Consulting Engineering

Greenville, South Carolina

In the fall of 2008 a plumbing contractor client asked about the WGBES knowledge and experience with solar thermal design. The reply was: “limited”. This would be typical for most mechanical engineering firms because solar thermal has been relegated to the niche category for the past generation. However, the client’s question was obviously timely for several reasons not the least of which are: 1) a growing interest in renewable technologies (particularly those deemed “zero emissions”), 2) the rising cost and general instability of energy prices, and 3) concerns regarding global climate change. So, in recognition of these facts and with a desire to move WGBES from solar thermal “limited” to solar thermal “knowledgeable”, some intensive study was undertaken. What we soon discovered is that the solar thermal industry and technology in particular have something of a third world nature: fairly undeveloped and scarcely documented. It quickly became apparent that the in-depth knowledge required to understand the technology and design systems would have to be gained first hand. So, with the interest and help of the original plumbing contractor and another, some vendors, and a few engineering associates, we set out to design, construct and operate a solar thermal installation of our own.

Our Solar Thermal Project...........

 We finished the installation in early August, 2009 and have been collecting data and making observations since. The system we have been testing is a single, conventional 4’x8’ flat plate solar collector (which was donated by a manufacturer’s distributor) connected to a 55-gallon storage system. The collector is mounted on the ground in a south-facing orientation and is constructed such that the angle can be adjusted (note that the “ideal” angle changes throughout the year as the position of the sun in the sky changes from low in the winter to high in the summer). We chose this feature for testing purposes; however, most systems would have a single fixed position which generally should be the degree angle relative to horizontal of the location latitude (for our site in Greenville, SC this would be about 35-degrees from horizontal). Also, we configured our system so that it can be operated in a direct arrangement during periods when there is no danger of freezing (direct means that the domestic water to be used is circulated and heated directly through the collector such that there is water in all pipes of the system at all times). However, during cold weather we can convert to an indirect arrangement which allows for “drain-back” through a secondary tank and heat exchanger such that there is no water in the collector or outdoor pipes except when the collector controller calls for circulation when temperatures are above freezing.

 In a given day most all of the contents of the tank are used; so, on average the daily use of our system is about 50-gallons. We have found that the daily tank temperature rise averaged across the year is 25-degrees (plus or minus). So, the average daily energy “collection” would be about 10,500 Btu (1.0 Btu/lbm-F x 50 gal x 8.34 lbm/gal x 25F). This equates to around 3 KW-hours per day. For the year this would be around 1,100 KW-hours which adds up to a savings of around $90 if the water were heated in a conventional electric unit at $0.08 /KW-hour. Obviously these results are fairly modest. However, we have kept the collector at about a 20-degree angle; this reduces its effectiveness somewhat though likely not more than 20%. Also, our site is such that the collector is in the shade until about mid-morning (so, maybe another 10-15% reduction). And, the recommended standard for a flat plate collector is 2 gallons of storage per square foot of net aperture collector area; our collector has an aperture area of 31 sq. ft. meaning that our ideal storage would be 62-gallons instead of 55— again, this would cause some reduction in effectiveness. The theoretical “harvest” of the collector based on the efficiency curves as tested by the SRCC is around 3,000 KW-hours per year for our location according to our calculations. So, our actual collection seems to be only about 1/3 of theoretical. However, seemingly with all things solar, it is rare that any installation is ever ideal (you might not be able to align to the perfect orientation, or there are shading factors, etc.). Consequently, our results are probably in the range of normal expectation. And, since our system is essentially configured in a fashion that scales directly, it is reasonable to extrapolate the results for larger systems. So, our system results would indicate that the average yearly collection of a flat plate solar thermal system in our geographical area would be ~22 KW-hours/gallon/year. By comparison, a 1,000 gallon system would collect 22,000 KW-hours which would be a converted value of roughly $2,000 in electricity or natural gas savings. This system would require sixteen 4’x8’ collectors, a storage tank the size of an average in-ground septic tank, and a pipe network and pumps to supply about 10 gallons per minute (0.6 gpm for each 4’x8’ collector). Such an installation would cost in the neighborhood of $20,000 or more. A straight-line 10-year payback.

So, our conclusions after a few years of testing would be:

-- Solar thermal works, but expectations have to be tempered— it is not big energy. For domestic water heating it is a supplement only and not a stand-alone system (meaning that there has to be a back-up heater). And, without significant cost off-sets (grants, tax credits, etc.) an installation is likely not going to be strictly cost-justified over the period of its useful life. Also, when you factor all of the carbon that goes into an installation (the energy required to produce and install the components), it will take some time before a system would actually begin to achieve a net carbon reduction.
 
-- A conventional flat plate collector system is not a “hot” water producing device if it is optimized for capacity. This means, that if you want to maximize it’s effectiveness, you are going to heat more water to a lower temperature— this is where the factor of 2 gallons of collection capacity per square foot of aperture area is derived. In winter the highest collector temperatures we have seen are about 95F-100F and in summer they are 130-140F; the average winter collector tank temperatures were about 80F and the average summer about 120F. [But, be warned, those are NOT the highest temperatures the system can produce as we have seen temps coming out of the collector as high as 200F]. The result of this is that a conventional flat plate collector system is not really suited for space heating since the average tank temperature is usually never more than about 75-80F in winter (and you won’t get a lot of air temperature rise out of that as a heating media without a tremendous storage capacity). In the future there is some promise that solar thermal could be used in conjunction with liquid desiccants to provide moisture removal in air cooling functions. But, the realistic current use of solar thermal is limited to domestic water heating (i.e., bathing, etc.).

--The system is not “maintenance-free” and would not be in any conceivable arrangement. Some companies install glycol systems that won’t freeze or use a drain-back arrangement all of the time to supposedly eliminate owner oversight. But even these are realistically going to require some attention, and the elements that may make them less of a headache (non-freezing solutions such as glycol or indirect arrangements or both) also make them less effective. When you add complication to something (which, of course you are doing when you add more pipes, pumps, valves, controls, etc. to your water heating system) it always requires more attention.

--The industry associated with this technology needs to be more robust for it to be broadly applied. The shared knowledge base from the manufacturers on down seems somewhat limited and definitely hard to access. On the other hand, this should not really be all that surprising. Given our enormous scale of energy consumption, the capability of conventional solar thermal is relatively miniscule. It has naturally been relegated to a niche element throughout its history of use. Certainly, this could change as it gains some greater favor. But that will only realistically happen when (and if) we ever reduce our relative consumption such that the “modest” production of solar thermal becomes relatively more significant. Until such time, engineers, installers and owners are going to be left scratching their heads and learning much by the seat of their pants as we have over the last two years. And, don’t misinterpret this observation, there are certainly individuals and entities in this industry that are highly knowledgeable. But, it seems that much of this knowledge is either concealed (after all, is a manufacturer really going to be anxious to tell you his product may not have a pay-back?) or it is tightly held by the few experts that understandably seek to sell their experience and technical know-how.

So, do these modest conclusions mean that our efforts have been a waste. Not at all! This hands-on effort makes for much engineering learning as the effort was definitely “do-it-yourself” from start to finish and all of the busted knuckles and singed arm hair in between (it takes time to become competent at sweating copper with a propane torch!). There are many engineering principles at play in our system; and it is good to turn paper into pipe, so to speak, on your own dime and then learn from your mistakes and successes. But, beyond these practical elements, this effort has certainly been enlightening with regard to the “sustainability” world we will increasingly face in engineering and otherwise. It is not enough to talk “pie in the sky” as we must quickly turn talk into results. As practitioners we need to be grounded and knowledgeable, and often that means telling people “hold on” when they are preparing to commit valuable resources to an endeavor. We have a LONG way to go to achieve a sustainable society. And, we (engineering professionals) have an obligation to test reasonable ideas and seek to find the best opportunities. Does this mean that solar thermal goes exclusively into the “hold on” category? Not necessarily; but we would definitely say with confidence that we have a long way to go to make solar thermal broadly applicable.