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Author Topic: Air to Air heat pump running off spare PV?  (Read 2264 times)
Sharkbait
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« on: March 07, 2016, 01:12:25 PM »

Sorry if this seems stupid but I've not found the answer on here yet.
We also have a holiday house by the sea that is heated purely by electric panel heaters and I'm quite keen to reduce the amount it costs to run. 

We have 3Kw of PV there and I was wondering if it was possible for an air2air to be run off the spare PV production while we weren't there?  Other than a fridge, a couple of Raspberry Pi's and the fibre router there's basically nothing electrical running at the house so there can be 1.5-2Kw going spare.  I already have an immersun type device heating the hot water but I can switch this off while we're not there giving all the spare production to the heat pump.

The house is upside down (so living/kitchen/1 bedroom upstairs and 3 bedrooms/bathroom downstairs.  At the moment only one of the bedrooms is used downstairs so ideally I'd have a multi split system with a single compressor outside and 3 indoor units, one in the downstairs bedroom, one in the hall (gets very little sunlight and is the coldest part of the house) and one upstairs in the [open plan] living/dining/kitchen.

I'm not bothered about the house not getting any heat on some days, as there's no heating running while we're not there, but it would be great to maximise the PV production and give a bit of background heat whenever possible.  For example, the PV is heating the hot water at the moment but it is probably already hot and so todays power could be heating the house to some extent.

So is this possible just using an immersun device or would I need to have a raspberry pi switch a relay when the PV is producing more than x amount of power - or can some heat pumps handle PV themselves?

(obviously I may be talking rubbish as heat pumps may not be able to handle the variable power from a PV installation)
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RIT
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« Reply #1 on: March 07, 2016, 01:52:43 PM »

You will need a configuration with some form of battery store as the heat pump when running will need a constant supply with a constant available current that can correctly operate from. The reason for this is that while you can use an immersun type device to heat water all it is doing is sending energy to an immersion heater which is basically just a metal bar with a high resistance. A heat pump is a far more complicated device that once operating needs a constant energy source and would not cope with being switched on and off as your RP sees different levels of PV output.

One problem I think you will find is that you will then have to take the battery bank and invert it back to 230V as the only 12v heat pumps on the market seem to roof installed caravan units. While Navitron lists a number of low cost off-grid 12V inverters, they are all modified sine wave units and I'm not sure they would work well with a heat pump. Pure sine wave devices are available from other vendors, but they have a much higher cost and the unit you select will have to cope with the peak current draw of the heat pump.

The aim of the battery store will be to balance the supply of energy from the PV rather than trying to run the heat pump without PV as such it can be small in size. You can then use your RP to monitor the batteries so that they do not discharge to much. With an RF transmitter your RP should be able to disable/enable the heat pump directly which will be far better than just turning it off and on at the socket.
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Sharkbait
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« Reply #2 on: March 07, 2016, 02:52:06 PM »

I was wondering about the intermittent supply issue! 
I think I could just keep it simple by using the rpi that monitors the inverter to just switch the heat pump on when it saw that pv production was over a set level.  The heat pump would then stay on for a set amount of time and if the PV output dropped during that time I would just have to suck it up!  If the PV production was still over the trigger level then the pump would be started again to run for another x amount of time.
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billi
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« Reply #3 on: March 07, 2016, 02:55:53 PM »

 www.china-aircon.com/detail-10016623/48v-dc-powered-solar-ac-new-version.html


Hi , I am in contact with those guy s  for direct DC  PV heatpumps with battery .....
Perhaps  something for you

Regards billi
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1.6 kw and 2.4 kw   PV array  , Outback MX 60 and FM80 charge controller  ,24 volt 1600 AH Battery ,6 Kw Victron inverter charger, 1.1 kw high head hydro turbine as a back up generator , 5 kw woodburner, 36 solar tubes with 360 l water tank, 1.6 kw  windturbine
RIT
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« Reply #4 on: March 07, 2016, 03:08:09 PM »

I was wondering about the intermittent supply issue! 
I think I could just keep it simple by using the rpi that monitors the inverter to just switch the heat pump on when it saw that pv production was over a set level.  The heat pump would then stay on for a set amount of time and if the PV output dropped during that time I would just have to suck it up!  If the PV production was still over the trigger level then the pump would be started again to run for another x amount of time.

The issue is that a heat pump is not going to like being stopped and started frequently (known as shortcycling). This will be made worse by the fact that you will be turning it on and off at the mains socket rather than via its own controls. Another problem is that the heat pump has its own current drawing cycle of on and offs so the RP will not be monitoring a constant draw.  

With batteries in the design the RP will have some certainty that it can allow the HP to operate for say 30mins at a time and maybe even extend that by telling the heat pump to switch to eco mode rather than off.


One key question - is this house also connected to mains electricity? It is not clear from your post. If it is then forget all the complexity and you just need your RP to control the heat pump via an IF or RF transmitter so that it can start/slow/stop correctly over the right time periods.
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Why bother? - well, there is no planet B
Sharkbait
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« Reply #5 on: March 07, 2016, 03:14:01 PM »

Sorry, I should have made that clear ...... yes, connected to the grid.  I've already found one chap who uses his rpi and a ir transmitter to switch his Mitsubishi heat pump on and off!

Thanks Billi - I'll take a look at that.
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Tinbum
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« Reply #6 on: March 07, 2016, 03:16:06 PM »

I was wondering about the intermittent supply issue! 
I think I could just keep it simple by using the rpi that monitors the inverter to just switch the heat pump on when it saw that pv production was over a set level.  The heat pump would then stay on for a set amount of time and if the PV output dropped during that time I would just have to suck it up!  If the PV production was still over the trigger level then the pump would be started again to run for another x amount of time.
I run heat pumps to heat a swimming pool on a similar basis as above but using an Arduino. I do have a lot of PV though and have the benefit of not having to heat when its winter and low PV output.

If you want to do it that way I would select a small heat pump that will run longer but with less input required.
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RIT
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« Reply #7 on: March 07, 2016, 03:26:37 PM »

Sorry, I should have made that clear ...... yes, connected to the grid.  I've already found one chap who uses his rpi and a ir transmitter to switch his Mitsubishi heat pump on and off!

Thanks Billi - I'll take a look at that.

Then I think you will have an easy project.

One thing about heap pumps, look for one that states that it is an "Air conditioner inverter"

      https://en.wikipedia.org/wiki/Air_conditioner_inverter

This will result in its current draw dropping as you select things like the eco mode, rather than it pulling the same high current for shorter periods of time. This will match what you are trying to achieve with your PV/RP configuration.
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Why bother? - well, there is no planet B
Sharkbait
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« Reply #8 on: March 07, 2016, 03:39:49 PM »

Thanks RIT, good info.
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mike7
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« Reply #9 on: March 08, 2016, 11:46:53 AM »

FWIW I have a 2.5kW Hitachi aircon which works well with my 3.6kW PV - it draws up to 1100W flat out, but mostly runs at 800W or so, a good match for the PV on a dull day like today. It is inverter driven and starts very gently - no big startup load - and it modulates down as the room temp approaches the 'set' temperature.

One further thought - it might be possible to get the current draw to follow the available PV output better than just switching on/off at set levels. The modulation could be controlled by adding resistance in parallel with the thermistor used to measure the room temp, so the aircon control would be fed a false idea of the temp so as to get the modulation you want.  I don't know enough about either electronics or the immersun/PV end to suggest what would need to be done there but maybe someone here would?

On both the Hitachi and a Midea aircon the thermistor was easy enough to find - it has to be somewhere reasonable well exposed to the incoming air, after all.
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RIT
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« Reply #10 on: March 08, 2016, 06:10:38 PM »

FWIW I have a 2.5kW Hitachi aircon which works well with my 3.6kW PV - it draws up to 1100W flat out, but mostly runs at 800W or so, a good match for the PV on a dull day like today. It is inverter driven and starts very gently - no big startup load - and it modulates down as the room temp approaches the 'set' temperature.

One further thought - it might be possible to get the current draw to follow the available PV output better than just switching on/off at set levels. The modulation could be controlled by adding resistance in parallel with the thermistor used to measure the room temp, so the aircon control would be fed a false idea of the temp so as to get the modulation you want.  I don't know enough about either electronics or the immersun/PV end to suggest what would need to be done there but maybe someone here would?

On both the Hitachi and a Midea aircon the thermistor was easy enough to find - it has to be somewhere reasonable well exposed to the incoming air, after all.

Inverter based air con units already control the compressor motor using the same method as immersun type devices control the energy flow to immersion heater elements so there is no advantage in going down that route. Also most air con units have parts that need a true 230v supply to operate correctly, so feeding a unit from an immersun device would most likely cause a range of problems.

One possible way to control the air con unit will come from being able to change its target temperature via IR signals. I do not know what format the signals are but if the remote sends comments such as "set target to 23c" all that will need to happen is for the RP to change the target temp to near the current ambient temp and the inverter air con unit will then lower its power usage. This maybe easier (if you are using a RP) than directly interfacing via the thermistor.
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Why bother? - well, there is no planet B
Tinbum
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« Reply #11 on: March 08, 2016, 06:21:46 PM »



Inverter based air con units already control the compressor motor using the same method as immersun type devices control the energy flow to immersion heater elements so there is no advantage in going down that route.

I thought Inverter based units are controlled by frequency change whereas immersun type units turn the power on and off at differing rates based on spare pv output.
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85no 58mm solar thermal tubes, 28.5Kw PV, 3 x Sunny Backup 5048, 3x Sunny Island 5048, 2795 Ah (135kWh) (c20) Rolls batteries 48v, Atmos wood gasification boiler, Brosley wood burner, 2000lt buffer tank and 250lt DHW
RIT
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« Reply #12 on: March 08, 2016, 06:37:14 PM »



Inverter based air con units already control the compressor motor using the same method as immersun type devices control the energy flow to immersion heater elements so there is no advantage in going down that route.

I thought Inverter based units are controlled by frequency change whereas immersun type units turn the power on and off at differing rates based on spare pv output.

The general way these things work is via what is known as Pulse Width Modulation (PWM), there are cheaper solutions but at high loads PWM is a good option. Immersum provides some details here

       https://www.immersun.co.uk/wp-content/uploads/2014/09/immerSUN-FAQ.pdf

This matches the description of how an inverter air con works on Wikipedia

       https://en.wikipedia.org/wiki/Air_conditioner_inverter


If you think about it by turning the power on and off at different rates (ON - delay - OFF - delay - ON  etc) you end up with a DC wave (zero to N volts) form of a certain frequency as detailed here (it is just not a nice sine wave)

       https://en.wikipedia.org/wiki/Pulse-width_modulation

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2.4kW PV system, output can be seen at  - https://pvoutput.org/list.jsp?userid=49083

Why bother? - well, there is no planet B
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