Don’t pay for capacity you can’t use.

Why do some solar geyser suppliers recommend bigger tanks than normal geysers?

These are invariably systems based on thermosyphon, the type of systems where you have a tank up on the roof.

This is because of the way thermosyphon works.

The fact that hot water rises and that the thermosyphon process is very slow, the hot water in such systems rises to the top of the tank leaving markedly colder water at the bottom. This design is unable to heat the lower volume of water in the tank.



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In large bodies of water such as lakes for example a similar effect is noticed. Divers will witness what is known as the thermocline, a point where the water temperature suddenly drops. The same thing happens in smaller volumes of water where there is no stirring of the upper and lower layers.

What this means is that if you buy a thermosyphon system you can’t really use the full capacity of the tank. So you need a bigger tank than you would have done.

One supplier states that their 300L product has a solar delivery of 240L and their 180 L product can only deliver 160L from solar. This suggests that approximately 20% of the capacity is not usable.

This phenomenon however only applies to thermosyphon systems.

The more modern designs which are actively circulated by the use of small pumps, does not suffer from this loss of capacity. As such it is perfectly acceptable to use your existing geyser or one of the more common geyser sizes that will suit your water demand needs.

NERSA consultation paper discusses revision of rebate rules.

How long will the current Eskom scheme last?

The National Energy Regulator of SA (NERSA) has published a consultation paper on the revision of regulatory rules for energy efficiency and demand-side management (EEDSM).

http://www.nersa.org.za/Admin/Document/Editor/file/Electricity/Electricity%20Infrastructure%20Planning/EEDSM%20Consulatation%20paper%20including%20the%20rules.zip

If it goes ahead it could potentially replace the current Solar Water Heating rebate scheme being run by Eskom

Whilst little is known for certain, the plan, it is understood, will remove the current scheme in favour of a flat concession of 200kWh per month per system, regardless of system efficiency or size.

The current concession value has been suggested as R1296 per year. Equal to approx R0.54 /kWh until such time as an actual “SWH saving” has been established at which time the regulator may adjust the concession value up or down.

This is a serious disincentive to the suppliers of efficient systems in the market and favours typically cheap. Underperforming, solar water heaters (SWH’s) as the suggested concession values are much lower that the current rebate value.

Given that the SABS test standards currently do little to ensure that a SWH is fit for purpose, with no real minimum performance being required* and its one size fits all approach with regard to geographic location variations. This will play into the hands of operators out to make a fast buck by supplying cheap underperforming systems.

With the same concession being offered regardless of solar delivery capability, systems of low performance will appear more attractive than those which actually displace useful amounts of energy.

An article published in March this year highlighted that as many as 40% of the systems offered under the current scheme delivered less than 60% of the energy required to heat the associated geyser by 40oC at the test datum.

With the advent of the new scheme and the continued refusal of Eskom and the SABS not to require that test performance results be published, selection of a SWH system remains a lottery. Unless of course you have done you research and found a supplier who will publish their Q factor and show their test reports.

As a simple analogy if you buy a car, you are entitled to know what size the engine is and what kW rating it has. Sadly this is not the case with SWH’s. Q factor, the SWH equivalent of kW in cars, is considered confidential and may only be disclosed at the option of the supplier.

I fear the current scheme could collapse in just a matter of months, possibly even before the end of the year.

My advice would be if you are considering investing in a SWH, do it sooner rather than later. But make sure you buy one with a high Q factor and therefore high rebate value.

*9MJ (2.5kWh)is the current minimum. This is only sufficient to overcome typical geyser standing loss not actually heat the water.

Solar Water Heaters Overheat

Overheating of certain solar water heating designs is a growing concern in the industry. Dangerously high water temperatures, even boiling can be achieved with the associated risks of injury.

Such system designs rely on a passive principle known as thermosyphon to enable heated water to naturally rise into a storage vessel. These are the systems that typically require a tank on top of the roof.

The problem with such simple passive designs is that there is no mechanism of controlling maximum temperature as the thermosiphon process cannot be turned off.

Example of the type of system which can be prone to overheating.

Particularly when the home owner is away, on holiday for example, without water being drawn off the system will heat untill the safety valve blows. At other times of low water usage tank temperature can reach scalding temperatures.

Whilst the problem can be reduced by adding thermal tempering valves, which mixes cold water with the hot, to avoid excessive temperature at the tap. Extra cost and additional maintenance are incurred which make these often low priced systems less attractive.

The challenge with using solar energy is that the sun is a highly variable power source, a fact which is exacerbated in a country such as South Africa, due to its great variances in latitude from the north to the south of the country.

Because of such variances the ability to provide control becomes all-important so that systems can deliver acceptable performance from the low sun conditions of winter whilst not overheating in summer. The alternative is to undersize collectors which reduces overheating but does not deliver any significant benefit in winter when hot water is needed the most.

Particularly prone to this problem are integrated or compact vacuum tube versions of these designs. (Pictured above) It should be noted however that it is the integrated close coupled system design, rather the vacuum tubes themselves, that create the problem.

Conversely the system design known as an "active split system" where in many cases the existing geyser can be employed and with the water circulated by use of a pump, is not prone to overheating since it is able to harness the power of the more efficient technology by providing failsafe control. It also has aesthetic benefits, not requiring roof top tanks.

This is the design advocated by Sky Power as it resolves the issue of overheating and simultaneously enables year round performance through its ability to provide control.

More advanced technology which permits control of maximum temperature safetly.

The ability to control circulation effectively provides the ability to throttle back thermal delivery once the desired temperature has been achieved. In this way systems remain safe even when left unused for extended periods. Such as when the owner is away on holiday for example.

Expected water temperature from solar geysers

Because of the fact that "solar insolation" levels, (energy from the sun), across the country are highly variable it requires that products be both correctly sized and oriented for their installed location if they are to be able to provide a useful year round energy source.

To highlight this phenomenon, Sky Power has launched a graphical utility, which attempts to predict water temperature achievable from a solar water heating systems.

By entering the size of the geyser the Q factor of the solar collector product and the location of the installation the tool create a graph showing expected water temperature for each month of the year.

The utility can be downloaded from: http://www.skypower.co.za/content/calculators/power_demand.asp

Whilst it is a very simple computer modelling tool, it presents graphically the expected water temperature achievable from installing SWH systems at different locations across the country.
Expected water temperature is plotted with consideration to the size of the geyser and the power delivery capability of the product, as measured by the SABS (Q Factor). It is further adjusted to reflect the effect of solar energy levels at the installation site, in conjunction with the elevation of the collector and the effect of both cold water temperature and thermal losses throughout the year.

In South Africa collector elevation is very important as its effects the energy available to the system and varies dramatically between the higher and lower latitudes.

Above all this utility helps to highlight three issues:
Firstly that a "one a size fits all" design is not suitable for South Africa. It demonstrates that solar collectors need to be sized according to the location at which they will be installed and matched to the volume of water they are associated with.
Secondly, that to be effective in replacing electrical energy for the purposes of water heating, the system is required to be sufficiently powerful to work in winter whilst having the necessary control mechanisms to prevent overheating.
Thirdly, that certain products currently being marketed will produce very little temperature rise in winter conditions even if optimised through correct elevation.

Modular system designs such as active split systems offer greater flexibility and more importantly control in achieving useful energy delivery.

Possibilities further exist to upgrade such systems to include pool heating for example. On large scale projects other techniques provide the means to dump energy when required by the use of thermal dissipaters which harmlessly redistribute any excess energy back to the atmosphere.

Surprisingly such powerful systems with suitable control mechanisms to eliminate potential overheating problems are often no more expensive than many of the older passive systems.

Response to Star Consumer Watch Article

In response to the Star Consumer watch article. Monday 28th June.


http://www.iol.co.za/index.php?click_id=3027


One critical point which has been omitted from this article is that the problems that have been encountered are entirely with flat plate systems. There has been no mention of that fact that Evacuated tube systems are more than adequately frost tolerant.

We have not had a single problem with the recent cold weather. Said Barry Cribb MD of Sky Power.

As mentioned in the article quite correctly, flat plate systems are notoriously prone to freezing because of their design. The very thin fluid channels in contact with the flat sheet of metal can only be protected by an antifreeze solution, typically propylene glycol and configured in what is referred to as an indirect configuration.

Evacuated tube systems conversely use a similar alcohol within a sealed copper pipe which will withstand temperatures down to minus 30 degrees. This is one reason why they are becoming more popular in the consistently colder climates of northern Europe.

The above two photographs above were taken in Rivonia, Johannesburg.

The tubes can be seen clearly covered in ice. Outside roof temperature was measure at -7 degrees Celcius.

No damage was caused to either of the systems inspite of the fact that the electronic frost protection mechanism had been disabled as proof of resilience.

It is important to understand the difference in performance of the various system designs and not brand all solar water heaters as frost sensitive. The more efficient evacuated tube systems are inherently frost resistant.

A word about configuration

A word about configurations

Technical Yes, but really quite simple.

There are many different types of Solar Water Heater now coming on to the market offering various levels of performance and various configuration possibilities.

Pre heat vs. Primary system.

A pre heater configuration heats water in a separate secondary tank using solar energy and then feeds this pre-heated water into your normal electric primary geyser. The theory being that this can reduce the amount of work your electric geyser has to do and so save some money.

Whilst this method can provide some benefit, the down side to this is that the pre-heated water is only transferred to the main geyser when you draw water. This means that the water in the electric primary geyser must still be kept hot electrically. Therefore still using quite a lot of electricity to keep the water hot, overnight.

The power rating, i.e the Q factor, is still just as important as you still need to know if the solar collector can produce as much hot water as your will draw each day. If not the electric geyser will have still more work to do than just keeping the water hot.

A primary heating configuration on the other hand can be configured to allow the water in the tank to be hotter than required from solar input prior to the evening. Thus providing some additional energy to stay hot over night or at very least, reduce the amount of electricity required to do so.

Conclusion: Wherever possible one should opt for a Primary configuration

How long untill I get my money back

What about financial payback?
Approximately 5 years

A typical 200L geyser uses an average of approximately 11-13 kWh per day to maintain the temperature at 60 degrees. At the current 39c/kWh this resolves to R4.29 per day or 1565.85 per annum.

With projected Eskom tariff increases of 30% per annum the recovery point for a system delivering 9.23kWh into a 200L tank will be reached between 5 and 6 years based on the installed price of the 9.23kWh product of R17, 300

Anything less powerful than 9.23kWh into 200L will take correspondingly longer to recover.

kWh Cost Annual Saving
Year 1 R 0.39 R 1,575.89
Year 2 R 0.51 R 2,048.65
Year 3 R 0.66 R 2,663.25
Year 4 R 0.86 R 3,462.22
Year 5 R 1.12 R 4,500.89
Year 6 R 1.46 R 5,851.16
Total R 20,102.07

See the Sky Power Payback Calculator: -
http://www.skypower.co.za/content/calculators/power_demand.asp

Any 200L product with a lower Q factor forecasting a shorter payback will be overly optimistic.

The true cost of an Installation

Cost per kilowatt-hour(for the capital cost of the system)The true cost of the system.

Using the Eskom web site as the source of data. Indicated installed pricing for solar water heater varies from R22, 700 down to R14, 900.

The Q factor explained earlier is used by Eskom to determine the value of the subsidy for which the product will be eligible. The subsidy has recently been revised and no longer applies linearly relative to Q factor.

Nonetheless a products Q factor can be determined from its subsidy value, though the calculation is more complicated than before.

Please visit our web site for our product performance evaluator. This allows you to calculate the Q factor and an estimate of how much energy any system will deliver from solar input.

The calculator also provides a "cost per kilowatt hour" comparison to help you select a suitable product by comparing apples with apples.

http://www.skypower.co.za/content/calculators/power_demand.asp

It is imperative that purchasing decisions be made on performance/price rather than just price.

But which is the most cost-effective product to buy? Divide the Q factor by the indicated pricing and you come up with a price per Kilowatt-hour. Figures range from R1874 to R3995 per kWh.

Still be wary however of any very low offerings as this might indicate the supplier is allowing insufficient funds to provide a reliable after sales service.

Retrofit or New Geyser

Retrofit to existing or complete replacement

Solar geysers for which you pay a premium are required to have a better standing loss performance than a standard geyser.

The SABS spec states that a 200L Solar geyser must achieve 2.26 kWh or better. That’s only 0.04kWh better than a popular brand of standard geyser. A good argument in itself to uses a system that will fit to your existing geyser rather than buy a new one.

In terms of losses there is often no significant difference between a solar geyser and a good standard one with adequate insulation. Greater losses come from un-insulated pipe work.

Almost invariably it's the suppliers who market integrated systems (though with built in tanks ) that decry the use of existing geysers.

Thermosyphon vs. Active Circulation

Thermosyphon vs. Active Circulation.

The most common image one has of a solar heater is of a roof tank with a flat plate collector positioned beneath it in what is known as a close-coupled configuration. This is the most common configuration. Known as thermosyphon, it relies on the fact that hot water rises.

Whilst this is the simplest of all configuration options, like many older designs, it has a number of disadvantages when compared to current technology.

Firstly the heat transfer rate is very slow. Secondly and more importantly it has no means to control top end temperatures.

For this reason many thermosyphon configurations are sized to prevent overheating and even boiling in summer conditions.

Being undersized to prevent overheating also prevents them from offering adequate performance in winter.

Conversely correctly sized systems having no control often present dangerously hot water temperatures.

Active circulation mechanisms however whilst being slightly more complicated are able to control temperature delivery and so can provide superb winter performance without the problem of overheating in summer or if the system is unused when the property is vacant for example during holidays, etc.

This is achieve by one of two methods, either diverting water flow to a heat dump to dissipate the excess energy on large scales systems or by allowing the collector to stagnate in the case of small systems.

Whilst it can be possible to locate a thermosyphon tank out of sight in a “separated” configuration, another benefit that active circulation brings is that the storage tank or geyser can be placed anywhere as opposed having to be above the collector.

In many cases active systems can employ the existing geyser as the primary storage tank rather than having to install a second or new one.

Although flat plate systems are more commonly used in close-coupled arrangements, certain manufacturers of evacuated tube systems also offer a close-coupled thermosyphon arrangements.

These systems however are equally as prone to overheating for the same reason having no means to turn off the energy delivery once the water is at the required temperature.
The result again is that such systems are under specified in order to prevent excessively high temperatures being reached in summer.

Not withstanding this these systems can still boil if left unused during holiday periods. This is of course is potentially dangerous as well as being wasteful of water, as ultimately the systems safety valve will discharge the entire volume of water to bring the temperature under control.

To borrow a statement from a Pirelli advertisement: – Power is nothing without control.

Conclusion: Active circulation provides control to achieve year round performance and protect against overheating.

Does Solar Really work ?

Whether a solar water heating system will provide any real benefit is purely a function how much energy the volume of water in question requires and how much energy the collector can deliver and when.

The energy required to heat for example, a 200L geyser by 40 degrees Celsius is 9.13kWh.
Systems currently listed on the Eskom web site show products with energy delivery ratings from 3.74kWh to 9.23kWh for 200L systems.

This rating is known as the Q factor. It is produced from SABS test measurements and indicates the heat output from an insolation level of 16MJ or 4.44kWh/m2/day of sunlight energy.

This means that the 3.74kWh product will provide 41% of the energy required whereas the 9.23kWh product will deliver 101% of the energy required to achieve a 40-degree temperature rise in the 200L of water.

Many systems available on the subsidy scheme are, in my opinion, under powered some of which are even S ABS Mark approved. Unfortunately, mark approval in the case of Solar Water Heating equipment does not indicate that the product is fit for purpose.

The minimum output energy rating required for approval being only 2.5kWh regardless of the volume of water into which the energy is being delivered. Typically in a 200l geyser 2.5kWh is just enough to keep the water hot. It will not provide sufficient energy to reheat the water used.
Make sure you select a product with a High Q Factor and compare it to the required to heat the volume of water in question.

See the Sky Power Product Performance Evaluator:http://www.skypower.co.za/content/calculators/power_demand.asp