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Thread: Advice on Thermal work done

  1. #1 Advice on Thermal work done 
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    Over the weekend I built a new solar hot water differential controller for my solar hot water system. It does all the normal things like turn on and off the circulation pump based on temperature differentials, turn off if it hits a programmable high limit. It has 5 temperature sensor inputs and records the highs and lows of each. More or less it's a pretty standard controller.

    What I was wondering is if anyone can come up with a good idea on how to calculate the work the solar hot water system is preforming each day. This doesn't have to be something like BTU, it just needs to be something I can reference each day to determine how effective the system was. The killer is that I'm very limited in the math functions I can perform on the PIC micro controller. The memory is very limited (8kb). So sticking with integer math would be ideal or using 16 bit integer shifted two digits over to give a decimal place. 32768 becomes 327.68. By the way I'm seeing tank temperatures at the top @ 165F and at the bottom 140F. This is an 80 gallon tank. The collector today got to 190F and for some reason the tank bottom got to 165F even though the pump shuts off when it hits 140F. I expect this is caused by the fact that it dumps the hot water in the mid section and the sensor is at the bottom.

    If anyone has any good idea let me know,

    Thanks

    IS


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    I don't think you mean work. I think you mean the amount of heat added by the solar system.

    If your water heater gets all of its heat from solar, then what you need to know is the temperature of the cold water going into the heater, the average temperature of the hot water coming out (that would be somewhere between 140 and 165, probably) and the flow through the water heater. So if your cold water is at, say 50 degrees F, and you used 100 gallons of water, which is about 800 lb, and if your outlet temperature averages 150 F (a 100 degree F increase from inlet to outlet) then you would have 100 degrees F*800 lb = 80,000 BTU.
    1 kilowatt-hour of electricity is equal to 3,413 Btu, so that would be 80000/3413=23.44 kilowatt-hours.

    If there is a supplemental electric heater or something to heat the water on cloudy days, you would have to measure the electric power going into the water heater and subtract it. Also, your circulating pump is using a little bit of electricity, so you would have to subtract that, as well.


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    Quote Originally Posted by Harold14370
    I don't think you mean work. I think you mean the amount of heat added by the solar system.

    If your water heater gets all of its heat from solar, then what you need to know is the temperature of the cold water going into the heater, the average temperature of the hot water coming out (that would be somewhere between 140 and 165, probably) and the flow through the water heater. So if your cold water is at, say 50 degrees F, and you used 100 gallons of water, which is about 800 lb, and if your outlet temperature averages 150 F (a 100 degree F increase from inlet to outlet) then you would have 100 degrees F*800 lb = 80,000 BTU.
    1 kilowatt-hour of electricity is equal to 3,413 Btu, so that would be 80000/3413=23.44 kilowatt-hours.

    If there is a supplemental electric heater or something to heat the water on cloudy days, you would have to measure the electric power going into the water heater and subtract it. Also, your circulating pump is using a little bit of electricity, so you would have to subtract that, as well.
    At the moment I have the backup electric heating element turned off. The circulation pump and the controller run off of solar electric, so I can pretty much just disregard them.

    What I know at this time is the starting temperature and the amount of seconds elapsed. I was hoping somehow to use the fixed volume of water along with the starting and ending temperatures to perhaps accumulate a reading. Sort of an odometer of hot water production.

    Still looking for more ideas.
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    Okay, how about this. The heat added by the solar collector, in BTU, can be calculated from the flow through the collector, in pounds, multiplied by the temperature difference (delta-T) across the collector in degrees F, i.e., the temperature of the outlet of the collector minus the temperature of the inlet. If you don't have a flow meter in the system, you could use the rated flow of your pump, or measure the pump flow by running its contents into a bucket and timing the run time of the pump. You could integrate the heat input and increment it each minute by multiplying the run time of the pump (1 minute) multiplied by the flow rate of the pump (pounds per minute) multiplied by the measured delta-T during that minute.
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    Quote Originally Posted by Harold14370
    Okay, how about this. The heat added by the solar collector, in BTU, can be calculated from the flow through the collector, in pounds, multiplied by the temperature difference (delta-T) across the collector in degrees F, i.e., the temperature of the outlet of the collector minus the temperature of the inlet. If you don't have a flow meter in the system, you could use the rated flow of your pump, or measure the pump flow by running its contents into a bucket and timing the run time of the pump. You could integrate the heat input and increment it each minute by multiplying the run time of the pump (1 minute) multiplied by the flow rate of the pump (pounds per minute) multiplied by the measured delta-T during that minute.
    Not a bad idea, the flow rate of the pump will be different however in a pressurized system versus free standing. It also sucks because the pump is microprocessor controlled (internal) to Peak Power Track and get the most wattage out of the solar panels. More or less a fancy impedance matching. So it tends to ramp up every 3 seconds before it stabilizes at a fixed RPM.

    This is the pump brochure

    http://www.altestore.com/mmsolar/oth...ng_D5solar.pdf

    I'm using the 090B

    Collector got to 196F today with the tank hitting 141F (bottom) . I'm still trying to figure out how it hit 165F the day before with the collector hitting 190F. It samples 18 times a second so it's not like it could have been left on for too long.
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    Yes, I can see where that would be a problem, unless you wanted to install a flow transmitter.

    Your idea of using the fixed volume of water in the tank, with the starting and ending temperature, could work but only if you shut the solar heating system down while you are drawing hot water out of the tank, i.e., for showers. laundry, etc. Probably not a practical solution, since the heater will kick on, in automatic, while you are using the hot water.
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    Quote Originally Posted by Harold14370
    Yes, I can see where that would be a problem, unless you wanted to install a flow transmitter.

    Your idea of using the fixed volume of water in the tank, with the starting and ending temperature, could work but only if you shut the solar heating system down while you are drawing hot water out of the tank, i.e., for showers. laundry, etc. Probably not a practical solution, since the heater will kick on, in automatic, while you are using the hot water.
    I'm might be out of luck. I've only got a few hundred dollars invested in resurrecting this old solar system. The panels I suspect are 20+ years old. Two 4x10 collectors, one of which is beyond repair due to a freeze a few years before I purchased the home. I guess I could calculate the temperature rise slope and accumulate that in such a way to have an end of day number. My goal is really to have a reference, It would be great to know how much better the system works in the summer vs the winter, etc. So far I have 80 gallons of 140-165F (bottom to top) water each day I come home. I've seen temps in the collector at 196F so far.

    One of the reasons I built a digital controller is so I could get an idea how well everything is working, so far I'm very happy with my decision. In case your wondering I have an open system where the water actually flows through the panel, living in Arizona I can get away with it. The controller will pump some hot water through the panels in the event the temperature drops too low. Not a perfect system, but ok for the climate.
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