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Wanting to get into solar? Think HOT WATER

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Some may be feeling that they are missing the solar power bandwagon and wondering how they can do something actually useful without spending a lot of money. Batteries and grid tie make a system expensive with a long payback. Creating hot water is the most effective way to use a couple of PV solar panels since close to 100% of the power generated will be used. This assumes you are currently using a resistive hot water tank and not on demand as the rest of the civilized world. Never could get used to climbing into a shower with a 240V line.I have been heating hot water for years with excess PV for years at my camp. With as little as 800WH, we can both get a long hot shower at the end of the day. Just a couple panels totaling 400W can make a significant dent in electric use for heating. Under powering the heater assures 100% of the panels output is used. That is much greater than any battery application and much lower in cost than grid tie which isn't even allowed in some locations. Typical heat loss during the daylight hours of a well insulated tank is more than 1KWH. Solar makes up for that loss.

Most tanks have two heating elements. The top is the primary and only one operates at a time. That allows the lower resistive element to be used for solar and still have grid power backup. At 240V even a 5500W element is 10 ohms, a replacement 120V 2000W is about 7 ohms. That requires at least a 36V panel string tog et any useful power. One positive is that may keep the system classified as "low Voltage". Connecting a panel directly to a resistance will result in power losses of more than half.  A simple PWM circuit keeps the panel voltage at the the power point. A small capacitor bank stores the panels power in the off cycle. A panels power point voltage is the same regardless of light level. It only varies with temperature. Attaching a couple of flat pack diode bridges to the back of the panel is an easy way to get a reference voltage tracks panel temperature. I use a $5 UNO to control everything and the 490Hz PWM makes driving a FET easy. Almost any switchmode chip would work just as well.

I'm not going to argue the economics of it. Watching solar work is just plain fun and panel prices are so cheap now. Everyone has some space they can stick a few panels on. It won't freeze and none of the plumbing issues of direct solar heating. I've built several versions with just junk box parts. I have more real world details if anyone is interested.  This discussion is NOT open to direct solar heating

I think this form of hot water assist will be commonplace in another ten years. Here is a "commercial" unit that sells for $270. I believe it is a very poor design and do not recommend it. The explanation is sufficient that I don't have to duplicate it. http://techluck.com/

The voltage at the maximum power point also depends on radiation levels. It gets high at more radiation. This gets obvious if you include the point were open circuit voltage gets below the assumed optimum voltage. Also PWM at only 500 Hz needs rather large capacitors to get ripple down. The way to go would be a normal switched mode regulator. Another option might be switching a different number of heaters in parallel  - so choose from 2 to 10 heaters in parallel, to get a useful power.

The downside of only using 36 V or so is that one needs relatively thick cables, not to loose to much in the cables. 400W peak power likely would not give enough for a shower - maybe not even compensate for the losses.

This is the difference between thinking and knowing.  The point of this discussion is that with as little as 400W of panels you can do something useful and that nearly all the power generated is used.  If you were to have enough panels to supply all the hot water needs a lot of the energy would be wasted.  I have a camp and 90% of the time we we can both have showers every day with about 600W of panels.  I have a 10 and 20 gallon tank in series.  When the first heats up power is diverted to the second.  Many times after taking a shower the water is hotter than when I started.

A 36V string has a power point of around 50V and cable size is only #12. This year it will be a 48V as I reconfigure the charging system.  Solar also run the fridge. Capacitor bank is about fifteen 330uF capacitors out of PC power supplies.  PWM is generated 30 feet away and driver is an opto isolator.  A good reason for slow and there is no FET heating. I monitor total power and 800WH gets me a shower, A little over a KWH a really hot shower.

You don't likely use a tank, but I have monitored heat loss during 12 hours of non use at night at 1600WH.  While it seems absurd to use solar just to cover heat loss during the daylight hours, this is reality in the US.  The controller is cheap (<$20) and easy to build.  This is the most cost effective way to use a small solar system. 

Sorry, but if you want to heat water, best to heat it directly with a solar water heater.


--- Quote from: Lightages on June 01, 2015, 11:05:33 pm ---Sorry, but if you want to heat water, best to heat it directly with a solar water heater.

--- End quote ---
It could be if you only wanted hot water, but using the excess electricity from a rooftop solar stored as hot water is a cheap way for most PV solar installs to reduce their payback period.


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