Boiler stove with gas combi boiler central heating and hot water 24 March 2022 at 2:03PM in Green & ethical MoneySaving 12 replies 205 views

Hexane

M0ney said:

The front is almost directly south facing, the problem is that we are limited by a velux and 2 dormers, another reason why I considered solar thermal because they need a smaller footprint, don't they...?

Yes solar thermal generally requires a great deal less space. It also suffers less badly from partial shading of one panel affecting an entire string of PV panels (if not using optimisers like SolarEdge). Dormer windows and similar irregularities are extremely limiting for solar PV installs - it's usually not really practical to just fit panels into the gaps between.

I wonder if it might be practical to fit a solar thermal installation on your south-facing (front) roof and some solar PV panels on the east and west roofs (if they're large enough).

I've had solar PV on a previous house, why is it so preferable over solar thermal?

Solar thermal was popular 15+ years ago when PV panels were much less efficient and much more expensive than they are now.

I don't have specific figures, but you can imagine in 2005 if you wanted a 4Kw solar PV array then you might need 20 x 200W panels and pay nearly £20,000. By comparison a solar thermal system large enough to be useful for heating domestic hot water (only) might have cost well under half that. Fast forward to today and you only need 10 x 400W panels to achieve the same peak power output, and might pay £5000 to £6000 for it. And the solar thermal system (partly because not especially popular now) might still cost you £7000.

The advantage of solar PV is the flexibility. So for example on a sunny day in mid to late March - a time of year when many of us still have some requirement for space heating - by lunchtime our over-sized solar PV array will already have heated the domestic hot water tank to its maximum temperature, it will have powered an entire "intensive" dishwasher run, powered the other less power hungry appliances and lighting etc., and now already be delivering 3kW to drive the electric underfloor heating - all for free. By contrast with all that, a solar thermal system would (in almost all cases) only be heating the domestic hot water tank.

Some have also questioned the long-term reliability of solar thermal systems (the possibility for a minor fault to cause major damage) but I don't have any figures or even anecdotal evidence on that.

Anyway the thing to bear in mind when designing your replacement heating and hot water arrangements is, if you later wanted to make use of solar PV power to heat your domestic hot water tank, then instead of having an immersion heater only at the top of the tank (quite common), you would ideally want one immersion heater at the top and one at the bottom, or at least a bottom-mounted immersion heater. The reasoning is that a top-mounted immersion heater will generally only heat the uppermost portion of the tank, and will reach maximum temperature very rapidly therefore reducing the amount of energy that can be used.

QrizB

Hexane said:

I've had solar PV on a previous house, why is it so preferable over solar thermal?

Solar thermal was popular 15+ years ago when PV panels were much less efficient and much more expensive than they are now.
The advantage of solar PV is the flexibility. So for example on a sunny day in mid to late March - a time of year when many of us still have some requirement for space heating - by lunchtime our over-sized solar PV array will already have heated the domestic hot water tank to its maximum temperature, it will have powered an entire "intensive" dishwasher run, powered the other less power hungry appliances and lighting etc., and now already be delivering 3kW to drive the electric underfloor heating - all for free. By contrast with all that, a solar thermal system would (in almost all cases) only be heating the domestic hot water tank.

I was going to make a similar point. Solar thermal is very much a one-trick pony, whereas solar PV generates electricity that you can use for any number of things - or sell back to the grid of you don't need it.

Also, unless you live in an exceptionally large property, there's only so much prime rooftop real estate. Do you want to give up, say 5 square metres of that space for solar thermal, or would you rather have an extra kilowatt of solar PV (and at a lower cost)?