PDD practice question about heating and cooling
In question below I don't understand why answer D would be wrong?
An architect is designing a single-family home in a location with cold winters and mild, dry summers. The homeowner is concerned about climate change and would like the home to have the lowest operating cost and the smallest carbon footprint possible. The local electrical grid is supplied with photovoltaic and nuclear power but not power from combustion sources.
Which one of the following systems is most appropriate?
|A.||Ground source heat pumps for space and water heating with reversing valve for summertime cooling.|
|B.||Local, sustainably harvested wood pellet space and water heating furnace with heat pump cooling.|
|C.||Direct-fired natural gas space and water heating with whole house heat exchanger forced air ventilation.|
|D.||Electrical resistance space and water heating system coupled with heat pump for summer cooling.|
Ground source heat pumps for space and water heating with reversing valve for summertime cooling.
While heat pumps require electricity to drive the refrigeration cycle, the electricity in this location is not generated by fossil fuel combustion, addressing the homeowner's concern about carbon footprint.
Not a distractor at all….making electricity from burning fossil fuel is wildly inefficient. It might be that more than half the energy is wasted as heat goes up smokestacks in the power plant and as electrons overcome resistance in the lines…so in those cases, directly heating your house with natural gas may be better for the climate than using dirty electricity made from coal…heat pumps have become much more efficient, but air coupled heat pumps still only do so well in very cold weather…so given the priorities here, using clean electricity more clearly disfavors the natural gas option
There are two criteria given:
- Operating cost
- Smallest carbon footprint possible
Because you are told that the electricity is produced without combustion, Options A and D satisfy criteria #2. However, Option D is inferior to Option A with respect to operating cost (criteria #1) because Option D requires the homeowner to buy more electricity: it involves electrical resistance space heaters (option A does not), and the heat pump in option D is not ground coupled, so it will require more electricity to provide the same level of heating or cooling.
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