So now I have a solar PV system how do I make the most of it???

grahamc2003

zeupater wrote: »

Hi

So, how do you keep the panels at 25C with water from a hot water cylinder when the water at the bottom of the cylinder soon reaches a temperature of 25C+ ? ..... Z

I know I mentioned 25C, but not in the context of the cooling/heating fluid flowing around. I'm not really examining things at the detailed level at all - just looking at a high level (a feasibility type analysis if you like) for any show stoppers. If I don't see any (and I haven't yet), then all I'll say is it's worth delving deeper with further analysis and finally, if hybrid panels still look a goer (ie.e perceived advantages outweighing the disadvantages/cost effective/amelioration of existing technology, no better cmpeteing technologies etc etc, then perhaps do the detailed thermodynamics/modelling and perhaps build panels. So I'm currrently saying, imo, it's still worth persuing deeper - I'm certainly not saying pv/t is the best thing since sliced bread, or am I able to answer any detailed questions without relying on google. But I can answer the question you posed, at the feasibility level.

Firstly, there's no reason to drive the system at 25C. It could be driven at 35C, or 45C to easily solve the problem you pose. Having said that, I'm not sure a full tank correctly sized to supply a reasonable amount of cooling would reach 25C 'quickly' at all. Obviously, raising the system temperature can be expected to lower both the pv and the thermal output, but that doesn't necessarily make it fail any feasibility - it's just a physical constraint which you always come up against when designing physical systems from theoretical ones.

Another simple solution (at this level) is delinking heat and temperature. I expect you know it's easy to keep things 25C while extracting and upgrading heat to a much higher temperature - it's simple to keep the cell temperature much lower than that (if that improves things) while providing a very hot fluid for heating a hot water tank (that's what heat pumps do). Even thermal heat pipes deliver heat at a far higher temperature existing anywhere else in the pipe, by evaporating, moving on eddies and condensing a suitable fluid on a bit of copper which can get very hot. Again, I'm not saying it's cost effective to design in a heat pump/heat transport system to solve your problem - just that it's a theroetical possibility worthy of investigation. It isn't a perpetual motion machine - just well known physics which may be applicable in this application.

On another point - I don't want to become some sort of champion for pv/t (which I think overall would be detrimental to society along with pv, solar thermal, windpower etc etc under current subsidy systems). But given that we have pv and thermal, then it makes sense (or rather it is less nonsense) to examine improvements in those technologies in an engineering manner. So, given that caveat, pv/t is still a goer for me (at this feasibility level) after your concerns, and deserves further consideration. Do you think pv/t should simply be dumped as a bad idea, and not examined deeper?

zeupater

Martyn1981 wrote: »

Can I chip in my two pence worth. And I'll admit to knowing very little about solar thermal.

Zeup, I understand what you're saying, and that the water would probably heat, not cool the panels. I also assume that as the panels hit max (normally around noon on average, though every day, and every system will be different), the water is probably also starting to get hot(ter). But .... come on, you knew there was going to be a but .... what about available roof space? Isn't that a factor important enough to influence the economics too?

I think you mentioned sometime back that you have a large roof, but most people probably won't have much or any spare from a 4kWp system. I'm also assuming that PV maximisation is important, given economies of scale, and its simplicity.

So if roof space is limited, would 'doubling up' some of the space, perhaps 2 to 4 panels worth (PV) be sensible?

I'm asking the question, as I'm sure this is going to get more complex. The affected panels could muck up the rest, so different strings, maybe separate inverters as matching voltages might also be a pain and so on. However, all that aside, might the performance loss from gradually warming some of the panels, be worth it in regard to doubling up the utility of the roof as Graham is suggesting?

I've got a gut feeling that this is getting a little complicated, but stranger things have happened.

Mart.

Hi

It all really depends on what you would be expecting to get out of the system. If I lived somewhere with a considerably warmer/sunnier climate then uninsulated hybrid panels could be an option and I would have a different viewpoint, however, within the UK uninsulated panels would make very little difference and I'm pretty sure that many/most would be very disappointed with their performance if fitted.

If flat plate solar thermal panels actually worked in the UK without glazing or insulation they would be pretty common, however, they're not. Any material in direct sunlight will heat to a point of equilibrium where the solar gain matches the heat loss, thats why flat plate solar thermal panels have insulation at the back and glazing at the front - even then the heat loss through the glazing is somewhere around 6W per degreeC difference between the interior and ambient per sqm, so, ignoring all other potential heat losses, 1sqm of glazed panel in 1000W/sqm sunlight in an ambient 20C could reach ~190C ((1000/6)+20), whereas an uninsulated pv panel in similar conditions, according to manufacturer data, would merely reach ~50C ... it's the potential for reduction of heatloss which is utilised by solar thermal manufacturers within their products to capture heat and transfer it to the HW cylinder.

Regarding roofspace .... if you were to consider that a pretty large solar thermal setup would cover around the same roofspace as four ~250W pv panels and would supply hot water efficiently, the trade off is simple - lose 4 pv panels and have both efficient hot water and electricity or have an inefficient hybrid system capable of supplying very limited heated water and gaining very little in efficiency (unless you have an effective heat dump such as a pool) ....

You mention the hot water within the cylinder heating the panels .... It's far more likely that the system would be designed to switch off the pump if the cylinder temperature is around 4C lower than the panels, so uninsulated hybrid panels would simply perform as standard pv when this happens.

HTH
Z

zeupater

grahamc2003 wrote: »

.... Do you think pv/t should simply be dumped as a bad idea, and not examined deeper?

Hi

I think that there is a total basic misfit/mismatch between solar pv & thermal at a basic level using current technology pv cells. In order for pv to work efficiently the temperature needs to be lower than typical domestic hot water, for thermal to deliver hot water it needs to operate at temperatures which are higher than domestic hot water, so any simple solution must be subject to compromise.

Of course, there are ways of engineering complexity into a solution which would separate the thermal issues within a single panel, but this merely adds cost, therefore a suitable solution would not be based on simply bonding some copper tube to the back of a pv panel .... likewise, cooling pv with a form of refrigeration simply consumes a proportion of the energy generated which, in the UK, would almost certainly be higher than the generation efficiency benefit ...

HTH
Z

Martyn1981

zeupater wrote: »

Regarding roofspace .... if you were to consider that a pretty large solar thermal setup would cover around the same roofspace as four ~250W pv panels and would supply hot water efficiently, the trade off is simple - lose 4 pv panels and have both efficient hot water and electricity or have an inefficient hybrid system capable of supplying very limited heated water and gaining very little in efficiency (unless you have an effective heat dump such as a pool) ....

HTH
Z

Cheers Z & G. Just enjoying the mental exercise of following your discussion.

So if adding T to PV is detrimental, would adding PV to T be worth it, just to get a little more oomph out of a product? Not sure that makes sense, what I mean is, if you're going for T anyway, could you maximise the space a little by also having some warm/hot PV too?

I'm probably answering my own question, but based on some of the numbers and temps you've quoted, I'm seeing a performance drop of perhaps 30+% in the PV. Don't know if this would have any detrimental effect on the T. So I suppose it might simply be easier to put the PV on the opposite roof, since even a north roof is 60+% efficient to a south (as long as it's not too steep).

Mart.

Ich_2

have you had your inveter cut-out. Mine started to cut out occasionally a few weeks ago - apparently it was due to high a/c grid voltages and my Inverter cuts out at 262v - the a/c grid should be less that 253v max - power company are investigating waht is causing the high grid voltages in my area - my inverter is working correctly so it is the power network that is causing the cut outs.

The excess voltage is actually being caused by the PV installation(s). To successfully export power to the grid the output voltage of the generator (PV system) has to be higher than the grid voltage, which causes grid voltage to rise.
This is something that is predictable and should have been foreseen by the installation company.

John_Pierpoint

My inverter handbook suggests that my inverter sees 264 volts as the over-voltage that will stop it working. [Perhaps I am your next door neighbour.?]

Your high voltage problem is similar to my question about "why should a neighbourhood shut down just because there has been a power failure "upstream" some where ?"

[I am the last house in my region where the electricity comes from the West. My neighbour is the last for a supply from the East. So I am particularly sensitive to the problems of power cuts]

See zeupater's explanation after my question.

https://forums.moneysavingexpert.com/discussion/comment/52538411#Comment_52538411

I am actually still not convinced that a major power cut [Such as the one caused by a feral gang of children in Tilbury, hammering a 6 inch nail into that challenging big black cable running along the top of the railway bridge parapet] could not result in a neighbourhood's PV panels staggering on for a measurable time, if the state of the sun and the density of PV happened to be within limits.
But perhaps that is because I don't understand the safety tolerances built into the inverters and their ability to reset and restart when switched off by excess voltage.

[Ironically at the same time as posting this, the BBC "Horizon" programme was reporting on the work of the solar weather forecasters - interesting clip of the man from the National Grid talking about "opening the storm drains" to get rid of the resulting power surge, before it fries our investment in electrical equipment - perhaps this thread needs a link to the solar weather forecast. to keep an eye on those sun spots as well as the chances of sunshine tomorrow.]

The list of threats to my £10k investment is growing:

We now have:
"Next door installs panels with a higher over voltage cut out"
&
"Northern Lights display signals inverter melt down"
to add to
"Flock of birds choose house for roosting"

Soothing videos here
http://www.youtube.com/watch?v=bJz3UCIj474
http://www.youtube.com/watch?v=dgRBollPLJs&feature=endscreen&NR=1

Martyn1981

Ich wrote: »

The excess voltage is actually being caused by the PV installation(s). To successfully export power to the grid the output voltage of the generator (PV system) has to be higher than the grid voltage, which causes grid voltage to rise.
This is something that is predictable and should have been foreseen by the installation company.

I appreciate that 'pumping in' PV will push the voltage up slightly, but the problem isn't being caused by the PV, it's the underlying high voltage that is already present. I'm not sure what the installation company could or should have been expected to do.

The DNO, as I understand it, have a responsibility to supply electricity within voltage limits. If the voltage is so high as to prevent an inverter operating, or shut down inverters once they add some over-voltage to the mix, then the voltage is far too high to start with.

The DNO should investigate, and monitor for a few weeks, before rectifying the problem if it's on-going. I appreciate that we can't expect to see voltages perfectly managed, but when they are consistently in the mid to high 250's then the issue should be addressed by the DNO.

Mart.

John_Pierpoint

An I right in thinking that over voltage means the consumer is paying for extra unneeded watts every time he uses his on switch for an appliance - the worst examples being appliances using motors?.

EricMears

John_Pierpoint wrote: »

An I right in thinking that over voltage means the consumer is paying for extra unneeded watts every time he uses his on switch for an appliance - the worst examples being appliances using motors?.

It's not quite that simple !

True, in a simple circuit power drawn is proportional to voltage squared - so raise voltage by 10% and you'll pay for 21% more power.

However, sometimes you actually get the benefit. e.g. in what seems to be our favourite appliance - the immersion heater - you'll actually use all your excess power so the thermostat will turn off sooner.

Even motors might benefit. e.g. if your hoover is drawing more power you may get 'more suck' so can finish cleaning quicker and switch off earlier.

There are devices you can add to your consumer unit to control voltage at lowest possible working level. However, they're not cheap and I really can't see them as an economic advantage.

Ich_2

I appreciate that 'pumping in' PV will push the voltage up slightly, but the problem isn't being caused by the PV, it's the underlying high voltage that is already present. I'm not sure what the installation company could or should have been expected to do.

The DNO, as I understand it, have a responsibility to supply electricity within voltage limits. If the voltage is so high as to prevent an inverter operating, or shut down inverters once they add some over-voltage to the mix, then the voltage is far too high to start with.

The DNO should investigate, and monitor for a few weeks, before rectifying the problem if it's on-going. I appreciate that we can't expect to see voltages perfectly managed, but when they are consistently in the mid to high 250's then the issue should be addressed by the DNO.

The maximum permitted is 253V and parts of the network will be running close to this as a legacy of when the declared voltage was 240V, it will be particularly evident near to substations as the higher voltage at the feed end is in part to maintain voltage at remote ends of cables.
The actual difference in a typical installation is about 5 volts (we measured and recorded this recently).

So if the voltage is within limits at, say, 250V adding 5 takes it out of limits. Fine we can reduce the voltage, and do but it does risk low volts for other customers.

As for what the installation companies can do, perhaps measure the voltage at the survey stage and flag it up the DNO then, rather than complete an installation and leave it to the customer to discover that the invertor keeps tripping and they have to start the process.

I would also suggest that the electrician who does the final connections should be checking the voltage to complete the documentation required by Part P of the Building Regulations.

As one of my colleagues commented recently that he wondered how many consumers are not checking the system is exporting and have paid out for a system that is not working.

We are now in the position on some parts of the network where we cannot reduce the voltage any further as we have transformers on the lowest setting. Of course this means that in poor weather customers might then start getting low volts!