Announcement

Collapse
No announcement yet.

Question about thermal vs PV

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

  • Question about thermal vs PV

    I have 22 PV panels and 2 thermal panels here in Colorado (9500 ft elev.). The PV at noon produce 5.5 kWh in an hour. The thermal panels can produce 100,000 BTUs in an hour. When I normalize this to "per square foot", the thermal produces about 30 times more than the PV. The PV efficiency is roughly 17%, while it is 84% for the thermals. Dividing by this ratio I find that the thermal panels are producing 5 to 7 times as much energy per sq ft as the PV panels. I asked some physicists about this, but do not have a definitive answer. The main reason would seem to be the fact that the solar thermal panels use infrared radiation. But can this account for a factor-of-5? Are there other things going on?

  • #2
    You have a great location. Right the PV may be only 17% efficient, because nobody knows an
    economical way to raise it much. Bruce Roe

    Comment


    • #3
      Those 2 thermal panels must be pretty big to produce 100,000 BTU under full sun, like maybe 70 or 80 ft.^2 or so each. Pool heaters ? That factor of 30 is simply wrong. Also, Infrared radiation has nothing to do with it in any way other than that's the frequency range that most common everyday devices radiate thermal energy at. (BTW, both solar thermal and PV radiate energy in the infrared.)

      For a cheap and dirty answer/explanation you may be looking for without the details: See #7 below.

      You have some confusion and confused information. Slowly:

      1.) Since the international standard is the S.I. system (similar but not identical to the "metric" system), I'll use that. Your mixed units between the British (sometimes called "customary') and S.I, or metric may be causing some of the confusion.
      2.) Assume that on a sunny day, 1,000 W/m^2 (or ~ 317 BTU/ft.^2) falls on a surface at an incidence angle of zero, perpendicular to that surface, That is, the surface is facing the sun, and for simplicity, will stay that way for about an hour. That stationary position of the sun won't happen, but assuming so will make things easier to explain, and won't change what follows.
      3.) A PV panel of, lets say, 1m^2 will sit it in the sun, and if hooked to a circuit with other stuff, will, by virtue of a potential difference in the guts of the panel, generate a current in the circuit. If all is well, the electrical energy generated by the panel that can do useful work or, if tied to an electrical grid, get exported to the grid.
      Taking losses from the wiring and other equipment in the circuit into account will result in something like, let's say 15% (or about 150 W) of the amount of energy (the 1,000 W/m^2) that falls on the panel being turned into electrical energy. The rest of the 1,000 W will either be lost as reflection off the panel glazing or as heat that raises the temp. of the panel. That heat is lost or dissipated to the surrounding by either convection loss to the atmosphere or as thermal radiation loss to the surroundings (including the sky) in a somewhat complicated way. There is also some heat loss/dissipation to the panel racking and supports by conduction, but that's usually a small % of the total heat loss.
      4.) A thermal flat plate device operates differently and has a different purpose. Basically, it's a device whose goal is to heat a medium, usually air or water. It does so by sitting in the sun and getting warmer than the surroundings. Then, a cooling medium (the air or water) is put in contact with the warmer surfaces of the thermal collector and heat is transferred, for the most part by thermal convection, to the (cooler but getting warmer from the contact) cooling medium. That circulated medium is then usually stored for later use. In a well designed solar thermal collector used to heat domestic water, for our example of 1,000 W/m^2 of incident solar radiation, perhaps 500 to 600 watts of the incident solar radiation might be transferred to the water over an hour's time as an equivalent amount of thermal energy.

      The 84 % efficiency you quote for the thermal is possible, maybe for a pool heater but not likely for DHW applications that would produce useful heat.

      5.) So, taking the generating devices only into account, in out example, the thermal collector is about 4 times more efficient at converting sunlight to some other form of usable energy (heat). ( 0.60/0.15). However, as always, it ain't that simple. The solar device's stored output (in this case warm or hot water) is hard to keep from dissipating (disappearing). Basically stuff cools down. Insulation and good system design helps but Thermodynamics (and entropy in particular as the driving force) will have its way with the thermal energy stored in the medium slowly and inexorably moving in the direction of temperature equality with its surroundings. As a rough 1st approximation, a well designed residential domestic solar water heating system will lose maybe 1/3 of its net useful collector output to standby losses. To simplify, that will reduce the useful hourly output down to say, very roughly, 550*(1-1/3) = 367 watt-hours of thermal energy for household use.
      6.) Bottom line, if we're talking about common residential systems and applications, On a per aperture area basis, solar thermal is ~ 2+ times or so more efficient at turning sunlight into thermal energy than PV panels are at turning sunlight into electrical energy.

      7.) FWIW, and this may be what you're looking for (?), that seeming disparity can be better understood in a Thermodynamic way. Electricity is a very high quality form of energy. Thermal energy is not very high on the energy quality scale. Put another way, Electricity is VERY versatile. You can do a lot of different things with it. Thermal energy will keep you warm and heat things up, but that's about it. Good as far as it goes, and that's good enough for a lot of purposes, but try (for example) operating the screen you're reading by putting a match under it. Quality costs money or effort or more resources or all 3 to produce.

      Well, all that versatility of electricity, while great, comes at a price (the no free lunch principle). The price is way back at the conversion efficiency as described above: The 15 % conversion efficiency of sunlight to electricity, vs. the ~ 60 % conversion efficiency of sunlight to thermal energy of my example is the price of making high quality stuff vs. low quality stuff.

      Comment


      • #4
        Quick response. More tomorrow. The two panels are each 4 ft x 8 ft. The 100K Btu/hour rate made sense to me on the basis of how much it is heating the water and the pump speed. It could be heating the water by 30 deg F. Anyway, the 100K BTU/ hour is what the control device reads (sometimes a little more) in the heat of the day.

        Comment


        • #5
          Originally posted by stanwagon View Post
          Quick response. More tomorrow. The two panels are each 4 ft x 8 ft. The 100K Btu/hour rate made sense to me on the basis of how much it is heating the water and the pump speed. It could be heating the water by 30 deg F. Anyway, the 100K BTU/ hour is what the control device reads (sometimes a little more) in the heat of the day.
          Not possible without a lot of irradiance concentration using mirrors or other means.

          Here's why: Using customary units, something called the "Solar Constant", that is, the sun's irradiance on a surface just above the earth's atmosphere, with that surface normal to the irradiance is ~ 428 BTU/ft.^2 (+/- ~ 3.0 over a year due to the eccentricity of the earth's orbit, making it, somewhat ironically, non constant).

          So, even in outer space, free of any atmospheric attenuation, scattering and absorption of the solar radiation, the most irradiation you'd see is (428)*(64) = 27,392 BTU/hr. on a surface normal to the sun, and that's before atmospheric attenuation is considered, which, even at 9,500 ft. above mean sea level, will reduce that extraterrestrial irradiance by probably something like 20 % or so if the atmos. is very clear. It also ignores aperture reductions for framing which will knock net area down by ~ 5+ % or so. But lets ignore those things. Those collectors you have are probably for DHW or space heating service, and if well made, and operating under common DHW service, will capture about 60-65 % of the energy that strikes them as an ~ max. under very optimal conditions.

          You will not see 100,000 BTU/hour output from those 2 collectors. Ain't gonna happen - unless you manage to use booster mirrors to increase the irradiance by something like an order of magnitude or so. At which time the collectors will either fail from excess energy input (bad things will happen from too much sun) , or what's more likely, you'll have what amounts to a boiler explosion.

          What's probably going on is that you are not using the correct/actual flowrate through the collectors in your calculation(s). The flowrate you are using is too high. Assuming the working fluid is water, I'd SWAG that your actual flowrate, if the 30 F. temp. increase is close, might be, roughly, somewhere between 3/4 and 1.0 GPM.

          Let me share a situation similar to yours, just closer to sea level.

          I've got 2 ea. 4 X 8 Sunearth selective surface water heaters. they are among the most efficient water heaters available, and I have them dialed in pretty good with flow meters, pressure gages, accurately attached thermocouples and calibrated thermometers. The best I've ever recorded for their output over an hour is ~ 12,400 BTU/hr., with a collector inlet to outlet fluid temp. diff. of ~5.2 F. measured at the collectors' inlets and outlets (plumbed in series, not parallel) on a late April day @ ~ 1300 hrs. solar time under unusually warm temps., clear skies and 320 BTU/hr. POA irradiance, and a flowrate of ~ 4.75 GPM.

          At 9,500 ft. el., and perfect conditions, you might do 10 % better than that due to less atm. attenuation. If so, and if that 30 F. temp. increase is still good, your flowrate would be ~ 0.85 GPM. But at that flowrate, your thermal efficiency wouldn't be as good as at a higher flowrate. But let's ignore that too as more muddying of the waters.

          Get a rotometer. I've got 2 made by (or more accurately sold by) Pentair. Paid ~ $95/ea. about 10 yrs. ago.

          I'll be leaving tomorrow A.M. for 3 weeks. Others may want to offer some guidance/opinion in the meantime. Lucman ? You out there ?

          Comment


          • #6
            You are correct in assuming that the thermal panels operate at a higher efficiency than the PV. You can go to the SRCC web site put in your panel model and find out what the output of the panel is in a standardized laboratory setting.
            But I doubt that any normal sized panel can put out 100K BTU per hr. Most 4x8 panels may output around 20-25k btu per day in optimum conditions like yours. Your controller is assuming an incorrect flow rate to come up with those kind of numbers. I set the flow rates for flat plates at around an 10 degree delta T at solar noon for max efficiency.
            Total BTU's per day can be calculated by the following equation, 1 btu is required to heat 1 pound of water 1 degree. To calculate your BTU's (gallons of water in your storage x 8.33 lbs. x temperature rise = BTU) 1 gallon of water weighs 8.33 #.

            Comment


            • #7
              Re the 100K BTU figure that the controller tells me. If the temperature difference is 30 degrees (which I am seeing via the gauges on the pipes to and from the tank) then the pump would have to move 6.7 gallons per minute to get 100000 BTU/hour rate. I think this is within the specs. of the WILO pump I have. This link

              http://www.wilo-usa.com/fileadmin/us..._ECORFC_11.pdf

              shows that the pump has a max of 15 GPM. Now this can vary with the height of where the glycol is going, but it seems plausible that the 100K/hour reading given is correct.
              Note that I have two Viessman thermal panels, at about 4 ft by 8 ft each.

              And I missed JPM's post: (I am in hospital recovering from a small "procedure") Of course if the 100K figure is very wrong then my puzzle completely disappears. I had the panels replaced recently (snow damage on the SolarHot ones, now Viessman) and the contractor put in two temp/press gauges on the copper lines to and from the tank. So I can see the temperature difference quite clearly there, and it is common for it to come in at 110 and return at 140. SO I think the 30 is a decent rough estimate. The controller is measuring the temps at the glycol line, but 30 is a good estimate. So the question is how much the pump is pumping. I see that a flow measurer would resolve this....interesting idea. Of course, my assumption was that this was being done by the controller in its computation of the 100k BTU/hr rate. [[I am not certain of the brand of the controller -- the installer of the SolarHot panels is not in business and it was a struggle to find someone to replace them.

              Aside: All this happened because of the BPSolar Class Action suit. I was the beneficiary of a new set of panels paid for by BPSolar. While on the roof the PV contractor observed the damage to the two solar thermal panels. Easy come, easy go. But it was pretty amazing to get a completely new set of PV panels for free after 10 years of use!

              More: JPM: But you have measured 4.75 gal per minute. I am surprised at the 5 degree temp difference you report. At noon here, the roof temp reaches 150 or 160 commonly and the water at the bottom of the tank is about 110 or so, so the increase in temp. does seem to be the 30 I reported. Plugging in a flow rate of 4.75 to a diff. of 30 gives 71000 BTU/hour, not that far from 100K. But I see that you are arguing from the physical knowledge of incoming energy to deduce that my flow rate would be 0.85 gpm. So, as you say, a flow measurer would tell the truth.
              Last edited by stanwagon; 05-23-2018, 09:59 AM.

              Comment


              • #8
                Like those physicists, I could care less about about correcting math. I am curious about how much this thermal system cost. Everything I have read indicates when panel costs drop below $1.50 a watt that PV beats out thermal. I heat water with PV and my system costs are very low.

                Comment


                • #9
                  Indeed, the total BTUs generated gave me a rough idea about when the system was paying for itself. 100000 BTUs = 1 therm and a therm costs, roughly $1. Heat is lost in storage but this value of $10000 tells me that the system, which cost me about 6500$ has paid for itself, or close. And of course, our bills for gas have become much smaller (but we still use gas to hear house, so I cannot quantify this). In short, it does seem like thermal pays for itself a lot quicker than PV. We generate $700 a year worth of electricity from PV panels but the installation in 2008 was quite expensive. As you all know, costs have come down a lot so payback time is a lot less for PV.

                  The specifics re cost: The actual cost of the thermal system in 2010 was 13000$ but credits brought my cost down to about $7000. But the two panels broke down and I just spent $5000 to replace them with two Viessman panels (and situate them more intelligently as regards snow/ice).

                  Comment


                  • #10
                    Originally posted by stanwagon View Post
                    Re the 100K BTU figure that the controller tells me. If the temperature difference is 30 degrees (which I am seeing via the gauges on the pipes to and from the tank) then the pump would have to move 6.7 gallons per minute to get 100000 BTU/hour rate. I think this is within the specs. of the WILO pump I have. This link

                    http://www.wilo-usa.com/fileadmin/us..._ECORFC_11.pdf

                    shows that the pump has a max of 15 GPM. Now this can vary with the height of where the glycol is going, but it seems plausible that the 100K/hour reading given is correct.
                    Note that I have two Viessman thermal panels, at about 4 ft by 8 ft each.

                    And I missed JPM's post: (I am in hospital recovering from a small "procedure") Of course if the 100K figure is very wrong then my puzzle completely disappears. I had the panels replaced recently (snow damage on the SolarHot ones, now Viessman) and the contractor put in two temp/press gauges on the copper lines to and from the tank. So I can see the temperature difference quite clearly there, and it is common for it to come in at 110 and return at 140. SO I think the 30 is a decent rough estimate. The controller is measuring the temps at the glycol line, but 30 is a good estimate. So the question is how much the pump is pumping. I see that a flow measurer would resolve this....interesting idea. Of course, my assumption was that this was being done by the controller in its computation of the 100k BTU/hr rate. [[I am not certain of the brand of the controller -- the installer of the SolarHot panels is not in business and it was a struggle to find someone to replace them.

                    Aside: All this happened because of the BPSolar Class Action suit. I was the beneficiary of a new set of panels paid for by BPSolar. While on the roof the PV contractor observed the damage to the two solar thermal panels. Easy come, easy go. But it was pretty amazing to get a completely new set of PV panels for free after 10 years of use!
                    Some things:

                    1.) Your temperatures are probably reasonably accurate, at least in terms of what they are rerading in terms of the difference between them. The estimated flowrates are not anywhere near what you think or what's being reported. Depending on how close those temp. gages are to the collectors, the readings will also reflect and include temp. losses in the lines. If the lines are not insulated and/or the temp. measuring devices are far from the collector inlet/outlet the temp. losses in the lines will DEcrease the temp. difference between the 2 thermometers. Also, a low flowrate, as I strongly suspect, will make that temp. drop in the lines greater than a higher flowrate will (although the actual heat loss in the lines will probably not change much).

                    2.) The flowrate, once flow is established and the system is running at something called "steady state", flowrate is pretty much independent of any height differences. The term "head" refers to the pressure drop induced as the pump forces fluid thorough the system. The longer the piping runs, more pipe bends, more valves and other devices in the glycol loop, etc., the more pressure drop, or "pressure head" you'll have in the system. That increase will decrease the flowrate. Also, as flowrate increases, pressure drop will increase, usually and as a 1st approx., proportionally to the square of the flowrate, but as pump characteristics and features, and as system particulars may alter as can be seen from the pump curve. See the pump curve for an estimate of how the pressure drop will influence pump flowrate.

                    3.) Glycol/H2O has different physical properties (specific gravity, viscosity, specific heat, thermal conductivity, surface tension and other things) than water. For fluids/piping engineering, those characteristics are called transport properties. Long story short, 50/50 glycol H2O mix. will induce more pressure drop in a system than an equal volume flowrate of water, mostly, but not entirely because it's more viscous than water. Looking at the pump curve. All that will change the flowrate and heat transfer calcs slightly, but the biggest (but still not more than maybe 10 % or so) effect will be on pressure drop ( or "head").
                    Last edited by J.P.M.; 05-24-2018, 11:09 AM. Reason: changed increase to Decrease. Apologies for the error.

                    Comment


                    • #11
                      The two recently installed temp/pressure gauges are on the lines from and to the storage tank and within inches of the heat exchanger.

                      Comment


                      • #12
                        Originally posted by stanwagon View Post
                        The two recently installed temp/pressure gauges are on the lines from and to the storage tank and within inches of the heat exchanger.
                        Then you'll be getting a better idea of the HX hot side inlet/outlet delta T, and a less accurate idea of the delta T across the collectors as the delta T will include most of the piping losses. Assuming the pressure drop across the storage tank is small compared to the balance of the system, use the pressure gages to get some SWAG at pressure drop and relate that to flow rate by using the pump curve. Just know that usual cheap pressure gages are often off by 5 - 10 PSI. If the gage taps have shutoffs/valves, measure the pressure at each point with the same gage by swapping and using one gage for both measurements and note/write down the diff. when the system is operating.

                        As for price, while it's not part of this conversation, although it's possible to get reasonable price for solar thermal, customer ignorance and other things makes it rather high $$. Even so, and as much as much as I like chasing BTU's, if I was doing solar thermal again, and I lived in a place that didn't freeze much, I wouldn't. I'd get a heat pump H2O heater and bump any PV system size. At the end of the day, the heat pump/PV system, in terms of output/input eff. would be about the same, the cost/and cost effectiveness would probably be a lot better, and things would be a fair amount easier to maintain, provided the heat pump maint. was not a problem. However, in a freezing climate, the economics would feed careful evaluation.

                        Written from Chicago.
                        Last edited by J.P.M.; 05-24-2018, 11:10 AM. Reason: spelling.

                        Comment


                        • #13
                          JPM

                          Latest thoughts:

                          0. The temp. diff I am seeing (in pic below) is surely a lower bound on the temperature changes caused by the sun. There are (small) losses in the glycol line, but what I am seeing on the pipes is a good estimate, and is in any case a lower bound. I see a 30 degF difference a lot, and the numbers are confirmed by just touching the two pipes in question. I think you indicated that the biggest temp. difference you have seen is 5 degF? That seems very small. Surely the point of solar thermal is to increase the water temp more than 5. I see you are in Chicago. Your incoming water might be warmer than mine, which is 50 degF. Maybe I misunderstand your 5 degF number?

                          1. Insolation is not the whole story. If the panels were insulated from the sun they would still warm up due to movement of the ambient warm air. They could be insulated from losses at night. Probably this effect is small.

                          2. A physicist at NOAA tells me the downwelling effect could be large. This is heat from the earth that is radiated up and then back down. He pointed me to some charts that quantify this. If correct, this would be input outside of the insolation figure.

                          3. I realize now that looking at the instantaneous readings on the controller is very misleading. Reason: The glycol heats up over time. Then at some point, the pumps turn on. So a lot of heat is transferred quickly but that rate of BTU/hour has a different time variable than the pure rate due to the sun, because the hours of pre-heating the glycol are ignored. So I will from now on ignore the instantaneous reading (the 131 below).

                          4. But the TOTAL reading on the controller could still be right. Your post indicates your view that such a large value is bogus. I really want to know if that 10^9 number is bogus. When I divide by years and square feet and compare to what my PV panels do (7500 kWh/year) this thermal number is 5 times greater than what one would predict, after taking the efficiency difference into account.

                          The first image below is the controller. 168 on roof. 118 and 125 in the storage tank top and middle (but the bottom is 110; see pic). 122 in domestic tank. 131 k BTU/hr instantaneous reading. Almost 10^9 BTU over 8 years.
                          Second image shows incoming water at 112 and outgoing at 142. Touching the pipes confirms these two numbers are reasonable. These thermometers go inside the pipes to the storage tank.

                          So I realize now that the 131 figure is a little bogus if one compares it to what the PV panels do at max. But the integral of those numbers (the sum) is what is going into the TOTAL BTU figure, and I do not see any clear reason why that should be bogus. I really want to know!


                          ControllerAtNoon.jpgTemperatures.jpg



                          Comment


                          • #14
                            And a new important observation. The controller has access to T1 and T2 (roof and storage tank temps). I thought it was using the actual temps. at the heat exchanger to get its BTU rate. But a study of how the BTU rate changes indicates it is linear in T1 - T2. So -- and this makes sense -- it is using the data it has, even if that gives an incorrect answer. As you can see from pics in previous post, T1-T2 is 50 but the actual deltaT at the exchange is about 30. So this gives, roughly, a factor of two overestimate in the BTUs stated. I am so used to the perfect numbers one sees in the PV electricity that I assumed the same for thermal. But I am learning that it is not so.

                            So I will that JPM is right: the numbers I am seeing are bogus. But not by a factor of 5, so interesting questions remain.

                            Comment


                            • #15
                              Originally posted by stanwagon View Post
                              JPM

                              Latest thoughts:

                              0. The temp. diff I am seeing (in pic below) is surely a lower bound on the temperature changes caused by the sun. There are (small) losses in the glycol line, but what I am seeing on the pipes is a good estimate, and is in any case a lower bound. I see a 30 degF difference a lot, and the numbers are confirmed by just touching the two pipes in question. I think you indicated that the biggest temp. difference you have seen is 5 degF? That seems very small. Surely the point of solar thermal is to increase the water temp more than 5. I see you are in Chicago. Your incoming water might be warmer than mine, which is 50 degF. Maybe I misunderstand your 5 degF number?

                              1. Insolation is not the whole story. If the panels were insulated from the sun they would still warm up due to movement of the ambient warm air. They could be insulated from losses at night. Probably this effect is small.

                              2. A physicist at NOAA tells me the downwelling effect could be large. This is heat from the earth that is radiated up and then back down. He pointed me to some charts that quantify this. If correct, this would be input outside of the insolation figure.

                              3. I realize now that looking at the instantaneous readings on the controller is very misleading. Reason: The glycol heats up over time. Then at some point, the pumps turn on. So a lot of heat is transferred quickly but that rate of BTU/hour has a different time variable than the pure rate due to the sun, because the hours of pre-heating the glycol are ignored. So I will from now on ignore the instantaneous reading (the 131 below).

                              4. But the TOTAL reading on the controller could still be right. Your post indicates your view that such a large value is bogus. I really want to know if that 10^9 number is bogus. When I divide by years and square feet and compare to what my PV panels do (7500 kWh/year) this thermal number is 5 times greater than what one would predict, after taking the efficiency difference into account.

                              The first image below is the controller. 168 on roof. 118 and 125 in the storage tank top and middle (but the bottom is 110; see pic). 122 in domestic tank. 131 k BTU/hr instantaneous reading. Almost 10^9 BTU over 8 years.
                              Second image shows incoming water at 112 and outgoing at 142. Touching the pipes confirms these two numbers are reasonable. These thermometers go inside the pipes to the storage tank.

                              So I realize now that the 131 figure is a little bogus if one compares it to what the PV panels do at max. But the integral of those numbers (the sum) is what is going into the TOTAL BTU figure, and I do not see any clear reason why that should be bogus. I really want to know!


                              ControllerAtNoon.jpgTemperatures.jpg


                              I am back in Chicago after being ingognito for ~ 3 weeks travelling in the boonies around Lake Michigan. I actually live in/around San Diego. I'll be home tomorrow and will read your stuff after resettlement and recivilization.

                              Don't know where your Physicist friend is getting those ideas. Perhaps you misunderstand. You will do better educating yourself about a few basics of heat transfer and the basics of how solar theremal works before proceeding further.

                              Comment

                              Working...
                              X