Air Source Heat Pumps

albyota

[FONT=Arial, Helvetica, sans-serif]Q. How do you calculate a COP?[/FONT]

[FONT=Arial, Helvetica, sans-serif]A. In its simplest form this relates to HEATING OUTPUT divided by the POWER INPUT. E.g. with a COP of say 3.6 to 1 the HEATING OUTPUT relates to 3.6kW of heat and the POWER INPUT of 1kW of electricity to run it.[/FONT]

[FONT=Arial, Helvetica, sans-serif](Note! Remember that COPs are instantaneous measurements of performance and are usually quoted by manufacturers to an international standard. COPs will vary according to the air or water entering the heat pump and with the temperature of the air or water being treated by the heat pump)[/FONT]

Air Source Heat Pumps (ASHP) are generally considered to be a more energy efficient option to gas boilers. By extracting warmth from the surrounding air, the heat energy released can be many times the energy required to power the equipment. In some instances however, a gas-fired condensing boiler may be a better choice, as ASHP systems are, by their very nature, dependent on the ambient temperature and more efficient at certain times of the year.


How ASHP's work
An ASHP system consists of a compressor and a carefully matched evaporator coil and heat exchanger. A refrigerant liquid, with a boiling point as low as 40°C, circulates within the system, evaporating when absorbing heat from the outside air. The resulting refrigerant gas is then compressed, adding more heat energy and raising its temperature to around 56°C. This heat is passed via the heat exchanger into water or air and used to provide space heating, for instance through low temperature convectors, or via under floor heating systems.

Calculating efficiency
The following formulae are designed to help installers make informed heating choices, analysing the relative efficiency by finding the break-even point, or point where both systems are as efficient as each other. The point can be calculated using either emissions or cost.
Gas Cost per Unit = Electricity Cost per Unit
Gas Condensing Boiler Efficiency = Electric ASHP Efficiency
The formula can be re-arranged to find the break-even CoP for ASHP as follows:
Electric air sourced = Electricity cost per unit x Gas efficiency
Heat pump efficiency Gas cost per unit

Putting typical figures in this place gives:
Break-even cost EVC =
0.09£/kWhr (electricity) x 0.9 = 2.3 CoP for energy equality
0.035 £/kWhr (gas)
The same formula can be used for emissions analysis:
Gas emissions = Electricity emissions
Gas condensing boiler efficiency = Electric ASHP efficiency
The formula can be re-arranged to find the break-even CoP as follows:
Electric air sourced heat pump efficiency =
CO2 emissions from electricity per unit x boiler efficiency
CO2 emissions from gas per unit
Result:
Break-even CoP EVC =
0.432 kg CO2/kWhr electricity x 0.9 = 2.24 CoP for emissions
0.193 kg CO2/kWhr gas

The figures used in the equations can be adjusted as prices, efficiencies and emissions change with time and consequently used to calculate the viability of gas driven heat pumps or any other similar comparison.
These calculations show that when an air-sourced heat pump has a Coefficient of Performance (CoP) of less than 2.3, its CO2 emissions are greater than a gas-condensing boiler and vice versa; when the ambient temperature is higher and the CoP is more than 2.3 the ASHP is the more efficient system. When choosing heating equipment, the questions that need to be asked are: at what ambient temperature does a system become more or less efficient and what percentage of the heating season is above or below this figure?
When used externally to a building, the outdoor unit ASHP provides a break-even point of about -3°C with an overall break-even point of 2°C, a temperature that is infrequently met in the UK.
With the vast majority of the UK heating season above this temperature, ASHP's provides a more efficient option compared with a condensing gas boiler, combining the functionality of cooling for warmer months.

some good info on GSHP here

http://www.gshp.org.uk/documents/CE82-DomesticGroundSourceHeatPumps.pdf

lomb

Can anyone with an energy meter hooked up tell what the standby consumption of these split or heat pumps is. This device is fitted to most and can be 40-100 watts on ALL the time.
Do a search for crankcase heater in wikipedia

That must surely cut into the figures

albyota

The crankcase heater is controlled by a thermostat and is only active when the thermostat calls for the crankcase heater to operate.

While the system is running, there is enough heat being generated by the compressor running to keep from creating refrigerant migration from occurring. Refrigerant vapor always migrates to the coldest part of the system (the compressor). The refrigerant migrates to the compressor crankcase where it is attracted to the compressor oil. This refrigerant vapor condenses and will return to a liquid in the off cycle. On the next start of the compressor, the oil is in a watery state and washes the bearings out, which leads to locked up, frozen or totally burnt out compressors. The temperature is sensed at the compressor, indoor coil, and outdoors. The sensed temperatures are compared and if the compressor temperature is not a specified amount higher than the lower of the other two sensed temperatures, then the crankcase heater is energized. When the compressor temperature rises to or is a specified amount above the lower of the other two temperatures, the crankcase heater is deenergized.

I'd rather have this on for short amounts of time as opposed to a massive bill for a replacement compressor....£900+ to replace, and yes this will be factored in to the COP of most manufacturers published specifications.

lomb

Hmm I wonder, do you have an energy monitor hooked up to a split? What is it drawing in standby and for how long at a time ie 100 watts for 20 minutes in each hour?
Also by defination mostly the compressor will be colder than the inside coil hence it will be on in our climate on that basis.
How can it be factored into COP if its a simple in and out,that cannot take account of standby. By defination the heater is switched off during running in any case.

albyota

I have an Air to Water Heat Pump and yes I have a Owl energy monitor connected and with all power off even to the fridge and freezer and only the heat pump connected but in the off mode, the monitor registers 0,(although these things are not very accurate) because the crankcase heater will only come on at very low temps, if the system has been off for a long time and ambient temps are very low.

albyota

100 watts for 20 minutes is an extreme, it will depend on what size unit it is.
100 watts for 20 minutes would equate to 0.0033p (at 10p/kWh) if my maths are correct at this time of night!!!

Cardew

Albytoya,

You have done a lot more research on ASHPs than I. Whilst not the same thing as the crankcase heater, I have read on 2 or 3 forums that defrost cycle is not factored into the quoted COP from manufacturers; and that this can be a significant factor in reducing the overall efficiency of a system.

I have always assumed that it would not be possible to calculate the power taken to defrost as conditions can vary. The humidity of the outside air will determine the ice build up for example.

albyota

cardew, when I first had my system installed, I had to wait 24 hours for the crankcase heater to get the compressor up to temp, I would not think this has needed to operate very often as the thermostat will control its operation only when the system has been in an off state for a while, (not sure how often I'll check this though) as for defrost cycles and whether it is factored in to the COP's or not, 2 minutes once in an hour only if conditions require i.e. 0 - 3 degrees foggy/moisture laden air at low ambient. yes you could probably factor in a loss....as for the manufacturers accounting for this, I wouldn't have thought it would impact the COP much, as they would have to say xxx unit has a COP of 3.59993 as opposed to 3.6.

richardc1983

lomb wrote: »

Can anyone with an energy meter hooked up tell what the standby consumption of these split or heat pumps is. This device is fitted to most and can be 40-100 watts on ALL the time.
Do a search for crankcase heater in wikipedia

That must surely cut into the figures

My split system uses about 8-15watts when they are turned off.

That is for a large 9kw outdoor unit. Obviously in summer when ambient temps are higher the crankcase heater doesnt need to operate so this will drop to about 6watts.

albyota

Cardew wrote: »

Albytoya,

You have done a lot more research on ASHPs than I. Whilst not the same thing as the crankcase heater, I have read on 2 or 3 forums that defrost cycle is not factored into the quoted COP from manufacturers; and that this can be a significant factor in reducing the overall efficiency of a system.

I have always assumed that it would not be possible to calculate the power taken to defrost as conditions can vary. The humidity of the outside air will determine the ice build up for example.

the defrost cycle will pull max power for 2 minutes (Ecodan) no heating being supplied for this short time..... never noticed with UFH...even with air to air not an issue, as I'm sure richard would agree....