Experience of air to water heat pumps
โขDownload as PPTX, PDFโข
0 likesโข302 views
Monitored energy use of air to water heat pumps in single Passivhaus dwellings is compared with the design estimates using PHPP. Heat pumps using conventional HFC refrigerants are compared with CO2 refrigerant. The latter gave improved performance for hot water heating but significantly worse for space heating. This is due to the very low heat load of Passivhaus dwellings being below the normal output range of available heat pumps.
Recommended
More Related Content
What's hot
What's hot (20)
Similar to Experience of air to water heat pumps
Similar to Experience of air to water heat pumps (20)
More from Nick Grant
More from Nick Grant (10)
Recently uploaded
Recently uploaded (20)
Experience of air to water heat pumps
- 1. Experience of air to water heat pumps for single family passivhaus dwellings 25th International Passivhaus Conference Alan Clarke @AR_Clarke
- 2. Why heat pumps? heat from methane Renewable electricity Year 2000 Year 2020 ?
- 3. Electricity generation carbon emissions heat from methane
- 4. Time to turn off the gas! Electrify everything
- 5. ???
- 6. Heat pumps Evaporation โ absorb air energy Compression โ add elec. energy Condensation โ release heat Expansion & back to start of cycle Refrigerant cycle: 1 kWh elec -> c. 3 kWh heat
- 7. UK heating system โCentral heatingโ Radiators are supplied with water heated by a gas-fired boiler Hot water cylinder Boiler Familiar, quiet, easy to adjust room temperature Hot water cylinder Air source heat pump
- 8. Air source heat pump + radiators โข Not too expensive โข Radiators are standard โข No noise indoors โข Similar installer skills โข Hot water provided โข More expensive than gas โข Radiators are bigger โข Some noise outdoors โข Specialists in demand โข Hot water 50-55 C
- 9. Passivhaus + HFC heat pump
- 10. Specifications Heat load (PHPP) = 1.5 kW Radiator @ 41 C, output = 2.8 kW Radiator @ 34 C, output = 1.5 kW Performance curve
- 11. So best to run at lowest possible temperature??
- 12. Heat pump compressor โ inside the box Rotary compressor
- 14. Typical compressor performance data
- 15. But ASHP data, at minimum output: Variable speed motor โ inverter driven
- 16. Heat pump out 2.8 kW Radiators out 1.5 kW -> heat pump โcyclesโ on and off
- 17. Cycling is bad Start-up energy loss is significant Multiple cycles per hour reduces COP
- 18. Final set up Flow temperature = 45 C PHPP seasonal performance factor (SPF) = 2.4 Actual season performance (SPF) = 2.8
- 19. Domestic hot water Storage temperature = 49 C โข PHPP SPF = 2.5 โข Actual SPF = 3.0 Combined heating and DHW โข PHPP SPF = 2.5 โข Actual SPF = 2.9
- 20. Passivhaus + CO2 heat pump
- 21. Carbon dioxide phase diagram More efficient for heating domestic hot water Hard to make work well with space heating
- 22. Specifications Heat load Design Heat load (PHPP) = 1.3 kW Radiator @ 41 C, output = 2.6 kW Radiator @ 33 C, output = 1.3 kW Predicted performance (PHPP) โข Heating SPF = 2.7 โข Hot water SPF = 2.9 โข Combined SPF = 2.7 Actual performance โข Heating SPF = 2.0 โข Hot water SPF = 2.7 โข Combined SPF = 2.2
- 23. Passivhaus + CO2 heat pump
- 24. Specifications Heat load Heat load (PHPP) = 1.5 kW Predicted performance (PHPP) โข Heating SPF = 2.4 โข Hot water SPF = 3.2 โข Combined SPF = 3.0 Actual performance โข Heating SPF = 2.0 โข Hot water SPF = 3.2 โข Combined SPF = 2.5
- 25. Observations on heating Conventional heat pumps donโt care about return temperature Carbon dioxide heat pumps need low return temperature โ hard to deliver reliably in practice โ performance hasnโt lived up to first expectations Minimum load COP on this ASHP much lower than at normal load
- 26. Radiator systems Go big! Must be sized to take heat pump continuous output 35 C will need big radiators even in a Passivhaus
- 27. Observations on hot water Conventional heat pumps fine if OK with low tap temperature Carbon dioxide heat pumps best for hot water alone โ communal hot water system โ sports hall showers
- 28. Conclusions Air source heat pumps can work efficiently with radiators Carbon dioxide refrigerant heat pumps have been disappointing The smallest heat pumps are really too big for 150m2 houses โ smaller houses are a problem we have yet to solve
Editor's Notes
- Hi โ Iโm alan clarke, designer of passivhaus heating and ventilation systems, and Iโm going to discuss our experience of using air to water heat pumps in single family dwellings
- Not so many years ago electricity was high carbon, from burning coal, and heat from direct combustion of methane was the low carbon option * Now we have large and increasing generation of renewable electricity, the UK has basically stopped burning coal, isnโt it time to stop using gas for heat?
- The green line shows historical carbon intensity of UK power generation, with projection of future progress * The blue line shows carbon intensity of heat from combustion of gas โ clearly in the last ten years the situation has changed rapidly
- So now its time to turn the gas off * And electrify everything
- However just using resistance electric heating has other problems โ with gas for heating the high winter peaks in heat demand have been manageable, but translated directly into electricity demand brings problems of supply capacity * How can we address this ?
- One part of the solution is heat pumps These use a refrigerant cycle to heat a space with much less energy than direct electric heating * In brief it works like this Low pressure liquid refrigerant absorbs energy from the outside air, and turns to gas, banking the latent heat of evaporation This refrigerant gas is compressed, using an electrically powered compressor. The refrigerant gains heat, and at higher pressure has a higher boiling point Inside the dwelling this hot gas is cooled by either air or water which transfer the heat to the space. The heat transfer temperature is below refrigerant boiling point so it condenses, releasing the latent heat previously absorbed from the outside air So the higher the heat transfer temperature, the higher the pressure needed and hence the higher the load on the compressor. To complete the cycle the refrigerant goes through a valve which allows it to expand and reduce pressure to restart the cycle outdoors * The typical energy use of this process is about 1 unit of electricity to deliver 3 units of heat. Some people like to think of this as magic, but there are good reasons to try and understand what is going on Also there are some climate implications of the type of refrigerant used โ these are normally hydroflourocarbons, which have very high global warming potential, so leakage would undermine the carbon benefit of using a heat pump. There are some refrigerants without this problem but they are new to this application The ratio of heat output to power input is called the coefficient of performance or COP This varies over a year and the overall ratio of heat out to power in is called โseasonal performance factorโ
- The usual system of space heating in the UK uses water-filled radiators in each room with the water heated by a central gas- fired boiler. This is familiar, quiet and easy to control. * The air source heat pump provides a simple replacement for the gas boiler. This is important for the large number of retrofits needed, but also works for new-build However the need to run at a low heat transfer temperature means that larger radiators may be needed compared with the gas boiler The other alternative, transferring heat directly from the refrigerant to air in a fan coil unit is not popular in the UK at present, requiring new installer skills and increasing noise in rooms
- So as a means of providing low carbon heating, air source heat pumps have some advantages * not as expensive as ground source, or biomass heating The heat emitters are standard products, the internal environment is basically unchanged Installers are doing much the same job as before The hot water system works as before And some disadvantages * The heat pump is more expensive than a gas boiler, especially when hot water storage needs to be added and before the boiler provided hot water instantaneously To run at lower water temperature requires bigger radiators The outdoor unit contains the compressor and a fan โ this will make some noise which might be a nuisance Specialists in heat pumps are in demand Many refrigerants are limited in the maximum temperature hot water they can provided
- Now to look at our experience of air source heat pumps with passivhaus dwellings. This project used a conventional HFC heat pump and radiators
- The design heat load from PHPP was 1.5 kW Following UK design guidance of lower external temperature and ignoring gains the radiator system was designed to provide 2.8 kW at a mean radiator temperature around 41 C, which is expected from a 45 C flow temperature. This is higher than we think we need, so could run at mean radiator 34 C, though as you can see from the graph as radiator temperature approaches room temperature the drop off in output is rapid. Here roughly halving the output reduces flow temperature by 7 K, but we would hope to see COP improvement
- However heat pumps have awkward characteristics Inside these heat pumps there is a simple rolling piston compressor
- The roller in the middle is off centre so as it rotates clockwise here the gas is compressed, Shown going from blue to orange to purple to red, Until it reaches exit pressure
- Increase sink temperature, ie increase output pressure, โ compression ratio increases โ more work for motor Increase source temperature, ie increase input pressure โ more gas in so higher mass flowrate So lower compression ratio but still more work for motor As source temperature increases the COP goes up but so does output This is opposite of what we want for a space heating application
- COP is similar But heat output is flat and compressor power reduces with increasing source temperature This is achieved with a variable speed compressor But note that minimum here is just under 3 kW
- We donโt want cycling Each time the compressor starts it needs to get the system back up to operating pressure and temperature โ the losses are significant Detailed monitoring by Limerick Institute of Technology showed COP could be halved with > 4 cycles per hour
- With a higher flow temperature the in use performance has matched the PHPP estimate The heat pump must run for < 50% of the day, but can run for longer periods Controls are for the comfort temperature to be maintained for 7 hours, then set back a little for 17 hours โ if the house is warm the heat pump will be off during this period
- Hot water storage temperature is set low to improve efficiency This works better than predicted More investigation needed to find out why
- In this example carbon dioxide is used as a refrigerant This has low global warming potential, which is good, but also interesting properties as a refrigerant
- Unlike conventional refrigerants carbon dioxide doesnโt have a clear phase change temperature at the heat output end of the cycle In practice this means that there is no condensation during heat rejection so the refrigerant can deliver heat over a range of temperatures, and the COP will be based around the mean heat rejection temperature rather than maximum. For hot water heating applications where cold water is heated from say 10 C to 60 C this offers a better COP than conventional heat pumps This seems like a nice fit for passivhaus โ we have relatively high hot water demand compared with heating demand
- So this one doesnโt work as well as expected, especially on heating. Here the occupancy and hot water use is less than usual, which means the hot water performance is not actually that useful
- We have another example with the same type of CO2 heat pump
- Here the hot water has delivered as hoped However again the heating COP has measured only 2 in reality
- Conventional refrigerant heat pumps maintain similar COP over the range of motor speeds CO2 heat pumps seem to suffer lower COP at lowest output โ we arenโt really sure why
- Radiators can work well with heat pumps, but make them big The system must be able to take the continuous heat output of the heat pump To improve COP with radiators at 35 C we will need big radiators even in a passivhaus
- With conventional heat pumps using a low storage temperature is key to good energy performance Carbon dioxide heat pumps have a role in large hot water systems such as hotels or sports halls
- ASHP and radiators can provide economical low carbon heating CO2 heat pumps disappointing The smallest heat pumps on the market are too big for these houses โ heating smaller houses effectively is a problem we have yet to solve