volcane

Solar Thermal cost effective?

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Until recently I have not considered Solar thermal but now I am, (PV had been a much better option but with removal of all subsidies and price remaining the same PV is no longer cost effective). With Solar Thermal having a lower capital cost is it worthwhile vs E7 electricity to a split ASHP to generate hot water?

 

My feeling is that it isn't worthwhile without some form of RHI (which here has gone up in smoke!)

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There is RHI on ST as far as I know.

But then there are maintenance and running costs too.

And a more complicated plumbing and storage design.

 

First things first though is to measure your water usage, and energy to heat that, without that information it is impossible to make a proper decision.

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I spent considerable time looking on various solar forums. I thought, considering we use a lot of hot water that solar thermal would be a good idea. However, most people seemed to agree that solar thermal had many issues and that PV was the way to go.

 

As well as maintenance issues solar thermal tends to generate way more hot water than people need when it is sunny and then very little when it is not. Depending on the design of your system, it is difficult to store excess hot water generation.

 

It is more likely that your house always needs at least some electricity and excess electricity can be directed to heat hot water, thus PV is more flexible.

 

It depends to some extent on what heat sources you have available. If you have mains gas available to heat water then this tends to be the cheapest way to do it due to electricity being so much more expensive than gas. However if you use oil then the calculations may start to change.

 

It seems like you plan to use an ASHP. From my recollection, this has similar costs to using mains gas, although higher capital costs than a gas boiler. However if you do use an ASHP then powering it from PV when possible would considerably cut costs to run it. One of the issues though with an ASHP it runs less efficiently when it is cold which is when you are most likely to use it. Also they are less efficient heating water to high temperatures.

 

 

 

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Unfortunately no mains gas and no RHI see here RHI N Ireland,

 

5 bed 5 bath house (1 bath, 5 showers). Plan currently is a split HT ASHP to a Sunamp stack to provide heat and hot water.

 

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@volcane, I have a similar setup to you, low heating requirement, and high DHW requirement 5 bath house.  I am getting a bespoke Sunamp Stack using a combination of PCM58 (the normal) cells and PCM34 (lower temp) cells, so using it as 2 separate systems the PCM34 is used to preheat the cold water (think Jeremy's buffer tank) and the PCM58 cells to provide the DHW top up.  The PCM34 cells will be charged by an ASHP (low temp high COE) and the PCM58 cells a gas boiler (I have mains gas).  If the HT Split ASHP you are looking at can do low and high temp you could charge each section from the respective flow to improve the efficiency of your DHW system.  Warning this is separate from the heating circuit as the cells are charged via a PHE and discharge directly into the DHW system, to separate primary and wholesome water, so your heating circuit must be separate, you can use a separate system (as I intend) (see attached)

Heating-DHW-Design.pdf

I can pass more detail if requested.

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5 minutes ago, le-cerveau said:

@volcane, I have a similar setup to you, low heating requirement, and high DHW requirement 5 bath house.  I am getting a bespoke Sunamp Stack using a combination of PCM58 (the normal) cells and PCM34 (lower temp) cells, so using it as 2 separate systems the PCM34 is used to preheat the cold water (think Jeremy's buffer tank) and the PCM58 cells to provide the DHW top up.  The PCM34 cells will be charged by an ASHP (low temp high COE) and the PCM58 cells a gas boiler (I have mains gas).  If the HT Split ASHP you are looking at can do low and high temp you could charge each section from the respective flow to improve the efficiency of your DHW system.  Warning this is separate from the heating circuit as the cells are charged via a PHE and discharge directly into the DHW system, to separate primary and wholesome water, so your heating circuit must be separate, you can use a separate system (as I intend) (see attached)

Heating-DHW-Design.pdf

I can pass more detail if requested.

 

Many thanks for this, I have spent a fair bit of time this weekend trying to work out what to go for, I'm drawn to a Sunamp stack but I can't convince myself that its worth the extra capital outlay.

 

 

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One thing to add (or is it take away) with the SunAmp is the cost of uncontrolled heat losses when compared to a regular water store, be it a simple E7 cylinder or an all singing and all dancing thermal store.

My basic cylinder can easily loose 2 kWh/day (have since reduced this with extra insulation).

But at 1 kWh/day and E7 at 8p/kWh, that is £30/year.

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+1

One of the reasons I am going with SunAmp, is the uncontrolled heat loss (Heat gain in summer).

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There is not much to dislike about the SAPV's but just look at the useful capacity if you have high DHW requirements / multiple bathrooms. 

I haven't yet dipped my toe into exactly how these units work, so I can't respond regarding how quickly they recover from part or full depletion, but that would be my biggest concern. 

Time for a chat with the tech guys at SA me thinks :S

Anyone have any downloads to share?

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I was originally going to fit 2 solar thermal panels and 23 PV panels, as I could get ones that looked identical and all the plumbing would be hidden by the system being in-roof.  The cost of the solar thermal was prohibitive (over £3.5k, compared with a few hundred for two extra PV panels.  The real killer was the very poor heat transfer rate when the tank is partially warm.  Solar thermal has a shorter useful window where it delivers practical heat levels compared with PV, that will deliver the full panel capacity for any given level of insolation, irrespective of the tank temperature.  This difference was enough to erode the apparent efficiency advantage of solar thermal to only a little bit better than PV, and that, coupled with the much reduced cost and very low maintenance of PV made the decision to fit 25 PV panels a no-brainer.

 

Ed Davies has done some excellent work comparing PV and solar thermal on his blog, and it's well worth a read, even though it's now a little out of date with some of the price comparisons:

https://edavies.me.uk/2012/01/pv-et-flat/

https://edavies.me.uk/2012/01/solar-per-area/

Edited by JSHarris

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1 hour ago, Nickfromwales said:

useful capacity if you have high DHW requirements

Why knowing what you need is so important.

Time for a jug, a clock and a thermometer.

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6 hours ago, volcane said:

 

Many thanks for this, I have spent a fair bit of time this weekend trying to work out what to go for, I'm drawn to a Sunamp stack but I can't convince myself that its worth the extra capital outlay.

 

 

 

Two things to check :

 

- Split ASHPs are very expensive compared to mono block. Consider two units and run them at different temperatures or drop one completely. £2k buys a lot of E7 KWH

- Can your ASHP get to the required temperature for the Sunamp to get a useful amount of energy in to it. At a CoP of 1.5 which is about what you get with a 7c/56c ASHP then it's only getting 50% more efficiency than a direct E7 connection. 

 

What is the space heating plan..?

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The American lads on their solar home brew scene tend to build a BFO storage tank themselves often in their basements or at the "rear" of the house. Often this is a wooden frame with an EPDM liner and up to 300mm of "poly iso" or EPS all round. Home wound copper heat exchange coils feature quite often too. This tank is used for DHW primarily and excess for UFH but sometimes simple ducted warm air space heating.

 

Different strokes!

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5 hours ago, SteamyTea said:

Time for a jug, a clock and a thermometer

That's some pregnancy test kit you have there @SteamyTea. The subsequent children will get through loads of DHW. 

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