Are there any studies or empirical evidence (or even guesses here) as to how solar gain at the outdoor coil may affect cold-climate heating performance of an air-to-air heat pump system?
I'm curious about this since I've noticed that there can be a significant spread between ambient air and surface temperature, simply due to the presence of direct sunlight. As an example, at my home my two heat pump units are located with a northerly exposure in the shadow of my house and never receives direct sunlight during the winter. The indoor thermostats will display outdoor temperature using the outdoor temperature sensor located at/in the heat pump. These readings I assume to be accurate and "as expected" since they read the same (+/- 1F degree), and correspond to the local weather forecast. Separately, I have temperature sensors located on the south-facing side of the house. These sensors are not shaded by structure, shrubs, enclosures or anything and receive direct sunlight as long as the sun is up. During the daytime, these south-facing sensors will read higher than those taken at the heat pumps, and in the wintertime the difference could be quite significant. For example, on a day forecasted to be 33F high, the outdoor temps at the heat pump read about 33F +/- 2, but the sensors under full and constant sunlight read 75 ~ 85F (depending on wind)! These readings are confirmed by two separate sensors, and in the absence of sunlight after normalization every outdoor temp reading overnight is approximately the same.
In reality it is obvious that the ambient (air) temperature is not so drastically different between sunny and shady sides of the house, but these observations show how solar radiation and solar gain can raise the temperature of a surface (i.e. the temperature probes) regardless of ambient temperature surrounding such surface.
This got me thinking, what if heat pumps were placed in a location that would receive the most direct sunlight to capture and benefit from the not inconsequential heat generated from solar radiation and gain, and thus improving wintertime COP. If the temperature on the fins/coil and any other heat exchange surface of the heat pump can be raised 40~50F above ambient by virtue of direct sunshine, that should seem to make the heat pump operate more efficiently especially in cold-climate regions. Defrost cycles potentially would be unnecessary during daytime. Even in the early evening when the sun is setting, there would seem to be an amount of retained heat in the heat pumps mass to aid in evening recovery.
On the contrary, how will such solar radiation and gain affect summertime performance when the heat pump is operating in air conditioning mode? I have noticed that although the temperature sensors in direct sunlight will still read higher in the summer, the spread between those readings and the temps taken at the heat pumps in shade is less, perhaps about 5-15F higher. In other words, the higher the ambient temperature, the lower the spread between surface temp and ambient temp; the lower the ambient temperature, the greater the spread. Thus any detrimental effects of placing the heat pump under direct sunlight for cooling might be less than any potential benefits gained for heating. This consideration may be particularly applicable for installed locations where annual heating demand is greater than cooling demand, timewise and energy-wise.