Posted By: renewablejohnFrom where will the energy for cooking and tea making come

That's a relatively small amount of energy in total compared with DHW and space heating, at least for me.

For all electricity use (cooking, refrigeration, lighting, computing, CH circulation, oil boiler power and *showering* [¹]) my electricity consumption in January and February was an average of a tad under 200 W.

I expect that my use in my new house will be similar so 6 kW of PV and 10 kWh of batteries should cover that. If not, the addition of either more PV or a small wind turbine should sort things out. Almost all of it (apart from the blower pump for the waste water treatment thingy) will contribute to the space heating, of course.

[¹] Yeah, my current rental house has a DHW tank heated by the oil boiler which I only use for washing and washing up so really only a few 10s of litres per day but still the house has a cold-fed 7 kW electric shower. Bonkers!

It only works when there's sun though, so lots of storage or some other energy source is necessary.

Thats the main difference between PV and Thermal. With Thermal it will still work on overcast days when PV will struggle. Obviously it works better with sun.

Posted By: Ed Davies

Posted By: djh…and then Ed said "I am already down that route with thermal oil."

No, John did. I won't be using it - the thought of having significant quantities of high temperature oil around the place scares the poo out of me. I can only see the point of it for energy storage for cooking but LiFePOâ‚„ seems a much better mannered storage medium for that.

Posted By: renewablejohnThats the main difference between PV and Thermal. With Thermal it will still work on overcast days when PV will struggle.

I'd say the opposite. It's under weak sun conditions when you want high temperature output that PV is cheaper per watt than thermal:

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

But is that solar tubes for thermal not flat plate collectors.

Posted By: renewablejohnBut is that solar tubes for thermal not flat plate collectors.

One of each.

Posted By: Peter_in_HungaryA good study, but I note that it was done in 2012. Have you updated this work because there have been some shifts in the price of PV since then and I suspect that PV might come out much better.

Yep, that might be worth doing though it's pretty easy to just scale the PV price graph by eye to account for the price change, a factor of about 2 for the actual panels though inverters, etc, have probably gone the other way (as would pumps, etc, for solar thermal).

Posted By: WillInAberdeenAlso you can now get more W per m² of PV panel, so more power from a limited roof area than previously?

Hot thermal oil is great for moving heat around, but not so great for storing it. The heat capacity on a volume basis is about one-third that of water, so you need your oil to be 3x hotter to store the same amount of energy.

Any liquid above ~50-60degC can scald you, irrespective whether it is oil or water. There is a safety advantage that thermal oil will not accidentally boil (steam is very dangerous), set against a slight fire risk.

But the point with thermal oil is that the higher temperature allows you to use conventional steam for power generation. Unfortunately water does not hack it with even fancy refrigerants you still need a mimimum of 80C for any sort of power generation. Storage of hot thermal oil was solved years ago and is used widely in the asphalt industry.

Posted By: renewablejohnBut the point with thermal oil is that the higher temperature allows you to use conventional steam for power generation. Unfortunately water does not hack it with even fancy refrigerants you still need a mimimum of 80C for any sort of power generation.

That does sound like a lot of fun, a home powered by a steam engine! Not sure about 'conventional' though... sometimes unconventional can be good!

Have you some details about the steam engine/turbine you have in mind : what temperature (pressure) steam are you looking at when your thermal store is heated to max temperature, and what about when your store is cooled down to min temperature? How much shaft power? Will you have a condenser, what will be used as the coolant?

Many years ago I worked on organic Rankine cycle generators, but with boiling temperatures less than 100⁰C the efficiency was fairly lousy (Carnot again).

Posted By: WillInAberdeen

Posted By: renewablejohnBut the point with thermal oil is that the higher temperature allows you to use conventional steam for power generation. Unfortunately water does not hack it with even fancy refrigerants you still need a mimimum of 80C for any sort of power generation.

That does sound like a lot of fun, a home powered by a steam engine! Not sure about 'conventional' though... sometimes unconventional can be good!

Have you some details about the steam engine/turbine you have in mind : what temperature (pressure) steam are you looking at when your thermal store is heated to max temperature, and what about when your store is cooled down to min temperature? How much shaft power? Will you have a condenser, what will be used as the coolant?

Many years ago I worked on organic Rankine cycle generators, but with boiling temperatures less than 100⁰C the efficiency was fairly lousy (Carnot again).

As a package system I have been looking at the Orcan 20.30 producing upto 30KWe plus hot water and central heating. As for it cooling down I do have a back up plan as Dunsley have already stated that their Yorkshire boiler is designed so that it can be used with thermal oil.

Thats interesting, thanks! The Orcan devices are organic Rankine cycles (ORC) using an air conditioning refrigerant, like big heat pumps in reverse. They will want a heat input at around 100-130degC and will have efficiency around about the 10% range.

So to provide 30kWe, they will need a heat input of around about 300kW, of which ~90% will be rejected, so around about 270 kW of waste heat will be available for space heating. About 20m³ of oil storage will be needed for each hour of max operation. (1.7kJ/litre/K)

Normally that much heat and electricity would be used in a commercial/industrial building, or maybe a housing estate? That could work nicely, as there would be plenty of roof to put solar collectors on, and perhaps space to store the hot oil.

I don't think there is a 'correct' solution for medium term solar thermal storage yet, Ed and John are both developing different ideas. I suppose the baseline is a plain roof with no solar collectors, which has 0% efficiency and 0 days' storage, and so anything at all would be an improvement on that.

Would be interesting if someone made a reversible ORC heat pump, that could be used when electricity is available (PV, wind, overnight, etc) to heat up to a 'hot' liquid store. The heat could be used in the house when needed. Or, the ORC could be reversed to make electricity when needed, releasing heat to a 'lukewarm' store that could be used for space heating, or topped up with ST collectors at an efficient low temperature.

Posted By: Pile-o-StoneI thought that the solar thermal vs solar PV argument was well and truly done with the victor being solar PV?

The main points being that once the hot water tank is full, the solar thermal panels at best are now useless and at worse, they start to destroy themselves. PV on the other hand can heat a tank of water, supply electricity to the house, charge a battery and charge your car, plus its 'fit and forget' so no leaks, no antifreeze.

One thing to remember when we build our own homes, we have to have an eye on a potential resale. I bought my current house and I've done a lot of work on it because it was our 'forever home', but we are now looking to relocate to a new area completely and it'll soon be on the market (once I've completed a ton of jobs). 'Muggles' will be put off buying if a house has a magical heating/power system (or even a non-conventional building design or construction).

Depends if you want to be energy self sufficient which includes both electric and heating. Think people are going to be in for a shock soon with high gas and electric bills. Think the thermal oil is the game changer for solar thermal.

Thermal oil is used in large systems with acres of mirrors and a solar tower collecting the radiation. They often seem to use molten salts of some kind to store the heat.

e.g. https://www.energy.gov/eere/solar/thermal-storage-system-concentrating-solar-thermal-power-basics

<blockquote><cite>Posted By: Ed Davies</cite>I can't see what using thermal oil helps with. Solar thermal works most efficiently at low temperatures, particularly when the outside air is cold and the sunshine is weak. People commonly claim that solar thermal hardly works in winter but I think that's because they're assuming DHW temperatures. At the temperatures where oil does have an advantage it seems to me it really wouldn't work. The only hope of it being usefully productive would be a low space-heating temperatures appropriate to a well-insulated house.</blockquote>

Maybe this article will help to explain the advantage of thermal oil in a Solar Tube rather than air

https://www.sciencedirect.com/science/article/pii/S2090447917301144

Its the storage capacity and higher temperatures which are important for power production after the sun goes down.

OK, got the spoons to look at that paper properly now. It's embarrassingly/depressingly bad. It'd not be great even as a school science project.

This research focuses on improving the thermal performance of evacuated tube heat pipe solar collectors.

However, they don't say what aspect of the performance they want to look at.

They imply that they're looking at the rate of heat transfer yet they actually measure the stagnation temperature and use that as a proxy somehow. They don't justify this leap, probably because it doesn't make any sense at all.

An experimental setup has been developed to study the influence of oil and foamed metals on the performance of evacuated tubes with heat pipes.

Their idea is that putting oil or oil + foamed copper actually in the evacuated tube (not just as the heat transfer fluid used to move heat away from the top end of the heat pipe) somehow improves the performance of the ET. Maybe it does in some circumstances but it's not easy to see what circumstances from this paper.

The irregular expansion of the fin causes the fin to loose contact with the evacuated tube and the heat pipe, which results in deteriorating the rate of heat transfer. Also, the fin surface can lose contact with the evacuated tube due to overheating due to excessive solar irradiance,…

Well, at least they provide a superficially plausible explanation for why they might get an improvement.

In each set of experiments the performance of the evacuated tube heat pipe has been examined in case of (i) a normal evacuated tube and this experiment is taken as the reference experiment, (ii) a normal evacuated tube but filled with oil, and (iii) an evacuated tube filled with oil and foamed copper instead of a finned surface.

Thermal oil, i.e. Mobil-Therm 605 [15], is used in the performed experiments.

OK.

They did two sorts of experiments with these combinations of pipe contents, 1) in front of an artificial 1000 W/m² sun and 2) outside in real Egyptian sun in September.

Hilariously, they write:

It can be seen that the temperature of the bulb of the heat pipe has reached to 166 °C in case of a normal evacuated tube heat pipe, and the temperature of the bulb has increased to 191 °C in case of filling the evacuated tube with thermal oil. The increase in the bulb temperature due to filling the evacuated tube with oil is about 15%, i.e. (191–166)/166 = 0.15.

Umm, true if they happened to start at 0°C (which would be irrelevant anyway) or if Celsius was somehow an absolute scale but if they were measuring in kelvins then the “increase” would only be (191-166)/(166+273.15) = 0.0569, i.e., ~6%.

The artificial sun is turned on until the temperature of the heat pipe bulb reaches a steady state temperature, i.e. there is no further increase in temperature and the slope of the temperature becomes equal to zero. Afterwards, the artificial sun is turned off and the temperature is measured until the temperature of the heat pipe reaches room temperature.

OK, but they don't seem to do much with the times other than:

Another important fact that has been realized during the performed experiment that the time taken for the heat pipe to cool down from the final attained temperature to room temperature is much longer in case of adding oil than the no oil case, which indicates that the thermal oil does not just improves the rate of heat transfer, but it acts also as a heat storage medium.

Here we're in PhD in the bleedin' obvious land. What they don't seem to realise is that heat storage in a solar collector is typically a bad thing. They write:

It can be concluded that thermal oil can be treated as a heat storage material, which stores heat energy during the Sunshine time and can supply that heat at night, i.e. when there is no Sunshine.

Remember, they're talking about oil actually in the evacuated tube so a signficant proportion of any heat stored there will be lost to the outside once the sunshine goes away.

Moving on to the real-sun experiment, again they concentrate on the steady-state temperatures except that their filled tubes don't seem to actually reach a steady-state but rather peak at about the time the insolation drops to around 750 W/m² (see their Figure 7).

What they ignore is that early in the day, when the insolation is below about 750 W/m², the temperature for the normal ET seems to rise quickest - i.e., it's transferring heat quickest into the litre of oil they use as the heat store above the heat pipe.

Even in Egypt in September insolation is below 750 W/m² for a large part of the day. It's above it for about 4½ hours and Sun seems to be up for about 11 hours.

Overall:

1) A poor paper.
2) Not terribly relevant to variable and weaker conditions seen in Britain, particular away from the peak of summer.
3) Doesn't bear much on whether harvesting solar energy to heat thermal oil then using the heat to generate electricity makes much sense compared with using PV panels and storing the energy in a battery.

Posted By: djhAt least I think that's what it all is.

Yes, that was my reading, too.

It's slightly confusing because they also, but separately, used thermal oil as the “working fluid” at the top of the heat pipe when they were measuring whatever they thought they were measuring for the real-sun experiments.

Posted By: WillInAberdeen: “I would have expected (without evidence) that the stagnation temperature at the top of the evacuated tube would depend on what fluid is used inside the heat pipe.“

Yes, I think that'll be one of the considerations but, if heat isn't being transferred away from the top of the heat pipe very effectively then simple conduction through the metal will allow higher temperatures to be reached. A bigger limit will be radiative heat losses from the tube resulting from the balance between the absorptivity of the selective coating at short wavelengths vs its emissivity at thermal wavelengths.

A previous experiment: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10441&page=1#Comment_172923