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For top notch performance, size really does matter

When it comes to heat pumps, size - or rather, sizing - matters. If the industry is to develop and grow to its full potential, this vital consideration must be addressed, says Maurice Diamond

Heat pumps must be sized correctly to ensure the correct choice of type of unit (air or ground source), type of system (monovalent or bivalent) and unit performance (system seasonal CoP and running cost).

To achieve this, climatic information about the location is as important as defining the operational parameters at the design condition. It is the climatic information that enables judgments about plant size, system type and supplementary energy to be made.

If the climate is temperate, an air source heat pump will at least match a ground source unit with a substantial reduction in capital cost. Such a system will require negligible top up energy from other sources. If a substantial proportion of the year is at low or very low ambient conditions, then the ground source performance is superior and an economic case for such an application may be made.

Lack of information

To design system pipework, pumps, valves and ancillaries on an incorrectly sized heat pump is not how professional engineers wish to operate, but they may be compromised by lack of information.

What is required is an energy approach based upon degree days at the heat pump location. My own company uses a 10-year average of the information that is collected at aviation recording stations across the EU.

This data, coupled with the building heat loss at two specific conditions, enables an annual heating load profile for the building to be generated. DHW requirements in the form of people numbers, incoming mains water temperature, volume of DHW per person and DHW temperature is then incorporated and the energy profile for this load is added to the heating profile to provide a total annual requirement.

The basic programme takes no account of internal or solar gains but a correction factor is provided that enables the total energy figure to be adjusted to match the determined annual load from the dynamic modelling that will have been performed by the consultant during the design. When these figures match, any running cost information that is generated can be taken as an accurate estimation of annual running costs.

The next information input required for the system is to define the operating water temperatures for the heating load, realistic source temperatures (if a ground source solution is being considered), set point weather compensation and the output performance for the selected heat pump. For ground source units the performance is a constant for the source temperatures that have been input and for air source it needs to be against varying ambient temperatures down to the minimum expected for the location.

Too often, for ground source units, optimistic temperatures for the source have been used. It must be borne in mind that if heat is constantly drawn from the source and that this process can only occur if there is a temperature difference between the source and the exchange fluid, then the temperatures used will be lower than the normal standing temperature of the earth.

When this is programmed, knowing the chosen heat pump performance against ambient temperature and incorporating this with the degree day information enables an output energy profile to be calculated that is superimposed on the load requirement profile. This provides visibility of when the heat pump will match the load or better, when there is a shortfall and the extent of this shortfall.

For ground source units, all that is required at this point to make a final selection that will cover all of the heating season with minimum running and capital cost is to reduce the size of the selected heat pump until it just matches the load with no supplementary energy being required. The cost of electricity per unit can then be input to the programme and estimated annual running costs can be determined.

For air source units the most sensible and commonly accepted approach to determining the unit size is to ensure that the output matches at least 95 per cent of the seasonal load requirement. The balance of the energy requirement is provided from integrated top up heaters that should be controlled by the unit microprocessor. This approach ensures that the heat pump is not drastically oversized (with all of the attendant issues relating to this) and that the annual running costs are minimised with no hefty use of direct acting energy. It is on this latter point that most mistakes appear to be made with heat pumps often being selected based upon their snapshot performance at 7 deg C ambient and 35 deg C water temperature.

All too often the Coefficient of Performance quoted is that for the heat pump alone at some snapshot condition. This is not only misleading but is entirely worthless as the energy (carbon) usage and the running cost are based upon the system seasonal CoP. This figure must include defrost energy (for air source) and supplementary energy use.

The only way that this can be calculated is by the energy approach described above that has been adjusted for the annual energy usage determined by the dynamic modelling software. When such information is generated then and only then can decisions be made about system type, unit size and pay back. It also ensures that the system will operate correctly under all situations and the horror stories that have been generated by installations that have been done with limited or misleading information can be eliminated.

Air source heat pumps have suffered from perception that they do not really work in the UK climate as a result of poorly designed domestic installations. This has been further exacerbated by the monitoring report of some domestic installations by the Energy Saving Trust all of which has probably influenced the authors of the RHI to preclude them from the scheme at this stage.

If the correct design approach as described above is used, it becomes immediately apparent that not only do they work perfectly well (proven by the performance of our installations across Europe during last winter) but unless waste heat or bore hole water is available, you cannot make an economic case for ground source heat pumps even on simple payback never mind on a discounted cash flow basis. This is due to the high cost of generating a suitable low grade source that will maintain performance through the entire heating season.

Exceptionally bright future

Heat pumps have no competitors when it comes to their ability to reduce carbon in a controlled and reliable manner. They have an exceptionally bright future but this could be adversely affected by those who wish to take the maximum profit for minimal input in this embryonic industry. Control is possible in the commercial market where professional engineers, acting on the clients behalf, are involved in the design however, the domestic market would appear to be open to the whims of the profiteer.

Furthermore, the constant statements about reducing running costs should be tempered and controlled as running costs depend totally upon the cost of fuel. It is obvious that a heat pump system will be cheaper than using the electrical energy directly but convincing people that the CoP will make the system cheaper than a gas system will depend upon the price being paid for a kWh equivalent of gas. If it is a third of the price or less cost savings are unlikely to be made. Again an energy approach for the entire season is required if sensible claims are to be made.

// The author is general manager UK & Ireland of Hidros SpA //
7 October 2012


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