SBMS Posted March 13, 2022 Share Posted March 13, 2022 Just wondered if anyone can confirm (or pull apart) any of my logic regarding Solar PV to run DHW vs a Heat Pump. Say that required DHW demand is 3888kWh per annum. Say electricity is 30p per kWh. Assuming a CoP of 3.5 on the heat pump, it would need an input of 1100kWh of energy. So to 'run' the system via the heat pump would cost £330 per annum. If this were run using PV generation on a 4kWh system, my estimate is that it could provide circa 3000kWh of this requirement, leaving approximately 888kWh. I'm not sure if this deficit would be picked up by the heatpump or if its either immersion or heat pump mode? If the remaining 888 had to be from the immersion, thats a cost of £266 per annum. If it can be served by a heatpump then the cost is £76. Assuming (optimistically) that the heatpump can pick up the rest, the PV would save £254 per annum over running DHW via the heatpump. Does this sound right? I'm guessing if it does the answer is "don't use PV to reduce electricity usage on things that your heatpump can power"? i.e. get the PV to bring down general electricity consumption that has a CoP of 1?? Link to comment Share on other sites More sharing options...
ProDave Posted March 13, 2022 Share Posted March 13, 2022 It is never as simple as you think. Solar PV is a good companion to an ASHP but it is wrong to think of it in terms of "solar PV will heat my DHW" The reality is "Solar PV will reduce my electricity bill" The trouble is solar PV is maximum in the summer and of course only in the daytime, with max output at mid day. so you have to think how can I shift as much electricity usage to close to mid day as possible? Three things work for me. No 1, I have the ASHP timed to start heating DHW at 11AM, there is a reasonable chance of decent solar PV generation by then. No 2 use the big appliances one at a time around mid day. and No 3 install a solar PV diverter, so if nothing else is using the PV generated power, it will go to the immersion heater. 3 1 Link to comment Share on other sites More sharing options...
sam Posted March 14, 2022 Share Posted March 14, 2022 It becomes even more complex if you have off-peak (e7) electricity as well! Do you set the heat pump to come on in the day when there might be solar generation or in the night when electricity is half the price? 1 Link to comment Share on other sites More sharing options...
J1mbo Posted March 14, 2022 Share Posted March 14, 2022 (edited) Battery storage is a key requirement. I'm looking at this as well, the idea is to set the ASHP to draw less than the inverter is capable of producing for DHW generation, then tweak via automation the cylinder set point (upwards) when batteries are approaching full and still charging, maybe 5°C up each time, to help use the energy on-site. Since the ASHP will multiply the PV energy generated, when eventually the batteries are full and the tank at 70°C the there will be more left over for feed-in than if an immersion diverter were to be used (and it will cost nothing to implement other than my time). At least that's the theory. Edited March 14, 2022 by J1mbo Link to comment Share on other sites More sharing options...
ProDave Posted March 14, 2022 Share Posted March 14, 2022 46 minutes ago, J1mbo said: Battery storage is a key requirement. I'm looking at this as well, the idea is to set the ASHP to draw less than the inverter is capable of producing for DHW generation, then tweak via automation the cylinder set point (upwards) when batteries are approaching full and still charging, maybe 5°C up each time, to help use the energy on-site. Since the ASHP will multiply the PV energy generated, when eventually the batteries are full and the tank at 70°C the there will be more left over for feed-in than if an immersion diverter were to be used (and it will cost nothing to implement other than my time). At least that's the theory. I find I can self use almost all of what a 4kWp solar PV generates just by self use and the immersion heater diverter, which makes the case for batteries harder to justify. It really only seems to be a benefit if you have a larger PV array so harder to self use it all. 1 Link to comment Share on other sites More sharing options...
SBMS Posted March 14, 2022 Author Share Posted March 14, 2022 4 hours ago, ProDave said: I find I can self use almost all of what a 4kWp solar PV generates just by self use and the immersion heater diverter, which makes the case for batteries harder to justify. It really only seems to be a benefit if you have a larger PV array so harder to self use it all. What's your occupancy pattern? Do you have people home during peak solar hours? Or is most of your PV generation going to the immersion heater diverter? Link to comment Share on other sites More sharing options...
ProDave Posted March 14, 2022 Share Posted March 14, 2022 2 minutes ago, SBMS said: What's your occupancy pattern? Do you have people home during peak solar hours? Or is most of your PV generation going to the immersion heater diverter? We are both semi retired so usually one of us is about for part of the day. If we are going to be out, we will set the dishwasher or washing machine on a timer to come on about mid day. something like 1/3 of what the panels generates goes to the immersion heater. Link to comment Share on other sites More sharing options...
joth Posted March 14, 2022 Share Posted March 14, 2022 8 hours ago, sam said: Do you set the heat pump to come on in the day when there might be solar generation or in the night when electricity is half the price? I've set our heat pump to do DHW on a number of criteria: 1/ between 0030 and 0430 (Octopus Go cheap tariff) 2/ whenever the PV has been putting out greater than 3kW sustained average over the last 2 mins. (It has a minimum "ON" time of 10mins, but remains on as long as the solar is >3kW and the tank is below the 50-deg target) 3/ whenever there's a surge in use of DHW (defined by the mid-point of the UVC drops below 40 deg C) 4/ manual "boost" button pressed in the loxone app. So far, this has never allowed us to run short, and never had to use #4. #2 is a rough heuristic for "is there enough spare generation to run the ASHP". it's not great as doesn't allow for the kettle or oven immediately going on, or sun dipping behind the cloud, but it seems pretty good on average. 1 Link to comment Share on other sites More sharing options...
SBMS Posted March 14, 2022 Author Share Posted March 14, 2022 2 minutes ago, joth said: I've set our heat pump to do DHW on a number of criteria: 1/ between 0030 and 0430 (Octopus Go cheap tariff) 2/ whenever the PV has been putting out greater than 3kW sustained average over the last 2 mins. (It has a minimum "ON" time of 10mins, but remains on as long as the solar is >3kW and the tank is below the 50-deg target) 3/ whenever there's a surge in use of DHW (defined by the mid-point of the UVC drops below 40 deg C) 4/ manual "boost" button pressed in the loxone app. So far, this has never allowed us to run short, and never had to use #4. #2 is a rough heuristic for "is there enough spare generation to run the ASHP". it's not great as doesn't allow for the kettle or oven immediately going on, or sun dipping behind the cloud, but it seems pretty good on average. What platform do you use to tie this automation logic together out of interest? Link to comment Share on other sites More sharing options...
SBMS Posted March 14, 2022 Author Share Posted March 14, 2022 3 minutes ago, joth said: 1/ between 0030 and 0430 (Octopus Go cheap tariff) Out of interest, under what scenario is 1/ not sufficient? I'm making an assumption but a 12kw heat pump would heat 300l of water from 12 degrees to 55 degrees in about 1h 15 mins. Just wondered why the 4 hour window wouldn't be enough? Link to comment Share on other sites More sharing options...
ProDave Posted March 14, 2022 Share Posted March 14, 2022 19 minutes ago, SBMS said: Out of interest, under what scenario is 1/ not sufficient? I'm making an assumption but a 12kw heat pump would heat 300l of water from 12 degrees to 55 degrees in about 1h 15 mins. Just wondered why the 4 hour window wouldn't be enough? A 300l tank heated to 48 degrees is not enough for 24 hours if 3 people all have a shower plus other uses so it may need topping up later in the day Link to comment Share on other sites More sharing options...
joth Posted March 14, 2022 Share Posted March 14, 2022 36 minutes ago, SBMS said: What platform do you use to tie this automation logic together out of interest? Loxone, with various home-brew integrations to make it play with the FTC6 acceptably 33 minutes ago, SBMS said: Out of interest, under what scenario is 1/ not sufficient? I'm making an assumption but a 12kw heat pump would heat 300l of water from 12 degrees to 55 degrees in about 1h 15 mins. Just wondered why the 4 hour window wouldn't be enough? As Dave says, if there's 10 people in the house each taking a bath it will obviously need more than 1 charge a day, but the point of #2 is to optimize for Solar self-use rather than avoid running short. Generally we shower in the morning, then if it tops up midday from solar it will be good through to the following morning. (Hence I sent the midday target temp a little higher than the overnight one, to help it ride through) (Arguably as octopus Go cheap rate is the same as my SEG export rate, 5p/unit, this is pointless, but I don't want to assume that will remain that way for any length of time) Link to comment Share on other sites More sharing options...
SBMS Posted March 14, 2022 Author Share Posted March 14, 2022 1 hour ago, ProDave said: A 300l tank heated to 48 degrees is not enough for 24 hours if 3 people all have a shower plus other uses so it may need topping up later in the day A family of 4 (baby and toddler) - would you spec a larger than 300l tank? Link to comment Share on other sites More sharing options...
SBMS Posted March 14, 2022 Author Share Posted March 14, 2022 43 minutes ago, joth said: Loxone, with various home-brew integrations to make it play with the FTC6 acceptably As Dave says, if there's 10 people in the house each taking a bath it will obviously need more than 1 charge a day, but the point of #2 is to optimize for Solar self-use rather than avoid running short. Generally we shower in the morning, then if it tops up midday from solar it will be good through to the following morning. (Hence I sent the midday target temp a little higher than the overnight one, to help it ride through) (Arguably as octopus Go cheap rate is the same as my SEG export rate, 5p/unit, this is pointless, but I don't want to assume that will remain that way for any length of time) Gotcha thanks. Link to comment Share on other sites More sharing options...
J1mbo Posted March 14, 2022 Share Posted March 14, 2022 1 minute ago, SBMS said: A family of 4 (baby and toddler) - would you spec a larger than 300l tank? How strong is your floor Link to comment Share on other sites More sharing options...
SBMS Posted March 14, 2022 Author Share Posted March 14, 2022 5 minutes ago, J1mbo said: How strong is your floor Cylinder is going on ground floor (solid block and beam) Link to comment Share on other sites More sharing options...
RobLe Posted March 14, 2022 Share Posted March 14, 2022 33 minutes ago, SBMS said: A family of 4 (baby and toddler) - would you spec a larger than 300l tank? Due to our first child having eczema when younger, we've mostly avoided the whole bathtime routine thing as it removes natural oils from skin, causing dryness and problems. It has also saved a lot of energy not doing it daily! Family of 4, now 1 teenager + 1 at Uni. We have a 200l tank, it's more than enough, we are probably light users though - the water meter says we average 50l/person/day for everything. Link to comment Share on other sites More sharing options...
SteamyTea Posted March 14, 2022 Share Posted March 14, 2022 59 minutes ago, RobLe said: the water meter says we average 50l/person/day for everything That is commendably low. Link to comment Share on other sites More sharing options...
JohnMo Posted March 14, 2022 Share Posted March 14, 2022 One thing not mentioned is during winter you will get very little hot water from PV. As it doesn't generate much. Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now