Solar Generation: Part Two

 More about solar power, including some basics of electriciity. There is a brief introduction to solar thermal and hybrid PV/T. 

Also watch out for my predictions on solar power.

Things we didn't do but maybe should have - Solar Thermal

I wrote a bit about hybrid photo voltaic thermal panels in April 2011. We looked into this, and even found a manufacture, Solimpeks in Turkey, and an importer, but in the end we didn't use these panels, and did not use solar water heating. The basic reasons were the uncertainty of maintenance costs, impacting the return on investment, and increased complexity leading most seriously to an ugly roof. In the case of the PV/T panels, which deliver both electricity and hot water, we would not have been able to connect them to the grid and sell the electricity, since the panels were not licensed in Japan. 

Photovoltaics have been pretty much a no-brainer to install on new builds here, as in many cases they have no moving parts and will still be producing electricity when the house is knocked down. Photovoltaics can literally be plugged in. 

Solar thermal, on the other hand, needs to store heat when there is more than you need, supplement it when there is less than you need, and deliver the heat where you need it. 

The most obvious worry is what to do when there is not enough sun, but the three big threats for solar thermal are freezing, overheating and hygiene. Overheating seems the most serious problem. When it gets hot nothing magical happens; water turns to steam, volume expands and pressure goes up. Watt solved similar problems for the steam engine in the nineteenth century, so it should be fairly predictable in a solar collector.

Although Watt is usually credited with having invented the steam engine, like many great inventors, he did not create it out of thin air, but innovated existing ideas to make them practical. Before Watt, the steam was thrown away after pushing the piston around in the cylinder. This was fine for a static engine by a big pool of water, and in the case of mining there were often big pools of water to get rid of. In the case of textile mills, irrigation was already in place to get water there. These engines wouldn't go very far without running out of water. Watt great innovation was to turn the steam back into water, and keep it in the system. When you are collecting heat from the sun, if you cannot guarantee that the heat will be taken away from the water, then it will inevitably turn to steam, and it needs a system like the one Watt invented all those years ago. It's not rocket science—that was Stevenson—but steam in your system could be explosive, and if any air bubbles get in there after it condenses, water may stop flowing. 

Unless your system can handle overheating, you either need a much smaller system than your needs, or you need to be able to store the heat for a rainy day. Heat storage strategies are a rabbit hole you can get lost in. Adding a supplementary heater will increase the cost, but it shouldn't be so complicated to add an electrical immersion heater, fixed up to a thermostat to switch on when the tank is too cold, and a timer to check that it's night time. Electricity is an expensive way to heat, but if you are getting most of your heat from the sun, and you are rarely using this heater, then it's not a big issue. In Japan systems are typically undersized, so and electrical heater would become very expensive, and effectively the solar panels are supplementing a regular heater system, which you need to spend more on to increase the efficiency.

There are seven different ways of getting heat from the panels into your hot water tank (according to Rob Harlan interviewed on Back Woods Home). Drain back systems are perhaps the most simple. Water is sent up to the panels when the sun comes out. When the water gets hot enough, or at the end of the day, it is all sent into a tank in the house to be used. No water is left out in the cold, so there is no chance of freezing. An alternative is a continuous circuit, either open-loop heating water directly, or closed-loop using a refrigerant that will transfer heat into water in the tank. Running domestic water through a solar panel is probably bad news, since there are risks of freezing, and of water standing at a lukewarm temperature that is ideal for legionnaires disease. The continuous circuit should probably have a closed loop, which will be slightly less efficient. Since you are not using the water directly, you can use a coolant instead, and make sure it has a freezing point below your minimum temperature.

The solar collectors can either be flat panels or vacuum tubes. An interesting twist on the storage problem is to add phase change materials into vacuum tubes, so heat is stored by melting one or two materials with a high melting point. Water can be passed through the panels any time, and it will be heated up from the phase change materials. There used to be commercial models here on Made in China,com, but all Google will find me now are research papers and patents.

References

Papadimitratosa, A., Sobhansarbandib, S., Pozdina, V., Zakhidovc, A. & Hassanipour, F. (2016).

Evacuated tube solar collectors integrated with phase change materials. Solar Energy, 129, 10-19. Available from: https://www.researchgate.net/publication/294111899_Evacuated_tube_solar_collectors_integrated_with_phase_change_materials [accessed Apr 21, 2017].

If NEDO had been done

I should write about the plan of the man who wanted to supply heat but not much light.

We had had reservations about his system from early on. During discussions about the chances of actually receiving the NEDO grant, I asked their boss and the builder's boss whether they would cover the grant amount in the event that we didn't get it because they hadn't built our house within the deadline. I think they thought this was funny. Evidently there was no concept of a penalty for not meeting a deadline. The Supplier of Heat but Not much Light suggested a place where I could get a loan to cover the cost. 

The radical part of it was a solar thermal system, providing hot water in the summer, and heating in the winter. 

Most people assume the problem with a solar thermal system is what to do on days when there isn't enough sun, but in fact that is a fairly simple problem. You need a back-up heating system. There are any number of ways to heat water from gas or oil burners, to a heating element from a kettle in the boiler. You don't see them so often but it's certainly possible to have an electrical heating element at the tap. A low-tech solution would be an electrical shower unit, which may be expensive to use but if you only need it a few days every year, then the total cost would not be so much.

The biggest problem with solar thermal systems is failure in the face of too much solar heat going in and not enough hot water going out. 

My simple question about the system was, what if the refrigerant boiled? This seemed likely to happen sooner or later. The behaviour of liquids heating up is quite predictable. As the liquid gets warmer it expands slightly. Then it starts to boil. The amount of gas steadily increases, with the pressure rising. When all the liquid in the solar collector turns to gas it will probably stop circulating and the system will stagnate. Gas tends to have a low thermal capacity so it won't take much more heat, and low conductivity so very little heat will get into the rest of the system, and the liquid in the circuit beyond the solar collector will not start boiling. The gas will reach some temperature at which there is an equilibrium between the solar heat getting in and heat radiating out. 

If the system survives this high pressure and high temperature, the next challenge is what to do as it cools down.

The gas will start to condense into liquid, and the collector must fill up again, so that it can continue to flow and get heat where it is needed.

A robust system should probably be able to cope with this. The Victorians were using steam systems for heating, so the pressure should not be a problem. 

In the plan of the man providing heat but little light, the solar elements used vacuum tubes, with a refrigerant pumped through them all the time. The pump was served by a dedicated solar panel, ensuring that the refrigerant would not stop, which would lead to disaster. This was his answer: it's not going to happen. 

That was one worry about the system.

In discussions around the same time as I found that penalties for not meeting deadlines were from an alien world, I asked what kind of guarantee they had on the solar thermal system, having heard that many solar thermal systems fail within about five years. He could only offer a one-year guarantee.  This did not reduce my worry.

Another minor worry, which I would have lived with, was that the back-up heating system used paraffin oil. Having spent too many winters lugging tanks of the stuff to my house, syphoning it into the small tanks and ferrying them to the heaters inside, and spilling several litres of it in the process, I really didn't want the stuff anywhere near my house. 

I know that in terms of environmental impact directly using fossil fuels is less wasteful than getting electricity off the grid that has come from gas-fired power stations, but given a choice I didn't want to build a house that took any fossil fuels. 

The delays meant that we were no longer going to get the 2.7 million yen NEDO grant, but once we had established this, it also meant we were no longer bound to this elaborate and expensive system that was included in the grant application. 

This system was certainly going to cost us at least an extra million yen, but if we had been able to get the grant, we probably would have been financially better off.

But, I suspect that if we had gone for the grant and followed their plan, by now we may have been stuck with a system that didn't work properly, and seeing a lot of their repair men, so it's probably a good thing that we got out of the grant.

Or perhaps if we hadn't even thought about applying for the NEDO grant in the first place, we may have bashed the plans around a bit more, changed the windows on the south side from triple to double and got some curtains for them, or given up on the concertina window, and found ways to save a great deal more than the 2.7 million of grant that we would have got.

And then it may have taken another year to build, and we would have paid another million in rent and bills on the old house. 

As it is, we have a great house where the good points far outweigh the bad points, so there is not much to complain about, and it doesn't help much to wonder what might have been.

The ideal solar thermal system

We ended up not choosing solar thermal, and just using photovoltaics which have no moving parts and nothing running through them that can freeze or boil. Below are some ideas from the planning phase of our building based on theory and various research.

The ideal solar thermal system stores heat in a single hot water tank which can collect heat from solar panels, add heat from a backup boiler or heater, provide hot water, and distribute heat to the radiators, underfloor heating or the ventilation system.

The problems and challenges facing solar thermal systems include overheating, chattering, hygiene and freezing. People often worry about not getting enough heat from their solar heaters, but apparently overheating is the biggest cause of failure for solar systems, and one reason why systems in Japan are usually under-sized, meeting demand only at maxiumum output. Overheating causes steam and high pressures, which can shorten the lifetime of elements within the system. It should be possible to design a system that can withstand a range of pressures and temperatures. After all, steam heating systems have been around for over a hundred years. Evidently they are not always designed for the pressures and temperatures that sooner or later they well reach, and solar systems often fail within five years.

Another problem is chattering. This happens when the hot water from the solar system is fluctuating around the level at which extra heat must be added. As with photovoltaics, sunny days are are no problem, as there is a large and constant amount of heat. Overcast or rainy days are not a problem, because there is a small and constant amount of heat. Partially cloudy days are a problem. Because solar systems cannot guarantee to supply enough heat all the time, there must be a backup system. If the solar heating is sufficient, the backup heater is not needed; if the solar heat is insufficient, for example on a snowy day, the backup must switch on. In certain weather conditions, the backup system may be switching on and off several times, rapidly wearing out motors and switches. A certain level of sophistication is needed in the control system. This problem should not be insurmountable, for example the backup system could only switch on after the sun has gone down.

Another issue is storage of water within the tank. The tank could most efficiently contain the same fluid used for the panels and for the domestic hot water. This is most efficient, however there may be issues with legionnaires disease, which thrives in the kind of temperatures that you will often get in a solar water system. In addition, tap water is liable to freeze, so any water left in the panels at night time could lead to problems, since freezing water expands and tends to burst pipes. Using a refrigerant within the solar circuit, and a heat exchanger for the domestic hot water also has advantages, for example in allowing higher pressure within the panels. Because of the heat exchanger it is, of course, less efficient.

One choice with solar systems is whether to use a drainback or continuous system. In the continuous system a refrigerant is used, and as soon as the water in the panels heats up, flow starts, transferring the heat from the panels outside into the tank inside. In the drainback system, water is sent to fill the panels in the morning, or at least when the panels start heating up. It is then returned either when the water has reached a certain temperature, or at the end of the day when it's got all the radiation it can get, before it starts cooling down. The advantages with this system are that overheating is avoided, since the hot water is sent into the house before it overheats. Tap water can be used, since there is no danger of the panels freezing as water will not be sent there if it is too cold, and will be sent into the house before the temperature drops.

Another issue is the performance of the hot water supply system. Ideally there should be time and volume settings so that the bath be filled to the desired volume and temperature. There should ideally be a means for re-heating the water within the tub, so that water may be saved as is customary in Japanese houses. Most commercially available hot water systems in Japan have all these functions ready fitted, although the controllers cannot necessarily be acquired separately. Designing your own system would be a challenge.

From the beginning solar thermal seemed very attractive as a way of reducing energy. Solar thermal panels turn over half the sun's energy into heat, and are three or four times more efficient that photovoltaics. In the end a major considerations for not choosing solar thermal were to keep the systems simple and the roof elegant. Thermally it may have made sense, but financially, in terms of initial cost and the relative costs of purchased and re-sold electricity, photovoltaics were the obvious choice. And if the solar thermal system was going to fail within five years, even the superior thermal efficiency of the system becomes doubtful. Another factor was our NEDO grant application, which I'll write more on later.