New installation PV, SHW, or both?

(Doug Allender) #1

Hi All
Looking for some group knowledge here. I have a house project starting next year, and would like to look at adding renewable energy to the mix. I live in Lancaster UK, and as part of the project need to renew the heating system. Any solar system would need to be an East/West system, and from what I have read we would probably need micro inverters for the PV system to avoid some shading of parts of the roof which occurs through the day. I do not want to spend vast sums of money on this for little payback, but would like to provide some buffer for the ever increasing cost of energy. Any comments, advise etc would be most appreciated.


(Moojuiceuk) #2

I think some of the best paybacks for little investment are the passive ones. Look to improve the insulation of the property first, especially if you are gutting a building and starting off with a bare shell or doing a “Grand Designs”. Get this right and you are then needing less energy in the first place to keep the building warm.

If there is mains gas available, then most people will take that option, a condensing boiler and where possible size the heat emitters so that they work well on lower flow temperatures, so the condensing boiler actually condenses and has a chance of being 90+% efficient! Water should return to the boiler at around 54C or less to stand a chance of getting the higher quoted efficiencies.

If there is no mains gas pipe nearby or you really want to go green then a heat pump would be a good option on a well insulated building. If you’ve got the land then a Ground Source machine is generally little more efficient (but the install costs are higher), but if you don’t have the land or are on a tigher budget, then an Air Source will do just as well. Heat pumps work more efficiently at lower flow temperatures, so if you can install underfloor heating (which typically runs on lower flow temps), it will be the most efficient use of the heat pump.

A heat pump can also heat a Domestic Hot Water cylinder too. This can be in addition to a PV optimiser, dumping excess PV energy into the tank rather than exporting that PV generated energy. Some cylinders have dual coils to connect solar thermal into the tank as well, but in my opinion, a good sized PV array and a solar PV optimiser / diverter is probably more flexible and slightly cheaper.

If you want solar PV, you might want to act quickly if you can as the Feed In Tariff currently available for a new install is only available until 31st March 2019. Installs registered after this date will not be able to claim the kick backs.

Regarding PV, I’m currently looking into it too for my own home and have yet to make my mind up what would be best. Given I have electrical qualifications but not MCS, I will probably do a DIY install to minimise cost, but not claim the FIT. If a roof is partially shaded at times, then a micro-inverter setup will be more flexible and efficient but may cost more to purchase.

I’m sure others who have more PV experience than myself will offer their thoughts on micro inverters vs two larger inverters for east-west setup. What I would say is, if you are going to have PV you should look at maximising your use of the PV energy, either via a PV optimiser (channelling excess energy into a hot water cylinder) or consider battery storage / Electric Vehicle charging when there is sufficient PV energy to minimise grid usage. The OpenEVSE project is also on my radar, as is a Renault Zoe one day!

Lastly, if you are radically improving the insulation levels of the property, this usually means reducing the amount of natural air movement / air changes within the building. Have a look at MVHR - Mechanical Ventilation Heat Recovery. Typically this will consist of a central fan unit with an air heat exchanger and duct work around the building to ensure frequent air changes whilst pre-heating the incomming air using the heat from the exhaust air. Too many new builds are well insulated, but full of stale stuffy air. I’m sure that’s given you plenty to think about :slight_smile:

(dBC) #3

Another option is per-panel optimisers, and a single inverter. That’s the approach I went with for my messy roof layout. More details here.

(Doug Allender) #4

Thx Moonjuice
The property in question is a 70’s Cow Shed conversion bungalow… which has stone walls with about 200mm air cavity then a brick inner wall as far as I can see. It also has a single rubble fill wall at the garage end of the house. The loft is well insulated with about 400mm of insulation in place. All windows are double glazed. So finding a suitable cavity wall fill might be a good idea.
The work we are looking at involves getting more light and hence solar gain into the living space, by adding a large window wall at the south end of the house. The house can feel cold in summer, due to the lack of solar gain, but most of that is just differential to the outside (i.e. its 18-20 inside, but warmer outside in the sun).
I have looked at the MHVR units due to current poor air flow in the house, and we have a client at work who produces these (we designed the control system for them), so I’m looking to add these to the modifications to aid air flow.
we have mains gas so will be installing a condensing boiler, and my heating company have already advised about getting the return temperature right so that the boiler will work in an efficient manner.

(Dave Howorth) #5

Hi Doug, and welcome. Congratulations on your project and good luck! PV is generally more cost effective than solar hot water these days, but needs more roof area. So consider SHW if you’re area-limited. Incidentally, a SHW system can work well mounted close to vertical on a south wall. The near vertical mounting evens out the summer-winter difference somewhat.

An east-west system can use a two-string inverter, but if you also have shading then microinverters or optimisers are a good investment. (I’m biased: I have Enphase microinverters and really like them). Note that if you want to claim FIT, you’ll need a completion certificate and an up-to-date EPC before you claim, so that might be pretty tight!

I second what @moojuiceuk says about insulation, insulation, insulation and also about airtightness and MVHR. If you’re looking at a wide cavity fill I’d suggest considering blown-in cellulose from a reputable supplier.

If you want to discuss your build project, I’d suggest the green building forum as a good place.

(Bill Thomson) #6

Micros enable system scalability. System not big enough? Add more PV modules and MIs.
No large inverter to replace or add. They do use special cables, so that’s a consideration.

I’ve got 42 Enphase micros. A mixture of M215s and M250s. I started with 4 PV modules, shortly after
that expanded to to 8 of them. A year later I bought a pallet of 20. Over the next two years, I bought
a total of 14 more, a few at a time. I wouln’t have been able to do that, at least not easily, with a
conventional string inverter.

They can significantly mitigate a shading problem.
Especially a partial shading issue. e.g. if you have only a panel or two out of an array that gets shaded,
a MI system won’t suffer a total array performance loss. Only the shaded panel(s) production is lost.
The rest of the array continues producing energy.