I'm contemplating installing a heat pump on our flat roof. We live in a row house so there's no other practical option for the condenser.
I've done a lot of research and it seems there are a lot of unsatisfied customers who have to deal with noise and/or vibration (start/stop, defrost mode, etc) from these types of units on their roofs.
This type of setup seems to be the best way to dampen the noise, though I'm not sure how to go about obtaining the materials:
I thought to expand a bit on user the answer provided by lshaver above. The following discussion concerns the use of an air heat pump though the discussion of some of the mechanics also applies to a geothermal heat pump. I would expect that a geothermal heat pump does not depend to any great extent on the placement of the outside unit since it uses geothermal source/sink for the heat rather than air.
The bottom line
The outside condenser unit should be in an environment that provides an air temperature as close to the desired air temperature of the inside evaporator unit as possible.
For warmth, a Southern exposure with provisions to create a warm micro climate in the area of the outside unit is preferable.
For cooling, a Northern exposure with provisions to create a cool micro climate in the area of the outside unit is preferable.
In both cases due to the amount of airflow through the condenser, the area of the micro climate will need to be large enough to handle the volume of air that flows through the unit.
However also see this article, Heat Pump Efficiency | At What Temperature Is a Heat Pump NOT effective?, about two types of heat pump units, the split heat pump and the central/whole-house heat pump which do have different characteristics.
The details
Looking at the link at Engineering Toolbox he provided there is an additional graphic that is important.
What this is saying is that as the temperature difference between the inside air temperature and the outside air temperature gets larger, the ability of the heat pump to move heat between the inside and the outside gets smaller.
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Using a heat pump in the winter to warm a home, the heat pump is more efficient when the outside temperature is warmer and closer to the setpoint for the internal air temperature. This is why heat pumps are not desirable in cold climates and why there is a radiant heat device built into the heat pump to handle when the temperature difference is large enough that the heat pump isn't able to pump sufficient heat from the outside into the inside.
The reverse is the case for a heat pump in the summer to warm a home. As the outside temperature becomes greater than the internal temperature, a point is reached at which the heat transfer from inside to outside is too small to be useful.
I'm in a warm climate, the southern part of the state of Georgia in the Southeast United States, where summertime temperatures (Fahrenheit) are routinely in the 80s and 90s. Winters are mild with the temperature seldom dropping into the 20s.
I found this article, Does Your Air Conditioner Need Shade? – Energy Efficiency Myths, which has links to a couple of research articles. The research question was whether shading the outside condenser unit of a heat pump improve efficiency by cooling the unit.
Research by the Florida Solar Energy Center, Measured Impacts of Air Conditioner Condenser Shading, indicates that shading the outside unit provides negligible benefit. They also found that plantings or surrounding walls/roofs to provide shade must not interfere with airflow through the unit nor redirect airflow so that air coming out of the outside (condenser) unit mixes with air entering the outside unit since that affects the temperature of air entering the unit.
The probable explanation for the lower than expected performance in the experiments is that outdoor AC units draw in a volume of air that greatly exceeds that of the nearby shaded air volume. Air-cooled condensers move a quantity of 600 - 1200 cfm of air per ton (80-160 L/s/kWt) of cooling capacity (ASHRAE, 1992). For instance, a typical 3-ton (10.6 kWt) air conditioner's 300-W condenser fan would draw 2,800 cfm (1321 L/s) of air at a very low static pressure across the coil (Proctor et al., 1994). Thus, the unit would process 168,000 cubic feet (4.76 x 106L) of air per hour. Assuming no mixing, this would represent a volumetric equivalent to a cube of air with sides measuring 55 feet (16.8 m).
The article doesn't mention the effect of turbulence on airflows but I would expect any installation that may generate turbulence within the airflows would reduce efficiency as well.
The article has a final conclusion for their results on testing cooling.
We conclude that any savings produced by localized AC condenser shading are quite modest (<3%) and that the risk of interrupting air flow to the condenser may outweigh shading considerations. The preferred strategy may be a long-term one: locating AC condensers in an unobstructed location on the shaded north side of buildings and depending on extensive site and neighborhood-level landscaping to lower localized air temperatures.
Looking at the average temperature in the UK, the range seems to be from around 40F (5C) to around 70F (20C) though it seems I've read of some above average days in in the 80s Fahrenheit.
Monthly average daily temperatures in the United Kingdom (UK) from 2015 to 2021
Looking at the above chart, it appears that average temperatures are well within the efficient operating range of most recent air type heat pumps and unobstructed airflow is the main consideration for the placement of the unit,
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