and water contain heat that is being replenished by the sun. By using a small amount of additional energy, a heat pump can capture the renewable energy and significantly increase the temperature to the required level.

Introduction

Almost every home has a refrigerator. Many homes and vehicles have air conditioners. Supermarkets, transport (ships, trucks, etc), factories and storage facilities use mechanical refrigeration technology. These are just a small cross section of how mechanical refrigeration technology is used.

Heat pumps also utilise mechanical refrigeration technology. It is believed that the first heat pump was manufactured in the 1930's. Through the use of advanced design techniques and specialised components, heat pumps can be very reliable and efficient heaters of air, water and other substances.

As a result of decades of ongoing development, mechanical refrigeration can be one of the most reliable technologies available in the world today. However, the actual reliability is very dependent on the product design and the quality of the components used.

Uses For Heat Pump Water Heaters

Heat pump water heaters are used for many applications, including: hot water, pool and spa heating, space heating, heating for industrial processes.

How Heat Pump Water Heaters Work

In 1848, Dr John Gorrie wrote: "If air were highly compressed, it would heat up by the energy of compression. If this compressed air were run through metal pipes cooled with water, and if this air cooled to the water temperature was expanded down to atmospheric pressure again, very low temperature could be obtained, even low enough to freeze water in pans in a refrigerator box".

In normal operating mode a heat pump uses the same process described in 1848, but in reverse. As with most current mechanical refrigeration devices, air has been replaced by refrigerant to increase the amount of heat that can be absorbed and transferred.

Heat pumps can absorb heat from various sources - typically air, water or waste heat. In this description we will focus on heat pumps which absorb heat from the air. These are the most suitable and cost-effective units for Australian conditions.

In the case of a heat pump the volume of the refrigerant is expanded by an Expansion Valve (1) which causes it to decrease to a very low temperature - typically below minus 20ºC. This cold refrigerant is passed through a coil. This coil is called an Evaporator (3). A Fan (2) draws a large amount of air through the evaporator. Even if the air temperature is zero degrees, this is considerably warmer than minus 20ºC. The refrigerant therefore increases in temperature, that is, it absorbs heat from the air. The refrigerant is then compressed by a Compressor (4). Compressing a substance increases its temperature. In the case of a refrigerant, the temperature can increase to over 100ºC. This hot refrigerant then passes through a Coil (5) in a heat exchanger. The Outer Shell (6) of the heat exchanger allows pool water to flow through. As the water passes through the heat exchanger it absorbs heat from the refrigerant and heated water returns the the pool or spa. The cooled refrigerant is then expanded again, the temperature reduces, it passes through the evaporator and the process continues.

Design Considerations

Life expectancy of the heat pump is dependent on the materials used in the construction and for the important components.

The important components should be chosen to ensure that they integrate to provide a heat pump which is efficient and in which none of the important components are unduly stressed.

A poorly designed heat pump can appear to provide good performance at higher temperatures, but performance can be extremely poor at lower temperatures such as 15ºC or below. That is, such a heat pump may have trouble keeping the pool at the required temperature and heating costs rise considerably because of the poor performance at these temperatures. Temperatures of 15ºC or below are most likely to be encountered during periods when a pool requires most heating.

Important Components

A heat pump contains some important components:

Refrigerant

R22. Most heat pump pool and spa heaters use R22. R22 is not considered to be environmentally friendly and will soon be phased out by the government.
R407C. This is more environmentally friendly and will be around for many years to come. R407C works at higher pressure than R22. This means that thicker refrigeration pipes are needed. This adds to the cost.
R410A. This is the latest environmentally friendly gas. Major components suitable for heat pumps, such as large single phase scroll compressors are not yet readily available for R410A. This means that design cannot be cost effective and the price for a heat pump which uses R410A is currently higher than it should be. As better components become available and have been suitably tested, THERMOswim will release R410A heat pump pool and spa heaters.

Compressor

Expansion Valve

Fan. The fan can be a major source of noise. The following factors are important:

The quality of the fan
How the fan is positioned in the heat pump. Manufacturers should adhere to known noise reduction techniques.
The speed of the fan

Evaporator

The evaporator is the "coil" on the side of the unit. A large volume of air passes through the evaporator. Refrigerant also passes through the evaporator where it absorbs heat energy from the air. A large evaporator will allow the transfer of more heat energy than a small evaporator.

Heat Exchanger (also known as a condenser)

Water Bypass Valve

Refrigerant Control Valves

Electrical Components

Controller

Heat Pump Performance

Pool and Spa Heat Pump manufacturers usually provide output power and COP.

Output Power is the addition of power consumed and free energy absorbed from the air.
COP is the "co-efficient of performance". It is a ratio of the output power compared to the power consumed. For example a COP of 4 means that the unit produces 4kW of heat per 1kW of power consumed. That is, for every 1kW of power consumed, the heat pump absorbs free energy from the air to produce 4kW. A heat pump with an output power of 20kW and a COP of 4 consumes 5kW of power to produce the output power of 20kW. That is, it absorbs 15kW of energy for free!

As described earlier on this page, heat pumps absorb heat from the air and transfer this heat to the pool water. When the air temperature is warm more free heat energy is available in the air and the output power and COP of the heat pump increase. This is shown on the following graph:

Unfortunately, output power and COP ratings shown on brochures are not always quoted under the same operating conditions. Different testing conditions can make heat pumps appear to have more heating capacity and higher COP than they actually have when operating under normal conditions. The accepted benchmark for quoting performance is as follows: Ambient temperature: dry bulb 15ºC, wet bulb 11.2ºC. Water entering temperature: 27ºC.

People must ensure that they obtain benchmark performance figures so that they can perform a meaningful comparison between different units.

Information on swimming pool and spa heat pump sizing and heating costs

Includes critical information about heat pump ratings and the factors to consider when determining the correctly sized heat pump for your needs

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