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Heat Pump


Heat Pumps explained (an insiders view).

 

Terminology

 

Kilowatt (KW) Unit of power.

 

Kilowatt hour (KWH) Unit of quantity of power ( 1 kilowatt for one hour = 1 KWH).

 

COP coefficient of performance. Tested according to specified parameters the COP is the multiple of the outputted energy times the inputted energy

Output (KW) / Input (KW) = COP.

 

SPF seasonal performance factor. The SPF refers to the performance over an entire season. The power input and output is cumulated for the season then the total outputted power is divided by the total inputted power to give the SPF.

Total Outputted power (KWH) / Total inputted power (KWH) = SPF.

 

The SPF is a better method of comparing heat pump performance as this figure will give a more accurate estimate of running cost over an entire season and also reflects the performance of the complete system including collectors, pumps and auxiliary heaters if employed. (please note that SPF is particular to the site and the customers usage and cannot be advised by the supplier).

 

Open Loop / Closed loop

 

This refers to the heat collector.

A closed loop is one which recirculates the same fluid constantly.

Advantages

Little chance of contamination to the heat exchanger.

Fluid can be modified for low temperature operation (brine).

Can use Soil or water as heat source.

Disadvantages

Requires large ground area and excessive excavation for horizontal ground loop or circa 3x 100m deep bore holes for vertical ground loops.

Requires excessive loop lengths.

Performance can be unpredictable due to soil types etc.

Ground usage is limited as trees, buildings and drive ways are prohibited above the collector. Even simple tasks such as erecting a clothes line could be disastrous to the system.

Loops contain refrigerant gas or water glycol mix which could contaminate local wells, water courses and the atmosphere if leaks occur.

Leak detection is impossible and will require complete excavation and replacement of the collector.

Closed loops are generally made from plastic and installed in soil this results in very poor heat transfer which lowers the efficiency of the heat pump.

 

A open loop entails a system which draws a fluid from the source to the heat pump and returns the fluid to the source once the heat has been extracted.

Advantages

Lower installation cost.

Little ground requirement.

Increased efficiency as the sources temperature is directed into the heat pump.

Greatly reduced risk of contamination, as the volume of refrigerant is only that contained within the heat pump.

Higher temperatures achievable where source temperatures are higher.

Performance can be determined prior to installation based on the source yield and water quality.

 

Disadvantages

 Every site is not guaranteed an adequate source. Ground water tables vary from area to area. Local investigation should be carried out also reference should be made to the relevant geotechnical bodies data for the area.

Water quality Issues may Increase maintenance and in extreme cases require pre-treatment.

Source supply may vary throughout the year. Recharge wells may be necessary where poor aquifers exist.

 

Direct Expansion, (closed Loop )

 This type of heat pump uses less heat exchangers. It can do this by eliminating the intermediate carrier fluid from the source (i.e. brine solution). In its place the refrigerant gas is circulated through the source (ground loop / lake etc.).

Advantages

It can achieve a higher COP than many other units on the market.

It can in some applications achieve domestic hot water at 60 C.

 

Disadvantages

Migration of lubricants. The refrigerant gas contains POE oils to lubricate the compressor during the operation. However when direct expansion systems are used often times the lubrication separates from the gas in the collector loop (particularly in vertical loops) and fails to return to the compressor. This leads to accelerated compressor wear and a shorter life span.

Poor SPF (seasonal performance factor) as the system is a closed loop when installed in a stationary heat source, it will readily draw any energy available close to the collector loop yielding an initial high COP however as the energy draw is prolonged over hours, weeks and months the available energy close to the loop has been consumed and energy from further away has to be drawn. This requires a greater temperature difference which requires the compressor to work harder. It has been shown that the COP of a system can reduce by over 50% due to this phenomena. Thus leading to an overall poor SPF.

 

Brine to Water, (closed Loop )

This heat pump is the most common Installation to date. The reason been that the industry trusted the closed loop better due to the lower risk of particles blocking the heat exchanger and fewer maintenance callouts. However the systems installation costs are high and its efficiency is on the low scale with a typical COP of approximately 4 . The SPF also is very poor often in the range of 2.9 - 3.5. Many of these units have been fitted with auxiliary heaters to combat the poor heat output. However when these units are activated it is equal to heating your house with the hot water immersion heater in your cylinder (very costly). Try to avoid heat pumps with in built auxiliary heaters.

 

Air to Water,

 Air to water heat pumps take heat from the surrounding air and deposit it into the water in the heating system. These units are quite cheap to install as they do not require much alterations or excavation. However their advantages stop there. They are the least efficient technology with a typical Cop of 3. This is mainly due to the low specific heat capacity of air which means that much more air is required to gain the energy from, this entails large fans to direct the air onto the fins of the evaporator. As electricity is approximately three times the cost of oil per KWH (kilowatt hour) A COP of 3 will mean that the energy cost will remain the same.

As the outside temperature drops so too will the performance factor this means that when you need heat most the unit will deliver less.

Another point to note is that in the calculation of the COP the defrost cycle is not accounted for this is OK in summer or when the outside temperature is sub zero (dry air) as the possibility of ice forming on the evaporator is reduced. However in Ireland we have a very wet climate which leads to humid (damp) air. When this air is cooled condensation will form on the evaporator this will then turn to Ice which will block the air passage. The unit will sense this and start a defrost cycle. This cycle reverses the flows in the machine which will extract the heat from the house to melt the Ice. Typically in Ireland as high as 25% of the operation will be a defrost cycle this means that only 50% of the operation is actually adding heat to the house. Therefore the actual efficiency can be as low as 150% rather than 300% dictated by the COP figure.

 

Water To Water (open loop) 

The water to water heat pump is as efficient as the direct expansion system as it employs the same number of heat exchangers (2 the evaporator and the condenser).

The installation costs are generally low as the source is surface or in a relatively shallow well (typically 30 m).

Cops between 5 and 6 are achievable.

The Heat pump realises the temperature of the source and not a lower temperature due to a previous heat transfer process as well as been more efficient the heat pump also can achieve higher heating temperatures up to and above 60 C. this temperature is adequate to heat domestic hot water and also supply radiators.

It should be noted that operating at higher temperatures reduce the performance of the heat pump.

The SPF depends on the Source temperature deviation. If the source is surface river, lake etc a SPF of 4.5 is typical however if the source is a well then the SPF will possibly match the COP. In excess of 5.

As stated earlier the water to water is susceptible to foreign bodies entering the heat exchanger where surface water is been used as the source extensive filtration will be necessary to ensure a clean supply. Well water is generally much cleaner and only serious contaminations of Iron etc would require pre-treatment.

In recent times Manufacturers have realised the importance of higher efficiency and lower Installation costs. This has lead to designing of heat pumps specifically for water to water applications.

 

 

When selecting a heat pump the following should be considered.

 

SPF (ensure that SPF data refers to the proposed design (i.e. similar heat pump, collector area, source temperature etc.)).

COP in accordance with EN 255 or EN 14511.

Excavation costs / Well drilling costs.

Heat Pump cost.

Collector cost.

Auxillary equipment costs (many systems require additional apparatus and equipment for hot water generation, surge protection also pumps, tanks and instrumentation).

Service Engineer cost / hour

 

A Low temperature heating system.

Heating storage capacity to maximise on night rate electricity.

Ensure that the capacity of the heat pump is adequate to provide in excess of 70% of the heating requirement during the night rate tariff.

Three phase availability for larger properties.

Expected pay back period.

Domestic hot water temperature.

Scheduled maintence period.

Aftersales service.

 

After Considering and choosing the supplier it is always wise to ask for reference projects prior to agreeing a contract.


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Enviritech Engineering Ltd,
Unit 6, Enterprise & Research Centre, Institute of Technology Carlow, Kilkenny Road, Carlow, Ireland.
Tel: +353 (0)59 91 76261 | Fax: +353(0)59 91 76377 | Email: info@etech.ie
 
       
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