From time to time enquiring clients, having spoken to other solar water heating suppliers or installers, have questioned whether the Ubersolar systems are overpowered.

From the outset the answer is a categorical “NO” but for those who want to understand ‘why’ this paper provides a technical explanation so that you can understand the reasons.

Further, when we are put into competition with other solar systems being considered, or other quotes, or even other hot water heating technologies such as heat pumps, or gas, false information can easily provide a distorted reality.

Where One Needs to Start

Any comparison needs to be done on a meaningful basis, which I believe to be the payback period, the ROI and the amount of hot water one gets, matching it all to the customer’s requirement.

The sticker price (the cost), normally being what people look at is misleading, in that until you have done a comparison on the factors above, you really have no idea what you are investing in.

So when you do the comparisons on solar systems, see if anyone gives you:

1. The expected temperature in the geyser in degree C,
2. The expected amount of hot water at 40C at the tap,
3. The deemed kWh output on an average day at 20MJ m2 p/d,
4. as well as the financial information,
   
   • Projected Payback Point,
   • Return on Investment,
   • Comparisons with heating by electricity.

To my knowledge solar water heaters are the only product that does not give its power output/consumption/saving in Watts or kWh output. Almost every other product specifies power data i.e. cars, vacuum cleaners, heat pumps, PV panels, microwaves, kettles, electrical elements, etc.

Ubersolar Technology Compared with Others

Our approach and the technology is different to almost every other high pressure solar water heating system, because the solar collector manifold is vented to the atmosphere.

This means that the solar collector manifold can never reach higher than 99 degrees C, in much the same way that a kettle or pan of hot water on the stove will never get hotter than boiling.

Consequently whether the solar collector is 18 EVT, 30,EVT, 60 EVT, or more the ‘stagnant’ water in the collector (no pressure) will never exceed 99 degrees C.

The next difference is that we have two completely separate areas of water

1. The ‘no pressure’ stagnant water in the solar collector
2. The ‘high pressure’ water in the storage tank (geyser) and in the circulation loop from the geyser through the solar collector heat exchanger and back into the geyser.
   
   • When fitted with the solar controller the temperature in the tank is limited to 70 degree C- 75 degree C
3. The two different water areas are never mixed, or put another way the water in the solar collector is not used in the hot water heating system for the house.

What this means when compared with other solar system types

We can never overheat even in periods of protracted ‘wet’ or ‘dry’ stagnation, or when hot water has not been extracted from solar system or tank.

In wet stagnation, for example, when away on holiday or absent for months, the tank water may reach 75 degree C every day (and then turn off), and the water in the solar collector may reach boiling point and result in evaporation from the vent pipes. This is automatically topped up maintaining an optimum level.

In dry stagnation, for example ‘IF’ the cold water mains had been turned off, the autofill mechanism would not operate and over a period of around 4-8 weeks eventually all the water in the solar manifolds and evacuated tubes would evaporate.

In this example, as the water evaporates, sooner or later there would be no solar heat going into the geyser, as the heat exchanger would be in air rather than water, and no heat would go into the geyser. The geyser however would still be full of water and the solar circulation pump would also be sitting in water.

When all the water in the solar collector has evaporated, it would be in a dry state of stagnation. Nothing happens other than the air in the solar collectors get hot and cools down again at night. It will remain completely safe and will not harm the solar manifolds or evacuated tubes, but dry stagnation is not recommended for more than a few weeks.

BUT, if the mains water is turned back on again when the tubes are hot during the daytime, cold water will go into the tubes from the autofill mechanism and they will be liable to crack. (CAUTION)

Other types of high pressure solar water heating systems in stagnation

The two main types of solar collectors for high pressure solar water heaters are evacuated tubes using heat pipe risers and flat plate collector types. Both potentially can suffer in stagnation and, depending on the size, can result in overheating and consequent failures in components.

As a result, the size of the collector is limited to avoid the overheating and in stagnation scenarios.

In turn, this limits the amount of electricity savings that can be achieved and the amount of hot water that can be generated by solar.

Most high pressure solar water heaters are NOT more than 70%-80% efficient, in contrast to Ubersolar where100% efficiency in replacing electricity is achieved on the solar collectors matched to the tank size.

Ubersolar Performance and Efficiency Compared

While Ubersolar systems can be 100% efficient in the goal of replacing 100% of the electricity used in heating water in the tank and producing the hot water volume required, (in contrast to the 70%-80% of other systems), it is a reality that the Ubersolar collector mechanism and heat transfer (in domestic systems) is not as efficient as the heat pipe or flat plate types.

Consequently, a larger number of EVT’s are needed to generate the electrical savings and hot water generated. We will use 30 EVT’s for 150l itres, while most others will use 16 to 20 EVT’s with heat pipe risers. But we achieve 100% electrical savings, while they will be 70%-80% and we don’t overheat and we don’t suffer in stagnation.

On many occasions, when we have explained the technology to interested parties, they come up with suggestions as to how to improve the efficiency of the heat transfer from the ‘no’ pressure water in the solar manifolds to the ‘high’ pressure water that is being stored and used for washing. We assure you we have tried many, many, permutations, and yes we know how to increase the heat transfer efficiency, but the cost increases. As we have already achieved the end goal of 100% electricity replacement at a lower cost than competing systems, giving a faster payback and better return on investment, until we can produce more power for less cost the technology will remain largely unchanged.

In Summary

When asked how we are different to other solar water heating systems, I draw the analogy of a lazy American V8 engine (ours) compared to a Formula 1 engine (others).

In essence, we are much simpler. We rely on stock components that are used around the world on solar thermal systems. We have one moving part, an impeller in a brushless motor solar pump.

By venting the solar collector manifold we achieve much lower temperatures in the solar collector system. At 99C, rather than EVT’s with heat pipe risers that can reach 275C during stagnation or flat plates that can reach 165C, we remove the opportunity for failure of components. This in turn results in greater reliability, none of the components ever being stressed, longer life, and reduced maintenance.

Having solar collectors that are modular, or ‘plug and play’, for example 12,18 24,30 EVT’s, we can build solar collectors of any size to match the required kWh savings, irrespective of tank volume and where the system is going to operate, in Johannesburg, Cape Town, London or the Sahara, all without the problems of overheating.

By addressing the conundrum of how to create more kWh savings, and more hot water for less investment, we in many respects went backwards rather than forwards. We made the solar systems simpler and more robust.

James Green
CEO Ubersolar.