Modeling Energy Consumption of the Heat Pump
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Can anyone recommend tools that an energy modeler may use to estimate the energy consumption (in kWs) to run a heat pump in a dwelling? We have created a heat loss calculation of the building in ManualJ. We have ONE heating load estimate for this building, which is based upon a single design temperature, but this is a point source heat load estimate. To know how much electricity the heat pump will consume at each outdoor temperature expected for the building site and the building itself is not known to us, but we need to know this information, at least roughly, to know how much the space heating will be met by the energy that is produced annually by the building’s existing PV system.
Is this something that can be quantified in OpenStudio or BEopt, and if so, is there an example workflow or plugins that would be needed to perform this modeling exercise?
Thanks for any suggestions
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Depends on how detailed you want. A simple way would be to take your heat loss in kw, 10kw for example, divided by (65F-design tempF). So if design temp is 15F, you’d get .2kwh/HDD hour. Then multiple that by annual heating degree days using a base of 65, then by 24. So 4000 HDD * .2 *24 = 19200kwh/year. Divide that by annual COP to get energy input needs. So around 6400kwh/year if average COP = 3.
What's tricky is that for combustion fuels a degree-day is a degree-day, but for heat pumps the COP varies with the outdoor temperature. It also depends on how the heat pump is sized, particularly if it has auxiliary resistance heat.
I'm not aware of any tool that takes that into consideration.
You could break it down by temperature bin. Or just use an average.
BEOpt (and OpenStudio, or any other tool built using EnergyPlus as the simulation engine) will vary heat pump capacity and input power as function of indoor and outdoor conditions (as well as partial loading and defrost) However, it is unlikely that the performance curves used in BEOpt align with your heat pump (not sure if the curves got updated with the recent BEOpt update, but default curves tend to be based on 10+ year old heat pumps).
i say give BEOpt a shot and see what happens, just don't make any significant decisions based solely on the results.
or just use a COP of 2.5 with either of the other options proposed
Sound like this is an existing building.
If so, I think you will get much more accurate data from your past fuel usage than from any manual J calculation.
Also this is a more accurate way to size your equipment. https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Take the amount of fuel used and convert that number to BTUs it put into the home. Divide the BTUs by 3412.14 =kWh of resistance electric heat. Divide kWh number by the COP of you HP and you have your answer.
Lets say you used 500 Therms of gas last year with a 60% furnace.
500 x 99976.1= 49988050 BTUs x.6= 29992830 BTUs delivered indoors /3412.14= 8790 kWh /4= 2197.5 kWh with the heat pump.
Walta
As someone who has converted from oil fired baseboard to mini splits, I think the problem in making this determination in actual practice is that "comfort" is so subjective. In most houses not built to very tight specs, heat from point delivery systems- like several indoor mini split units -blowing hot air is less uniform than say, well designed radient or baseboard systems (or even a well designed ducted forced air). Thus, you may need to generate significantly more btu's from the mini splits to make up for this comfort differential.
For instance, in our household, the primary "comfort" decision maker (the "boss") seems to insist on much higher temperatures coming from the air handlers than would the occupant (the "supplicant") who is more interested in energy efficiency/CO2 reduction. The result is that although our overall efficiency has improved over the oil fired baseboard, the supplicant suspects that we are using more kwh than we would need to just to keep up with the total btus in heat output or than an energy calculation based on Manual J or prior energy usage would have suggested.