Wednesday, July 01, 2009

Performance of the DIY tracking solar parabola array

Many of you have asked about the performance of my DIY (Do It Yourself) solar heating project.

Performance has been somewhat of a moving target since I have been making various changes and improvements but I have enough experience now to provide some detail on the measurements that I have made, to comment on some of the factors that influence the results and to show the calculations that I make to determine the heat capture of the system and to estimate it's efficiency.

With apologies to my metric readers, I will be using Imperial measures.

My solar pool heater project currently consists of 13 parabolic trough reflectors faced with acrylic mirrors which rotate automatically to track the sun over 50 degrees east or west from vertical. At the focus and mass center of each parabola is a matte black coated copper collector tube through which the water flows and receives the sun's heat which is concentrated by the reflector. The reflectors hang down from the collector tubes and simply pivot on them. The reflectors are pushed or pulled into position by means of a slender steel rod which couples them all to a small gear motor drive.

The collector tubes are connected in series so that the pool water flows through each collector one after the other.

By measuring the flow rate and the inlet and outlet temperatures of the water, I can calculate the amount of heat which is being added to the water by the solar array.

Inside the pump shed, I have installed a pitot tube acrylic flowmeter from Blue-White Industries Ltd ( model F-300 with a range of 20-100 gpm as well as a dual differential thermometer from Fluke Electronics ( model 52 II, shown here (click on any picture to enlarge it).

The Fluke thermometer is at the top, the Blue-White flowmeter is to the left.

The temperature sensors for the thermometer are located on the white ABS pipes under the green bands. The bottom pipe is the water feed to the array, the top pipe is the return line from the array.

The Fluke thermometer measures temperature to one tenth of a degree for each of two sensors and will conveniently display the temperature difference between the two, also to one tenth of a degree. Fluke claims an accuracy of plus or minus 0.2% when used with K type thermocouples (the sensors that I am using) when under 100 degrees C.

The Blue-White flowmeter is inserted in a hole that I've drilled in the ABS pipe and is clamped to the pipe with the band clamps. It protrudes slightly into interior of the pipe and draws a small sample of the flow which is diverted up through a channel in which rides a small steel puck. By reading the height of the top of the puck against the scale, I can read the flow rate, in this case 37 gpm.

Blue-White claims a full scale accuracy of plus or minus 10% for this meter. I would have preferred to use a more accurate type of flowmeter but the approx $75 cost of the F-300 fits the project budget.

The water flow rate is fairly constant at about 37 gallons per minute. It will fall slightly to about 35 gpm when the sand filter needs back flushing but after that operation is done the flow rate will return to 37 gpm. Because of the +/- 10% tolerance, my actual flow rate could be as low as 33.3 gpm or as high as 40.7 gpm. I've used 37 gpm for the calculations.

The temperature difference readings I have taken (between the inlet and outlet) over the past two years vary considerably from a low of about 1.8 degrees F to a high of 2.6 degrees F.

Obviously, a bright midsummer sun high in the sky (ideally at solar noon when it is directly overhead) against a crystal clear blue sky gives the highest reading. Passing clouds impair the result as does a hazy sky.

Dirty reflectors covered with dust or pollen give lower readings. My best results are obtained after the reflectors have been washed by a recent rainfall. I seldom wash the reflectors myself since I've found that once the spring pollen season is over, the reflectors pretty much keep themselves clean.

Poorly focussed reflectors (because the reflector drive linkage or the sensor aim has not been properly adjusted) will give lower results.

Reflectors that do not properly conform to the parabolic shape of the ribs due to deformation will give lower results. I have been experiencing some deformation of the acrylic mirrors due to backside heating which I have recently solved by painting the backs of the mirrors with white paint. Only five of the 13 mirrors have been treated this way and the results so far are that problem has been solved for those five reflectors. The other eight show various amounts of deformation so that I cannot achieve the best results now until those eight are replaced.

I will write more about this problem and it's solution shortly.

The high reading of 2.6 degrees F was reached last year when all mirrors were clean and not deformed in any way and the array and sensor focus had been carefully done. That reading was taken in late August. Had it been done on a clear day around the summer solstice (June 21) it might have been higher still. Nevertheless, I have used 2.6 degrees F for the calculations even though I have not recently seen a value that high for the reasons mentioned.

So, using a temperature rise of 2.6 degrees F at a flow rate of 37 gpm:

One BTU (British thermal Unit) is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. Heating is usually expressed in BTU/hr.

At a flow rate of 37 gpm, in one hour (37 x 60) = 2220 gph.

The weight of one gallon of water is 8.345 pounds, so the weight of water which flows through the array in one hour is (2220 x 8.345) = 18,526 lbs.

If the water is heated by 2.6 degrees F, then the heating power of the array is (18,526 x 2.6) = 48,168 BTU/hr.

One BTU/hr is approximately 0.0002931 kWh so the array generates approximately (48,168 x 0.0002931) = 14.118 kWh.

There are 13 sections in the array, so each contributes:
(48,168/13) = 3705 BTU/hr
(14.118/13) = 1.086 kWhr

I estimate the efficiency as follows:

The sun energy (insolation) in my area (43.7N -80.0W) that falls in one square meter (1550 square inches) is approximately 1150 watts or (1000/1550) = 0.645 watts/ The optical size of each section of my array (the aperture) is 19.375" x 96" or 1860 sq. inches and I am capturing 1086 watts per section or (1086/1860) = 0.584 watts/ So the capture efficiency is approximately (0.584/0.645x100) = 90%.

This result does seem to be a bit high and probably not right. I will continue to work on this.

As always, your interest and comments are welcome. I have much more to tell you about and you can watch this blog for updates.

The plans are coming along and should be available shortly. Thank you for your patience.

Update: My plan book is available here.


Ben said...

Your figures add up, though I'm sure you meant to say kW instead of kWh. 90% doesn't seem too high for once-through efficiency close to ambient temperature. Obviously this system has little conversion loss.

That sounds like a rather high flowrate. I wonder what is the energy usage of your pump?

george plhak said...

Thanks Ben.
I'm not a thermodynamics expert by any means but if I am talking about the heat to raise one pound of water by one degree and then go on to calculate the number of pounds of water in an hour, I am talking about kwH in a conversion, no?
The flowrate is high and is the entire flow from the pool pump and filter. The pump has a nameplate of 1 hp, or about 750 watts. I am planning to put a smaller, solar powered pump in parallel with the main pump and use the main pump only for vacuuming.

gittel said...

Ben is correct. A BTU is a unit of energy. BTU/h is a unit of power (which is a rate of energy usage). 1 Btu/hour = 0.000293071 kilowatts

However, don't feel too bad. The BTU nomenclature is totally confusing! Check this out from Wikipedia: "When used as a unit of power, BTU per hour (BTU/h) is the correct unit, though this is often abbreviated to just "BTU"." WTF? So how am I supposed to know if someone really means BTU, or if they really mean BTU/h?

To make things even more confusing, the air conditioning industry uses the acronym "BTUH" to mean BTU per HOUR (divide BTU by hours), which is completely different from the usage of most other energy terms, such as kWh, which is kilowatts multiplied by hours, not kilowatts divided by hours.

Ourstralian said...

Very interesting.
I am designing a 'precinct scale'composting process that outputs heated water and/or air. I haven't built a prototype yet.
I intend to use the heated water as an input to a water heater. Given the energy curve for boiling water, there should be substantial energy savings.
Would a system like that be a good "pre-heater" for your system,with yours then being a preheater for my landlord's electric water system?
I am, for a number of reasons, putting the composting system into a greenhouse.
Would your system Function even more efficiently inside one? I know there are condensation issues.

george plhak said...

Certainly a solar trough heater could function as a pre-heater or as a final heater since it will effectively raise the temperature of the inlet water in either case.
I am pretty sure that placing the solar heater inside a greenhouse will reduce it's efficiency since the glass will filter some of the sunlight reaching the solar heater.
Thanks for your interest and comment.

harishdarji said...

Hello Georges I am very interested in your parabolic trough solar water heating. I wanted to purchage your guide. how I can get E book for this.

george plhak said...

Thank you harishdarji. I have added a link to the book at the end of the article.