Brookforge swimming pool engineers

Geothermal & Ground Source Heat Pumps

2. Save energy

 

REDUCED CENTRAL HEATING CIRCUIT WATER TEMPERATURE :

A characteristic / limitation of a ground heat pump system is that the temperature of the water circulated through the central heating circuit is very much lower than that achieved by a conventional fuel boiler.

In essence, this means that devices used to dissipate the heat from the LPHW pipe circuit into the occupied areas, such as radiators etc, need to be 'oversized' in order to compensate for the much lower LPHW circuit temperature.

Whilst a fuel boiler may operate at a flow temperature of 82°C, a ground heat pump would normally operate at flow temperatures as low as 35 to 45°C. It is not that a ground heat pump cannot generate hotter circuit temperatures, as they can, the problem is that the efficiency of the heat pump is adversely affected and the coefficient of performance is reduced.

The issue of the reduced water circuit temperature is particularly exaggerated within a swimming pool application. A typical domestic dwelling would be heated to around 22°C, whereas a swimming pool room would require to be heated to a significantly warmer temperature, say 30°C.

This has a marked impact on the ability of a heat exchange system to transfer heat into the occupied areas.

Therefore, a realistic heating circuit water flow temperature of 55°C is required if meaningful heat exchange for a swimming pool area is to achieved.

For example :

Conventional fuel boiler system :

Heating circuit water flow temperature : 82°C
Heating circuit water Return temperature : 71°C
Mean average in-circuit temperature for heat exchange : 76.5°C

Typical pool room air temperature : 30°C
Temperature difference for heat exchange : 46.5°C (76.5°C – 30°C)
Heat exchange coil performance of rating : 100%

Ground heat pump circuit :

Heating circuit water flow temperature : 55°C
Heating circuit water Return temperature : 45°C
Mean average in-circuit temperature for heat exchange : 50°C

Typical pool room air temperature : 30°C
Temperature difference for heat exchange : 20°C (50°C – 30°C)
Heat exchange coil performance of rating : 43%


From the above comparison, we can see that, with a ground heat pump, we are left with less then half of the heat exchange duty previously possible with a fuel boiler.

In respect of pool water heating ability, the scenario can be simply overcome by significantly increasing the selected capacity of pool water heat exchanger to compensate, as the normal flow of pool water from the pool filtration pump would normally still be adequate to transfer the required heat. This is possible because water has a comparatively high specific heat capacity, i.e. it can absorb a lot of energy before any temperature increase becomes apparent.

However, with air heating it is not so easy. To achieve the same heat input to the pool room, the quantity of warm air flow into the pool hall has to be increased to compensate for a lower delivered air temperature on a pro-rata basis.

For example :

Example air heat loss from a typical pool hall : 15kW @ -5°C outside temperature.
Pool hall air temperature : 29°C

Fuel boiler :
Average LPHW in-circuit temperature for heat exchange : 76.5°C
Increase in air temperature through heating coil : (Max.) : 26.6°C
Temperature of air delivered into the pool hall : 55.6°C (26.6°C + 29°C)

Quantity of Air flow through air heating coil necessary to
transfer sufficient heat to match room heat loss : 1800 M³/Hr.

Therefore : 1800 M³/Hr. of air, increased by 26.6°C in temperature = 15kW of heat input into the pool hall.

Ground heat pump :
Average LPHW in-circuit temperature for heat exchange : 50.0°C
Increase in air temperature through heating coil : (Max.) : 12.0°C
Temperature of air delivered into the pool hall : 41.0°C (12.0°C + 29°C)

Quantity of Air flow through air heating coil necessary to
transfer sufficient heat to match room heat loss : 4000 M³/Hr.

Therefore : 4000 M³/Hr. of air, increased by 12.0°C in temperature = 15kW of heat input into the pool hall.

Considering that more than half the heat exchange potential has been lost through using the lower circuit temperatures of a ground heat pump, this potentially has a notable affect on equipment selection and air duct work capacity and physical size.

BROOKFORGE SWIMMING POOL TIPS 2009 - CHELTENHAM - 01242 573572