1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I Sizing a Solar Array To Boil Water

  1. May 4, 2017 #1
    Hello,

    I am having a hard time deciding the size of a solar array to boil water for a school project. Before putting any thought into the subject I simply thought that I would just take the wattage of the panels and add them up since they are added in both series or parallel and then select an element close to that wattage. Apparently this is not the case.

    I am trying to do my calculations for a regular household element that is rated for 1500W at 120vac with panel specifications as 31.5V and 8.89A. My calculations go as follows:
    With 5 panels connected in series=157.5V, 8.89A
    Element Resistance: 120^2/1500= 9.6ohms
    Element Power: 157.5^2/9.6= 2,583.98W
    Element Current Draw: 2,583.98/9.6= sqrt269.16= 16.41A *This value cannot be supplied*

    How would I go about calculating the voltage drop caused by the current draw being higher than the panels are capable of handling?
    regular vdrop calculation would be: IR= (8.89)(9.6)=85.34V
    if the power is recalculated with the new voltage of 72.16V I get a wattage of 542.4W. This seems way too low...also, performing this vdrop calculation does not seem to make sense in this scenario. If it is correct, then why are panels that are rated to supply 1400W providing a value so small? Would rearranging the panels make any difference (1 in parallel and 4 in series)?

    Thanks for any help you can provide.
     
  2. jcsd
  3. May 4, 2017 #2
    There's no need for photovoltaics if you are boiling water. You can use a parabolic reflector to heat much more efficiently.
     
  4. May 4, 2017 #3

    russ_watters

    User Avatar

    Staff: Mentor

    You can't connect an AC load to a DC source directly. The panels should be wired in parallel to a controller/inverter to change the current to AC.

    What level of schooling is this and what level of supervision are you getting? This can be dangerous for someone lacking the required knowledge.
     
  5. May 4, 2017 #4
    Are you talking about a store-bought heater? If it's a simple resistive heating element, then it can be made to work with AC or DC at various voltages, but a store-bought heater will probably only work with 120VAC.
     
  6. May 4, 2017 #5

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    If this is a school project then there is absolutely no need to be trying to mimic a domestic installation. It is potentially very dangerous and needlessly expensive. PV panels are available in all maximum power ratings from 1W to 1kW and a small version would be a far better project.
    There is a range of low cost panels will provide you with 12V DC at a few tens of Watts. That voltage would operate many readily available pieces of DC equipment, including lamps and radios.
    But it is very inefficient to use PV as a source of heating only. Direct heating with a parabolic style reflector is far better value and a lot cheaper.
    I used an old 0.9m microwave dish with cooking foil stuck to it to boil a few hundred cc of water in a black can in a very few minutes. It needed full sunlight, of course. But so would your PV version. The focus was not at all critical because it was better to heat most of the can's side surface with the defocussed beam. A perfectly adequate reflector shape could be made with paper mache´ built up on an exercise ball or any other handy shaped object.
    I have to point out the danger of direct sunlight, focussed onto a small area and the risk of fire and blinding. A diffuse reflector is easier to make and much safer but would produce the same effect without a dangerously bright focussed image of the Sun. You should not consider using more than, say 1m2 of area, which would give just a safe few hundred Watts of heating. Your teacher should be aware of this in any case (I would hope)
    Edit: If you really want to go for the PV solution, you need to use an appropriate load. If you try to load a PV panel with a lower resistance than corresponds to its rated power, the power output drops drastically (the Volts drop like a stone) You should ask teachers for specific help with this.
     
  7. May 5, 2017 #6
    Good advice - if this is all about demonstrating principles, keep it scaled down. As others mentioned, the OP is proposing dangerous voltages.

    Go with a 12V panel, and if you still want to boil water, boil a single cup with one of these (~ 140 watts):

    https://www.amazon.com/RoadPro-RPBH...001AQZI64/ref=cm_cr_arp_d_product_top?ie=UTF8
     
    Last edited by a moderator: May 8, 2017
  8. May 5, 2017 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    That could be a fun project - if you have the money for 0.5m2 with of panel area (Around £200). But what would you do with the panel afterwards, to justify that cost? You would need a battery and controller if you wanted it for other uses. I have such a system which lights my shed. It has actually worked for over a year (all seasons) to power internal lights and security lights. It uses only a 40W panel.
    A much smaller solar reflector than 0.5m2 would do the same job as 140W worth of panels and the parts would be very low cost.
    I could suggest that boiling water would not be the best way to demonstrate a PV based system; it't not the tool for the job. People can be very impressed by moving parts, light and sound, powered from a solar panel. Going for more than a low power panel would increase impact because 'everyone' knows about 10W panels charging phone batteries.
     
  9. May 5, 2017 #8

    OmCheeto

    User Avatar
    Gold Member
    2016 Award

    Interesting question.

    My guess is no.

    ps. I'm not willing to do the experiment, as I only have 3 of 4 of my original solar panels left.
    pps. Here's a graph of me trying to analyze my PVs last summer:

    2017.05.05.pf.coffee.pot.png
     
  10. May 5, 2017 #9
    Typically solar panels are rated for max power, open circuit (max) voltage (I=0) and short circuit (max) current(V=0).

    Below is a typical characteristic curve for a solar cell for different light intensities with different loads showing the resulting currents and voltages. The max power occurs near the top of the knee around 0.8 Vmax. You can see that the power produced is highly dependent on the choice of load resistance.

    pviv.gif
     
  11. May 5, 2017 #10

    davenn

    User Avatar
    Science Advisor
    Gold Member

    you are approaching this the wrong way

    you only need around 12VDC .... there are 12V water heating elements available
    they are used for water heating to run off a car battery used for camping :smile:

    depending on the wattage of the panel(s) and the wattage of the 12V element .... you may only need 1 panel or maybe a couple in parallel

    as others have said, keep the project small scaled to keep it safe

    most of them are plain resistive element .... it will work on AC or DC :smile:


    Dave
     
  12. May 5, 2017 #11

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Unfortunately, the OP ( I now realise) seems already to have 30V panels (?) and they would be difficult to butcher to work at lower volts.
    But power resistors are not that expensive and could be used as heating elements. Perhaps two 12V heaters could be connected in series. For Auto use, any heater would be capable of running on 15V. If you chose to use a Maximum Power Point Tracking (MPPT) Solar Charge Controller you could optimise the available heat.
     
  13. May 5, 2017 #12

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Doh .... yeah you are correct, I did read that but overlooked it in my reply.....
    don't know what immersion heater elements are available around that voltage .... google haha


    or put 2 or 3 12V elements in series, problem solved :smile:

    actually probably only need 2 elements as panels usually specify open circuit voltages
    so it will drop under load
     
    Last edited: May 6, 2017
  14. May 5, 2017 #13
    It's not clear to me if this is still in the planning or hypothetical stage, or OP actually has these panels and is trying to configure them.

    Hopefully, OP comes back to clarify. IMO, this whole thread is rather muddy. Is this a learning exercise, or an application, or both? OP should have learned it takes a lot of PV panels to boil a significant amount of water!
     
  15. May 5, 2017 #14

    OmCheeto

    User Avatar
    Gold Member
    2016 Award

    "still in the design phase" according to "PV panels to boil water 1.0" thread.
     
  16. May 6, 2017 #15

    davenn

    User Avatar
    Science Advisor
    Gold Member

    good grief .... I doubt he's trying to heat a swimming pool ! .... it's a school project, so you could safely assume a litre or 3 of water

    my suggestion above will do that very easily with just one of the panels that he stated
    for an element rated at 24V up to ~ 200W

    ebay and other places are full of examples

    Dave
     
    Last edited: May 6, 2017
  17. May 6, 2017 #16

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    . . . and that's quite enough if you want to avoid hot wires[(use really fat connecting wires - 10mmSUP]2[/SUP] ) and too much hot water. If this is for a demo, I would think regularly boiling small amounts of water would have more impact, rather than many litres at a time. A glass beaker with Good thermal insulation (with a hole in the insulation for people to see the bubbles) would add to the impact. Audience concentration span is a factor here.
     
  18. May 6, 2017 #17
    I agree, but the OP seemed "hot" (pun intended) on using a 1500W element to its full rating, and that takes a lot of panels Also, those elements need to be fully submerged when used near their rated power or they overheat and burn out quickly (seconds?), that adds to the set up complexity, along with safety issues of 120 V and water.

    Also agree with sophiecentaur, small volumes boiled quickly would make a better presentation. Heck, I'd go all the way down to a 12V 30W soldering iron, and boil a shot glass sized (should be an SI unit!) amount of water. That should be cheap and small enough to make the point. And a small panel can be easily handled, people could easily experiment with changing the angle, shading a bit of it, etc.
     
  19. May 6, 2017 #18

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I still think that PV for heating is not a good message to be giving. Anyone who is serious about a domestic PV supply in the absence of the Feed In Tariff that totally distorts things, should plan a dual system. Solar thermal for heating and PV for when you actually need Electricity.
    If the OP has the toys already, there is no argument that he should use them. But why use them for an inappropriate purpose such as heating? If this project will be presented to 'the public' then he should be prepared for someone to bring this up and to have a good answer ready to an embarrassing question and have demos of other more impressive uses for the power from his PV. 140W can do some pretty impressive mechanical tasks.
     
  20. May 6, 2017 #19

    OmCheeto

    User Avatar
    Gold Member
    2016 Award

    It seems to me that we've exhausted the ohmic logic here.

    Solar panels are not ohmic devices. They are ≈constant current devices on one end of the curve, and ≈constant voltage devices at the other end of the curve.

    I really liked mcharbs55's question; "Is there a way to hook these things up, like resistors, or batteries?" [paraphrased]

    2017.05.06.still.dont.know.how.pvs.work.png

    The above schematic is grossly simplified, in the context of this question, of course.

    The question is, "Does the single panel on the left, limit the current capability of the two panels on the right?"

    I spent about an hour yesterday trying to find an answer, and could not find one.
    Hence, my "No" guess.

    ps. I have no idea how solar panels work, other than; "quite well".
    pps. If the panel on the left is NOT capable of carrying 17.8 amps, then the problem becomes very trivial.
    8
     
  21. May 6, 2017 #20

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    That should be carved in stone, somewhere. I love it.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Sizing a Solar Array To Boil Water
  1. Boiling Water ? (Replies: 8)

Loading...