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Simulating Solar Heating System Performance by Computer
We've compiled verifiable predictions of what a properly sized solar pool heater will do for your pool based on typical weather for your region. This monumental task has been the result of 15 years of hard work on the part of many people in Canadian government, universities, independant consultants and industry experts like myself. I finally got around to publishing some of this capability. I had to learn how to extract numerical data from the computer simulations and plot it onto a graph that would make sense to a pool owner. I enthusiastically invite further contributions and suggestions from any scientists, engineers, government people and even competitors. We all need to work together to let the public know the truth. Credibility sells and credibility comes from a concerted and honest effort to get at the truth.
I've spent my entire career with nothing much to show for it other than expertise in solar pool heating. In the 1980's and most of the '90's I was the president of one of only 3 manufacturers of this type of technology in Canada. The market for solar in Canada is weak so I had a lot of time to exercise my engineering degree and investigate new applications for the technologies and better ways to bring it all into the mainstream. The Canadian government was very co-operative. In 1989 the NRC, Canada (Natural Resourses Canada) tested all the collectors sold in Canada at the time in Canada's indoor solar simulation laboratory, then Ortech International at Queens University in Waterloo Ontario. We got our collector, the Powermat, included in that testing and a long attempt at collaboration with government began. I learned that the needs and wants of a small solar company trying to exist in the free market are quite different that the needs of politicians and the government officials assigned the task of spending large sums of taxpayer's money to stimulate an industry, however, this story was very positive. Leading up to this time, a group of engineering masters students at Waterloo University had created a DOS based program that would simulate the energy balance on a swimming pool and what a solar heater would do for it in direct comparison to a gas heater. This computer program called "Watsun" needed the results of NRCan's performance testing. At the time I barely knew how to use a computer but I jumped on board eager to learn how to give people a reasonable prediction of what a solar heater would do for their pool. 14 years later I'm reasonably certain we've arrived at that goal. Adding this sentence another 10 years later I can say I'm now positive!
The Canadian testing facility, now called "Exova" uses an indoor solar simulator and the testing includes the effect of wind. The collectors are actually moved through the room to simulate air movement. The only other accredited solar testing agency in North America is the Florida Solar Energy Center. That test facility is outdoors and has no way to include the effect of wind. The Canadians are way ahead of the Americans in solar pool heater testing. Here are recent test results showing the Powermat collector's performance curves and the effect of wind.
On the y-axis we have efficiency. That means the fraction of the solar radiation exposed to the collector that ends up as heat energy delivered to the water leaving the collector.
On the x axis we have the difference between air temp and pool temperature divided by the solar intensity on the collectors. Note that pool temperature is the temperature of the water entering the collector. It is the collector temperature. Depending on flow rate the average collector temp is actually a little higher but that is taken into account in these tests and the computer simulation that uses them.
The Canadian tests show our Powermat collector if anything is more efficient than most. Below are the efficiency curves at low wind speed (1 m/s) for three of the collector brands that were sold in Canada in 1990. Powermat is the one marked AO370. This is my defense against a poor FSEC rating given Powermat back in 1980 when the FSEC didn't even try to limit the wind effect. There is no question about this flaw in the FSEC tests.
At the time SG358 (now extinct) had the highest FSEC rating. As you can see solar collector performance isn't a number like 1000 BTU/sq ft/day. It is a curve that depends on several factors. In fact there are no technical differences between any of these collectors that would cause one to be more efficient than another in all situations. If it's black and cold it gains solar as efficiently as possible. That's all there is to the physics of this technology. The challenges are in the longevity, and compatibility with the pool system and the roof they attach to. These are the things consumers need to pay attention to, not the FSEC performance rating. Even when we do see marked differences according to the Canadian or the US testing we can easily attribute these to wind effects. Wind is what takes the heat off the collector and sun is what puts it in. FSEC ignores wind in their testing although at the time of this edit (2010) we have some indication they have finally started to fudge the wind effect somehow. When you look at the poor performance curve of the Enersol collector above (EN363)you can atribute this to the fact the collector is moved past the artificial sun in a direction normal to the flow cores exposing more effective cross sectional area to the wind and resulting in higher wind losses. If the wind was across the collector the gaps between the tubes would protect the flow cores in between from wind loss and the performance would be higher, perhaps higher than our Powermats. Our newer product, Powerstrip in fact has a very similar profile to Enersol's because we believe its actually an advantage in real life. We were also trying to be compatible with Enersol because we mistakenly believed they had solved their issues with breakdown from exposure to chlorinated water. EPDM solar collectors are fine as long as you don't treat your pool water.
Enormous effort has gone into that original software then called Watsun and now called Enerpool Pro available freely to the public at the NRC web site but it aint simple. It helps to be an engineer. For those of you who insist I do provide a full guide on what to input and why here so you can make use of this powerful software if you're so inclined.
In 1997 the Canadian government made a further effort to help the solar swimming pool industry in Canada by putting high level monitoring on three pools in Canada with solar heaters and comparing the results to what the Enerpool simulations told us. We participated in this study, providing one of the solar heated pools. We provided help hooking up monitoring equipment and with co-ordination. Bruce Sibbitt MASc PEng ,partner in Caneta Research of Mississauga Ontario was contracted to carry out the monitoring and analyze the results and co-ordinate changes to the Enerpool Pro software to reflect what was learned and make the software more accurate. Bruce did a great job.
The above photos show the Powermat system undergoing this high level monitoring. You can see an anemometer measuring wind speed at pool level. There was also one on the roof. There's a pulse width modulating flow meter and a main computer hooked up to a modem making a call every night. There are expensive pyranometers on the roof measuring solar radiation horizontally and on the roof plane, temperature sensors and much more, even a flow meter and temperature sensor on the hose that refilled the pool. Data was collected for a whole season and used to validate the computer simulations. This was at least a $30,000 endeavor and it was repeated with two other systems in different parts of Canada. This was the best money the Canadian taxpayers ever invested in Solar Energy. Cudos to NRC CanMet Alternative Energy Division for spearheading it and Bruce Sibbitt for his work.
Bruce Sibbitt through his work was able to tell us the values we should input into the Enerpool program for things like the emissitivity of the pool cover, the wind speed correction factor for the roof and the pool etc. As a result of Bruce's good work and the efforts of NRC, the Enerpool software that we've used to create the predictions found on the links in this section is thought to be quite accurate for outdoor residential pools. We've verified many simulations with real systems and every indication is a close correlation with reality. Since we publishing it in a form that the general pool owning public can understand easily our existing customer base was able to compare what they found on their own solar heaters to what our predictions were telling us here. The correlation has been amazingly close. I'm editing this in 2013 and I still agree with myself.
Back to Sizing Go here definitely if you didn't come from it in the first place. You've got to see the graphs.
In 2009 we have suffered a major setback in all of this work. Just as we were launching a major initiative to start demonstrating and proving solar heating on commercial pools with our special web based monitoring and control capability NRCan sabotaged our efforts with a rebate program for commercial solar pool heaters that relied on performance ratings as the measure of the rebate allocated to each product. This inaccurate testing gave our competitor Enersol a rating of 69% compared to Fafco and others. We missed out of the testing because we improved our product from Powermat to Powerstrip and our old testing didn't qualify. That was our own fault but the end result was that many many commercial pools in Canada installed expensive solar pool heaters, many with the wrong technology (boxed and glazed collectors) and some with evacuated tubes and some pool building roofs actually have PV now! None of them with the superior and appropriate Canadian technology. You have to use flexible collectors in Canada. Start at the beginning of our web site if you don't understand why. This rush to spend tax dollars without any consideration with anyone other than industry associations with their own objectives, brought our movement to a halt. In 2010 the conservatives got elected in Canada and this spending frenzy ended. This has allowed some room for us to demonstrate what we can really do in a sensible free market viable way. We're all for a major investment in solar energy to save the world but its important to realize that in subsidizing one favorite technology you inadvertently hurt another.