Would this work?

[John Keith]

Please excuse my ignorance, I'm just trying to learn as much about ASHP's and MVHR systems as possible, prior to building.

I understand that when the outside temperature drops in winter, heat pumps are not as effective but, could a polycarbonate / plexyglass box be placed around the input?

Having worked on building sites in winter, driving old plant machinery, I know that it's warmer inside the cab even without a heater. My thought is, assuming there is winter sunshine, whilst not being 'summer hot', the sunlight still has a modicum of heat to it, and a transparent plastic casing would encourage heat capture. I've never seen it done so I assume there's a reason it hasn't, but wondered why?

[Iceverge]

Nice idea but unfortunately The heat pump would move far too much air far too quickly through your poly-carbonate box to allow for any noticeable gain in intake temperature.  

 

A 10kw heat pump can move up 5000m3 air per hour through the heat exchanger. Thats about equilivant to an average houses volume every 4 minutes. 

 

Now if you had an industrial glass house handy it might work. 

 

However in that case you'd be better off putting the house inside! 

 

 

[joe90]

This efficiency drop with temperature is not as bad as some make out, it’s more about relative humidity and low temperatures that tends to drop the COP. Someone will be along in a moment with actual figures but I did ask this question some time ago ( @SteamyTea ??).

yes, enclosing will not work because of the huge amount of air required.

[ProDave]

@John Keith where abouts are you that you should be worried about an ASHP not liking the cold?  If mine works okay in a "sheltered Highland Glen" with a cold winter climate, I am sure they will work anywhere in the UK.

[Thedreamer]

The approach seems  similar to what we have achieved with the joule aero.

 

[SteamyTea]

There are three aspects to how the air part of an ASHP work.  One is the relative difference between the cold side of the HP and the external air.  Two is the amount of air moved. Three is the chances of the external radiator frosting/freezing up, that is related to humidity.

 

The amount of specific energy in air is pretty fixed at the temperatures they work at in the UK (there is a slight drop as the temperature drops, but not much, about 0.8PPM over 60K  https://www.engineeringtoolbox.com/air-specific-heat-capacity-d_705.html).

What can happen is the if the ASHP is running at close to full capacity when highly water saturated air hits the cold side, it can freeze, this both impedes the airflow and insulated the fins.  This is why there is a defrost system built in.  This uses little energy in the scheme of things, so not worth worrying about in a  correctly sized system.

The most likely temperature to cause freezing is not 0°C (273.2K), but ~4°C as this is when water is at its most dense, so there is more to condense out than at 0°C or below.

 

As @Iceverge points out, they shift a lot of air, so location i.e. sun or shade, makes no real difference.  Fit them where there is good airflow and the external pipework/wiring is short.

Don't try to run an ASHP at its highest rated temperature, that is like driving a car at top speed all the time, it will end in problems (I suspect a gas or oil boiler would break, if it was run 24/7 at full rated power, fairly quickly).

 

If you get into calculating thermal efficiencies, they stop using the celsius scale and use the kelvin scale, the kelvin scale is a truer model of the kinetic energy of air molecules, and there is no minus temperatures to fool you.

 

Edited September 1, 2021 by SteamyTea

[SteamyTea]

1 hour ago, Iceverge said:

A 10kw heat pump can move up 5000m3 air per hour through the heat exchanger.

Should really convert that to kilograms, then it is pressure and temperature independent.

PV/T =C