Green Building Forum - PV+ASHP - dream team or what?

PV+ASHP - dream team or what?

ratmin — Tue, 23 Jun 2009 15:09:07 +0100

I'm looking for research and monitoring info into real life use of PV combined with ASHPs. I'm working with people who want to use this combination but have no hard evidence to back up my personal doubts, which are based on my understanding that on grey cold days when heat is most needed PV produces nothing and COP of ASHP plummets. Any comments, ideas or links please?

PV+ASHP - dream team or what?

GBP-Keith — Tue, 23 Jun 2009 15:25:14 +0100

Your doubts are well founded ratman. Your people are asking too much. Heat pumps need far more electricity than any sensibly sized solar pv system can produce in the UK. I am totally off-grid using pv, wind and small hydro and I can only 'dream' of ever running a heat-pump.

PV+ASHP - dream team or what?

ratmin — Tue, 23 Jun 2009 16:24:28 +0100

thanks Keith for the speedy response - this is not a domestic situation and there is room for loads of PV (if they can afford it) but....
I guess I need to analyse their figures and do a reality check.

PV+ASHP - dream team or what?

sinnerboy — Tue, 23 Jun 2009 23:40:45 +0100

Posted By: ratmin Any comments, ideas or links please?

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=4174&page=1#Item_5

PV+ASHP - dream team or what?

CWatters — Wed, 24 Jun 2009 08:52:14 +0100

I think you would have to be very rich to size the PV array to supply the peak power needed by the ASHP in winter. Wouldn't it be better to size it to suit the average household consumption and use the grid to decouple supply and demand? For example in the summer sell excess electricity to the grid and buy it back in winter?

PV+ASHP - dream team or what?

djh — Wed, 24 Jun 2009 13:19:57 +0100

Posted By: CWattersFor example in the summer sell excess electricity to the grid and buy it back in winter?

That'll work well but it doesn't solve the planet's problem of reducing peak demand!

And it may not work so well if demand pricing becomes widespread! Sell it cheap in summer and buy it dear in winter. There's an interesting consultation at http://www.nationalgrid.com/uk/Electricity/Operating+in+2020 that talks about pricing among other things.

PS Welcome back "Keith"

PV+ASHP - dream team or what?

CWatters — Wed, 24 Jun 2009 13:41:03 +0100

Posted By: djh
Posted By: CWattersFor example in the summer sell excess electricity to the grid and buy it back in winter?

That'll work well but it doesn't solve the planet's problem of reducing peak demand!

Then have to hope they are rich :-)

PV+ASHP - dream team or what?

SteamyTea — Fri, 26 Jun 2009 16:16:35 +0100

If this is for a UK installation have they thought about using the electricity from the PV to directly heat storage heaters. Remember it is the control of the heaters that lets them down not the fundamentals of heat storage.
Reason I ask if it is a UK installation is that when it is cloudy here it tends to be warmer (in winter) than when it is clear so the usable power from PV can be better used.

I know that people hate storage heaters but if someone is going to spend thousands on PV then they can easily modify a standard storage heater to respond better to temperature differences.

PV+ASHP - dream team or what?

GBP-Keith — Fri, 26 Jun 2009 16:28:17 +0100

The real acid test would be how much heat the building actually needs. if it is a super-insulated building then a heat pump would hardly ever be on. In fact one would not be needed as the heat from the lights, appliances and people would be enough.

So if they have lots of money, get them to build/renovate to passivhaus standards or better.
hiya djh

PV+ASHP - dream team or what?

SteamyTea — Thu, 16 Jul 2009 09:42:10 +0100

Ratman

Have been pondering your original question and though I have no evidence, and now that the university is almost totally empty I cant ask anyone there either, I have thought up an idea. Hear me out and then lets rip it to shreds in the best academic fashion, then we can put numbers to it.

Option 1: PV charges batteries-->batteries run inverter-->inverter runs heat pump

Now we know that charging batteries is pretty inefficient (0.5) and ASHP have a COP of about 2 (open to debate I know), so no net gain there then.

Option 2: PV runs Storage Heater (can get elements made to run at low voltage)-->Storage Heaters heat house

Trouble with storage heaters is they are uncontrollable on a short time scale (I know as I live with them) but they do convert near enough all the electrical power to heat so efficiency pretty high (maybe 90%) and a cheap installation.

Option3: PV charges batteries-->Inverter (or re-wound heaters)--> Fan Heaters (or convection) when needed

Reasonably cheap option and controllable with switches, sensors and timers.

Could do a combination of Option 2 and 3, some storage heating, some 240V AC heating and some battery storage.

Comments anyone?

PV+ASHP - dream team or what?

CWatters — Thu, 16 Jul 2009 15:13:05 +0100

Posted By: SteamyTea Trouble with storage heaters is they are uncontrollable on a short time scale (I know as I live with them) but they do convert near enough all the electrical power to heat so efficiency pretty high (maybe 90%)

As far as I can tell electric heating (storage or otherwise) is 100% efficient. That includes fan heaters because stiring up the air with a fan also heats it. Even the fan noise is ultimately converted to heat (if you discount noise that escapes the house).

The "losses" from a storage heater are mainly due to heat leaking out at times when they are meant to be off.

PV+ASHP - dream team or what?

SteamyTea — Thu, 16 Jul 2009 16:03:38 +0100

CWatters

I tend to agree with you there, there are some losses in the cabling but they are not worth bothering with.
There are entropy losses, but again still very low in converting electricity to heat. I just picked 90% as a figure, no real basis for it other than it was not 100%.
More interested in the concept of turning PV electrical power to heat, either via a heat pump or through direct heating.

PV+ASHP - dream team or what?

pmcc — Fri, 17 Jul 2009 21:34:33 +0100

This looks like a technology solution looking for a problem. Why not just use a cheap flat plate solar panel to heat water? Flat plates will collect up to 5 times the amount of energy for a given area than PV. PV is an expensive way to acquire high-grade energy, so there must be better things to do with it than turn it into low grade heat.

Coupling an ASHP with solar thermal looks like an increasingly good option, assuming renewables make up a fast increasing proportion of electricity generation mix and the grid remains reliable. During the day water is heated by the sun and at night it's heated by the heat pump powered by the grid.

If current research bears fruit PV will come into its own. In a few years time PV panels may be much cheaper through new mass production techniques. And home electrical storage options may be much improved, with technology such as ultra-capacitors possibly allowing electrical energy collected during the day to be used at night.

PV+ASHP - dream team or what?

SteamyTea — Sat, 18 Jul 2009 07:55:42 +0100

PMCC

I think your right if you have the room to store the energy as hot water. What is really needed is better storage rather than better collection. Solar thermal collection has probably gone as far as is practical (except for micro concentrated) and PV does have a fundamental quantum limit of just over 30%. Water Source Heat Pumps are a very underused technology, every time I look into Penzance Harbour when the tide is in, I wonder why the town is not heated by it.
Greening up the grid does seem to be the way we are heading slowly along with reduced consumption through better insulation on dwellings.

The point I was making I suppose was that coupling ASHP with PV is am expensive option and there are alternatives.

PV+ASHP - dream team or what?

RobinB — Mon, 20 Jul 2009 09:56:40 +0100

ASHP and PV is the combination I'm going for, with Solar HW as well. Although the contribution from PV will be very small initially, hopefully as panel prices fall (I'm sure they will -but when?) I'll be able to add more.

PV+ASHP - dream team or what?

SimonH — Mon, 20 Jul 2009 13:07:02 +0100

How about an oversized solar thermal system, sized to support most of the winter heat, then...

A low pressure steam turbine or sterling engine to make the "waste" heat in summer into electricity and sell electric back to the grid or use it on site.

Given solar thermal is 5 times more effective at colleting suns energy(figure above not verified), and it's heating you actually want then the money may be better spent on lots of solar thermal which is also much cheaper per KW to buy. Rough guess less than Â£1000 /kw for tubes VS more than Â£3-4,000 per kW for PV.

I had a look a while ago and unformatuntely this kind of thing isn't available "off the shelf" yet, but a few people have been having a go. A really promising set up is from these guys... http://matteranenergy.com/

A set of clever plumbing stuff near to the themal collectors, and a new kind of valve which removes the losses of the feed pump of in a the condensor of a normal turbine. Also can produce cooling - ideal if you have a comms room that needs cooling too!

Alternatively ....

Even if you can't get electric generation yet, you could bury pipes in the ground, and store heat in the soil for the winter. Like this... http://www.icax.co.uk/. UK Based = UK jobs etc ;-) the advantage of this one is they have a working system. Soil is free, plumbing won't cost loads (compared to fossil fuel running costs) - it's the design and knowledge you'll probably pay a lot for!

You might then want space for a heat pump as heating of last resort, if the building design fails to live up to expectation.

I'm currently in a 1960's building with no insualtion, and poor quality secondary glazing. With 50 pc's monitors, lights and people, it overheats in february so we have the windows open as there's no air con :-(

PV+ASHP - dream team or what?

pmcc — Mon, 20 Jul 2009 14:37:59 +0100

"The point I was making I suppose was that coupling ASHP with PV is am expensive option and there are alternatives."

Yes indeed, feeding a heat pump with PV collected electricity looks like a poor option all round - very inefficient and expensive.

Assuming heat is the goal here rather than electricity, then solar thermal is the most efficient way to collect heat from the sun. Solar concentrators yield the most per unit area, but nonetheless simple flat plates are pretty efficient, durable and cheap, particularly where space is not severely constrained.

Coupling an ASHP with solar thermal looks like a good combination. The ASHP will only be needed as top-up most of the year. As Nick and Simon mention, long-term storage (earth, rocks, water, etc) is the only way to get a large fraction of heating from solar during our gloomy winters.

Simon, the Matteran idea looks very interesting. Surplus low-grade heat could be used to generate electricity during the summer. My concern about all these ideas usually ends up the same - too much complicated equipment! Unless we believe that the grid is going to fail some time soon I reckon we're better off looking at efficient centralised generating plant and heat pumps to supplement solar on a domestic/small business scale (sorry Keith).

PV+ASHP - dream team or what?

SteamyTea — Tue, 21 Jul 2009 08:36:01 +0100

Does anyone have any data logged from an ASHP that has been running for a few years. Would need more than just the kWh consumed and the COP. That is the best way to determine the workability of a system. It does seem that there is the risk of heading down "the road to hell is paved with good intentions" all for the sake of decent data analysis. Sometimes people put up charts of their turbine/PV output but what is really needed is the raw data so that standardised techniques can be applied.

The debate is heading in the right direction though.

PV+ASHP - dream team or what?

Rwatking — Thu, 01 Jul 2010 15:40:33 +0100

I know this is an old thread but has anyone got any updated views on this given such developments as the Feed in Tariff for PV and the Renewable Heat Incentive for ASHPs?

PV+ASHP - dream team or what?

SteamyTea — Thu, 01 Jul 2010 17:09:22 +0100

Interesting question, will know more when then RHI kicks in, but will work best in an old house with less insulation if the stories that the RHI is based on the SAP rating of a house are true.

PV+ASHP - dream team or what?

Viking House — Fri, 02 Jul 2010 23:27:39 +0100

This got me thinking of the heat we could have harvested and stored from the tarmac and sewer line in the Foxrock Mixed Development. Does anybody know if they are insulating their Thermal Store?

PV+ASHP - dream team or what?

Suz — Mon, 26 Jul 2010 00:41:28 +0100

PV for heating ... we had PV panels commissioned early June. By lucky chance an innovative heating engineer had installed a DPS thermal store a couple of years ago for gas central and water heating. It's got an immersion heater so I switched off the gas water heating (night and morning) and switched on the immersion heater midday. Turns out running the immersion heater for 2 hours 13.00 - 15.30 provides hot water right through the 22 hours to next boost. I'm going to try less time still. This sunny weather we use practically no electricity for water heating alone.

Now in winter when we run the central heating radiators I plan to continue this. At least we should get some free electric heat. Radiators seem a much better bet than storage heaters, at least if there's an existing system. We'll see!

PV+ASHP - dream team or what?

DamonHD — Mon, 26 Jul 2010 08:19:57 +0100

In terms of carbon footprint, using an electric immersion heater is horrible, more than twice that of using (say) mains gas or a heat-pump for a given amount of energy. It would be better to export excess energy to the grid and trim someone else's footprint than use direct electric resistance heating!

Rgds

Damon

PV+ASHP - dream team or what?

SteamyTea — Mon, 26 Jul 2010 11:40:58 +0100

Damon
Do you mean let someone else heat their water with it after transmission losses.

Suz
You don't say what size system you have installed and what your water usage is, but personally I think you are doing the right thing. Assuming your cylinder is well insulated. Use as much of your own as you can is the way to go. Have you thought of putting your fridge/freeze on a timer. If that can store enough 'coolth' during the night then you can recharge it during the morning after the early flurry of use. Think I may try turning mine off tonight and logging the temperature rise.

PV+ASHP - dream team or what?

DamonHD — Tue, 27 Jul 2010 06:47:30 +0100

Transmission losses are <9% on average for the whole grid (~2% high-voltage transmission, 7% local distribution IIRC), but you only have to be pushing power far enough up the local distribution network to stop your neighbours putting from the grid, so such losses will be tiny. One potentially great feature of distributed generation. And no, your neighbours shouldn't be running an immersion heater either.

Rgds

Damon

PV+ASHP - dream team or what?

Gavin_A — Tue, 27 Jul 2010 11:56:58 +0100

Posted By: SuzPV for heating ... we had PV panels commissioned early June. By lucky chance an innovative heating engineer had installed a DPS thermal store a couple of years ago for gas central and water heating. It's got an immersion heater so I switched off the gas water heating (night and morning) and switched on the immersion heater midday. Turns out running the immersion heater for 2 hours 13.00 - 15.30 provides hot water right through the 22 hours to next boost. I'm going to try less time still. This sunny weather we use practically no electricity for water heating alone.

Now in winter when we run the central heating radiators I plan to continue this. At least we should get some free electric heat. Radiators seem a much better bet than storage heaters, at least if there's an existing system. We'll see!

presuming you've got a bog standard 3kW (or probably 2* 3kW in a DPS thermal store set up) immersion heater, even if you've got the biggest standard domestic pv set up (4kWp) there are going to be very few days even in summer when the PV will be generating all the electricity needed to run both the immersion heater and the rest of the houses base load.

Heating water by imported electricity will cost you in the region of 3.5-4 times the cost of heating water via a high efficiency condensing gas boiler, so you'd need to be averaging at least 75-80% of the electricity used being generated by the solar PV simply to break even on cost terms. In environmental terms, the carbon impact isn't much different either in that gas used in condensing boilers is around 1/3 the carbon impact of the average grid electricity per kWh.

On environmental terms, ok there could be nominal carbon savings within your household, but given that the PV generated electricity would have been used by your neighbours to offset high carbon grid electricity, while you used gas to heat the water, the overall impact would be negative. Then there'd be the impact on increased grid demand if lots of people started doing this, which could result in extra fossil fuel generators needing to be used.

Even thinking about running your space heating from immersion heaters is simply going to mean you're costing yourself a fortune in imported electricity when you've got a a much cheaper and more efficient option available to you in the form of a (presumably) condensing gas boiler.

Switching the use of non-time critical demand such as washing machines and dishwashers to daytime to make use of pv generated electricity is one thing, switching from a condensing gas boiler to immersion heater for your hot water though is IMO not likely to be a sensible idea on either financial or environmental grounds.

PV+ASHP - dream team or what?

pmusgrove — Tue, 27 Jul 2010 13:20:30 +0100

Gavin, all very well argued which is why we fit gas boilers to all our new properties where has is available. At less than 3p per kWh there is no argument BUT what happens when you are not on the gas main? That is where alternative ideas really come into play such as using PV in line with ASHP or simple immersion heaters.

PV+ASHP - dream team or what?

Gavin_A — Tue, 27 Jul 2010 15:59:52 +0100

solar water heating backed up by a secondary source that'd usually be the same as the heating source unless in a genuinely passive house, or could be a biomass boiler in addition to any existing oil boiler / heat pump.

if the second source were ASHP / GSHP then an immersion heater on a timer once a week to boost the gshp/ashp/solar upto 65 degrees to reduce legionella risk / comply with regs would be sensible.

if it were a genuinely passive house with no heating source at all, then there could be an argument to be made for oversizing the solar, and putting it on a steeper angle for peak winter impact, then using an immersion heater for backup water heating, as the cost / benefit of using ashp when at it's lowest efficiency in winter would be negligable, and GSHP would be overkill for such a relatively small annual energy use.

PV+ASHP - dream team or what?

pmusgrove — Tue, 27 Jul 2010 17:09:26 +0100

Thanks Gavin for the 3rd paragraph; I am doing just that on my own place! Putting in the pipes for UFH heating though just in case (and a chimney).

PV+ASHP - dream team or what?

Rosco_82 — Sun, 05 Jan 2014 21:45:08 +0000

I'm looking to replace our old oil heating system this spring. Currently looking at a wood burner with a wetback and a thermal store as the primary source of heating and hot water in the winter.

The question is what to do in summer to produce HW?

Solar thermal is an option.

We have 6 kWp PV on site already, so could put peak generation of PV into thermal store by way of an immersion element on a timer.

The other option that I've been told about is locating the intake of a ASHP behind the PV panels to remove the warm air from behind the panels (I've got some data loggers installed to monitor this over the winter) and use it in the ASHP which should help COP and in theory if air removed is of higher temp than ambient then help reduce inefficiencies PV panels experience when they get too hot.

Anyone have any experience of this relationship working in practice?