On Thu, Sep 19, 2013 at 11:57:34AM +1200, RussellMc wrote: > > 2. By using solar PV to heat hot water* and thus replace an > > equivalent amount of grid energy I can achieve a payback period of > > about 5 years with no subsidies. Also no battery, no grid tie or other > > inverter. Very simple pseudo MPPT. (Actual panel only payback time is > > about 2.5 years) > > * This is an unintuitive use of solar PV but has a number of > > advantages over solar thermal and makes more economic sense than any PV > > alternatives I've seen so far. On 19 September 2013 12:21, James Cameron wrote: > Sounds interesting. Have you written this up anywhere? Only the calculations to see how it proved out *BUT* somebody else has. When I thought of it as a viable means of energy storage and grid offsetting I realised that the idea was hardly original and that many others would have considered it BUT I did not expecting a full blown NZ paper on it. A friend recently came across one written in June this year. See below. > My gut feel > is that 250 W during sun delivered into an insulated water container > would certainly achieve useful storage. Response to over temperature > is easy; open the circuit. I've seen the thermal transmission and > over temperature controls on solar thermal and found them complex. As long as you max input is less than you would otherwise put in in a day or so then there is no problem.If you have excess energy you can just turn the PV energy source off - unlike with some solar thermal systems which will be damaged if you do not dissipate the energy elsewhere. (I have about 9 such which I got free after parts of some melted in the sun :-) ). If you have too much energy you can eg run your freezer from it with an inverter (depending on practical aspects.) If PV energy ALWAYS exceeds cooling load averaged over say 2 or 3 days then you can island a chest freezer and run it solely on PV with any excess going to water heating. As long as your freezer can withstand the longest period that PV will not deliver enough cooling energy then this is workable. It would be easy to have an automated switchover between either grid or PC island although odds are there will be regulatory issues. Note that this does NOT involve grid tie at all. Worst case you could have the freezer manually unpluggable and transferable en extremis. If you are away for a weekend the freezer will not be being opened and a properly insulated one should last 2 or 3 days. If necessary add thermal mass (add water inside freezing space) as a thermal buffer. The water heating works well $ wise because you need no battery, no grid tie inverter and indeed no inverter if you are happy to have a heater element rated AT panel peak power voltage. You then add pseudo MPPT which simply PWM's the resistive element onto the panel to maintain the panel at Vmp. It is claimed that this allows tracking within about 1.5% of true MPPT. Looking at typical MPPT and pseudo MPPT curves I believe that by adding a slight current sense component to the maintained panel voltage that even closer tracking can be obtained. During PWM off periods the panel voltage is maintained close to Vmppt with "adequate capacitance". I've priced good quality caps with adequate ripple current ratings and believe that the cost of requisite capacitors is modest compared to eg panel costs. Getting 25 year lifetimes wit electrolytic capacitors in heavy ripple applications would be "challenging" [tm] no matter what Arrhenius says, but DCF calculations are hardly affected by returns beyond 15 years at typical interest rate and are reasonably insensitive at 10 years. More on that anon if the discussion develops. If you are willing to go to a tapped or multi resistor element then pseudo MPPT can be achieved with FETs and resistors alone. More steps =3D closer tracking to actual MPPT curve. PV versus thermal =3D no header tanks, pipes, plumbing,ease of control, lower roof loadings, easier maintenance, .... . Lower energy conversion efficiency. Top thermal solar can achieve around 75%+ efficiency Report below. They go into great detail in some areas and less than I have in others. A competent friend of mine takes issue with some of their assertions. ______________________ University of Otago, NZ report June 2013 http://www.epecentre.ac.nz/docs/media/Wood_EEA_2013_PV.pdf They say: Water heating is not a well established option for PV panels. However, connecting the PV panels to the heating element, and storing the energy as heat for use in the evening or the following morning has some advantages over grid connection. Grid connection equipment and compliance costs are avoided, and higher value avoided energy usage results. PV based water heating has no pumps, plumbing, freezing and boiling issues, is potentially simpler to install than a thermal collector based system, and should require less maintenance. However, much greater roof area is required, in summer excess energy goes un-utilised, and currently there is no established standard for this sort of system. Nevertheless it is included in this paper, as the idea has merit. Consistent with solar thermal systems, the size of the system is very important - too many panels and the un-utilised summer energy reduces the return on capital, while too few results in the fixed installation costs reducing the return http://www.epecentre.ac.nz/docs/media/Wood_EEA_2013_PV.pdf June 2013 > > -- > James Cameron > http://quozl.linux.org.au/ > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .