Author Topic: Wanting to get into solar? Think HOT WATER  (Read 27506 times)

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Offline Seekonk

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Wanting to get into solar? Think HOT WATER
« on: June 01, 2015, 01:35:29 pm »
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/




 

Offline Kleinstein

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Re: Wanting to get into solar? Think HOT WATER
« Reply #1 on: June 01, 2015, 08:51:20 pm »
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.
 

Offline Seekonk

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Re: Wanting to get into solar? Think HOT WATER
« Reply #2 on: June 01, 2015, 09:57:36 pm »
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. 
 

Offline Lightages

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Re: Wanting to get into solar? Think HOT WATER
« Reply #3 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.
 

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Re: Wanting to get into solar? Think HOT WATER
« Reply #4 on: June 01, 2015, 11:34:00 pm »
Sorry, but if you want to heat water, best to heat it directly with a solar water heater.
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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Offline moffy

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Re: Wanting to get into solar? Think HOT WATER
« Reply #5 on: June 01, 2015, 11:53:27 pm »
Sorry, but if you want to heat water, best to heat it directly with a solar water heater.
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.

I agree with Someone. I have a direct solar powered hot water system and it is great, but the idea suggested doesn't require any special installation apart from the PV cells which are installed for other purposes. Using excess PV capacity this way appears very sensible and economic.
 

Offline Lightages

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Re: Wanting to get into solar? Think HOT WATER
« Reply #6 on: June 02, 2015, 12:16:05 am »
The topis is "Wanting to get into solar? Think HOT WATER", not "Want to use your current system more efficiently?".

I agree. If you have a solar system right now that is sitting idle some of the time, then divert the output to heating water if you can. Not all grid tied system are allowed to do that in all jurisdictions. So yes, divert your lost production to heating water or some other useful thing.

If you are "starting", then the best way to heat water is directly with a solar water heater.
 

Offline mtdoc

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Re: Wanting to get into solar? Think HOT WATER
« Reply #7 on: June 02, 2015, 12:32:05 am »

If you are "starting", then the best way to heat water is directly with a solar water heater.

If you had asked me this question a few years ago, I would have had the same response. That has been the conventional thinking for a long time. But the equation has changed recently with the extreme decrease in PV prices and also with the introduction of heat pump water heaters.

Direct solar hot water heating has a lot going for it. Simple and low tech.  But - the plumbing is susceptible to corrosion issues  and these systems are not maintenance free. 

Using PV to heat hot water is starting to make more sense - both economically and practically.  I considered adding a direct solar hot water system a few years ago. But looking into it I realized I could add more PV and a Nyle Geyser heat pump to my existing system for a lot less money and with less long term maintenance issues.

As far as using excess PV production to heat water (for those who can't sell excess to the grid) several people are using the programmable auxillary outputs from their MPPT charge controllers (eg the Midnite Classic) to drive SSRs which control power to resistive heating elements.

As far at the OP's idea - sounds reasonable but the link provided  leads one to a web page promoting this simple board selling for $275!!!!   :wtf:
 

Offline Lightages

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Re: Wanting to get into solar? Think HOT WATER
« Reply #8 on: June 02, 2015, 12:51:00 am »
Anything that has water in it is susceptible to corrosion.  The other thing is that adding an electrical system to heat water is going to need permits and certified installers or an inspection afterward. I see your point about the dropping prices making PV production lower I still think that direct heating i the most economical and easiest way. Without hard numbers from a a properly conducted study, I am just going by experience. If you have a something with hard numbers I am very interested.
 

Offline rx8pilot

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Re: Wanting to get into solar? Think HOT WATER
« Reply #9 on: June 02, 2015, 02:13:36 am »
I cannot wrap my head around using super low-efficiency PV to heat water. Trying to get it, but it seems so senseless.

I have been experimenting with a solar steam generator for fun. From my minimal knowledge, it seems that you need a LOT more PV surface area and complexity to heat water electrically.

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Online Someone

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Re: Wanting to get into solar? Think HOT WATER
« Reply #10 on: June 02, 2015, 03:08:34 am »
I cannot wrap my head around using super low-efficiency PV to heat water. Trying to get it, but it seems so senseless.
Watch some of the eevblog videos on Daves solar install, noting the tariffs paid for electricity. During periods of high PV solar generation the excess flows into the grid and those without artificially inflated feed in tariffs get paid a fraction of the price they buy electricity for, much less than the "off peak" discounted rate that some people heat their hot water from.

Assuming that:
a) the household needs a daily supply of hot water
b) the electricity from the solar panels is sold for much less than what you could buy the same energy for
c) the house already has a storage type water heater.
Using that excess cheap power to heat the water that would have been heated by some other energy source anyway is a quick way to improve the cost effectiveness of the system. You can assess the investment of a small solar system to do this against the cost of changing to a different hot water heater, direct solar hot water (most have boosters) or a heat pump, and the answer is different for different people. But as a typical case for most people adding some solar capacity or extending their solar system to the hot water storage is a very short payback.
 

Online Someone

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Re: Wanting to get into solar? Think HOT WATER
« Reply #11 on: June 02, 2015, 03:12:36 am »
If you are "starting", then the best way to heat water is directly with a solar water heater.
For a completely greenfield install, solar heated thermal storage for both climate control and domestic hot water would be a strong contender but very few people get the chance to start from the ground up.
 

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Re: Wanting to get into solar? Think HOT WATER
« Reply #12 on: June 02, 2015, 03:34:33 am »
I see your point about the dropping prices making PV production lower I still think that direct heating i the most economical and easiest way. Without hard numbers from a a properly conducted study, I am just going by experience. If you have a something with hard numbers I am very interested.
Lets do some numbers here.

http://users.tpg.com.au/users/robkemp/Power/HotWater.htm
Daily losses of 2kW/h give or take.

http://reneweconomy.com.au/2013/graph-of-the-day-australian-retail-electricity-prices-in-2020-2020
A typical electricity price in Australia was 25 cents per kWh.

We can keep the estimates round at $200 per year, just to maintain hot water on standby (not counting the used hot water).

Delivering 2kW/h per day average from 500W of panels is typical, to smooth it out and ensure you can keep up the 2kWh through winter you might want to bump that up to a 1kW system. ***Insert your cost estimate here and divide by $200 per year***

http://www.sustainability.vic.gov.au/services-and-advice/households/energy-efficiency/at-home/hot-water-systems/hot-water-running-costs
Looks like a winner to me, even more so when you count the offset energy consumption of the hot water used (which would be another 4-6kWh additional).
 

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Re: Wanting to get into solar? Think HOT WATER
« Reply #13 on: June 02, 2015, 03:40:10 am »
The other thing is that adding an electrical system to heat water is going to need permits and certified installers or an inspection afterward.
In Australia (and I assume many other nations) the elegance is you can install a solar PV system and connect it to anything you like other than the power grid, without any need for licensed tradespeople or permits.
 

Offline miguelvp

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Re: Wanting to get into solar? Think HOT WATER
« Reply #14 on: June 02, 2015, 03:57:08 am »
This worked for me for camping:

http://www.amazon.com/Coleman-Sun-Shower%C2%AE-Portable-Shower/dp/B000NVC1JY

But if you want to actually drive a generator, well a sterling engine with a big parabolic mirror to boil the water to create the steam needed.

https://www.google.com/search?q=stirling+engine+solar&tbm=isch

But you need a sunny place, PV works on cloudy days, that wont.
 

Offline Lightages

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Re: Wanting to get into solar? Think HOT WATER
« Reply #15 on: June 02, 2015, 04:16:05 am »
The other thing is that adding an electrical system to heat water is going to need permits and certified installers or an inspection afterward.
In Australia (and I assume many other nations) the elegance is you can install a solar PV system and connect it to anything you like other than the power grid, without any need for licensed tradespeople or permits.

I am not trying to be a naysayer. What I am trying to say is that if you already have a system hooked up to the grid, and wish to use it to also heat water, you now have to deal with the regulations of using the grid tie system to do other things. I am not saying you could not use a stand alone solar system to heat water, just that I am still not convinced it is more cost efficient than heating water directly with the sun.
 

Online IanB

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Re: Wanting to get into solar? Think HOT WATER
« Reply #16 on: June 02, 2015, 05:05:02 am »
But if you want to actually drive a generator, well a sterling engine with a big parabolic mirror to boil the water to create the steam needed.

On a point of information, Stirling engines don't run off steam and don't need a boiler.
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Online Someone

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Re: Wanting to get into solar? Think HOT WATER
« Reply #17 on: June 02, 2015, 05:11:39 am »
The other thing is that adding an electrical system to heat water is going to need permits and certified installers or an inspection afterward.
In Australia (and I assume many other nations) the elegance is you can install a solar PV system and connect it to anything you like other than the power grid, without any need for licensed tradespeople or permits.
I am not trying to be a naysayer. What I am trying to say is that if you already have a system hooked up to the grid, and wish to use it to also heat water, you now have to deal with the regulations of using the grid tie system to do other things. I am not saying you could not use a stand alone solar system to heat water, just that I am still not convinced it is more cost efficient than heating water directly with the sun.
If you've already got a grid tie system (with net metering) then its simply modifying/building a plug in appliance, again not requiring any licensed tradespeople in the countries I am familiar with, you lose some efficiency going through the inverter but the implementation is cheaper than those solutions adding a DC heating circuit.
 

Offline Lightages

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Re: Wanting to get into solar? Think HOT WATER
« Reply #18 on: June 02, 2015, 05:18:50 am »
Sorry I wasn't clear in my definition of "grid tie". Many people just have solar panels that push back to the grid, without any useful connection to their house. This is what I meant and is not really what should be called a "grid tie system". My error, sorry.
 

Offline miguelvp

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Re: Wanting to get into solar? Think HOT WATER
« Reply #19 on: June 02, 2015, 05:27:45 am »
But if you want to actually drive a generator, well a sterling engine with a big parabolic mirror to boil the water to create the steam needed.

On a point of information, Stirling engines don't run off steam and don't need a boiler.

I thought it was due to the expansion of a gas and usually they use water for it, I guess I have to read up on them.

I do understand they don't need a boiler but I thought it was steam powered but not in the conventional way.
 

Offline jwm_

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Re: Wanting to get into solar? Think HOT WATER
« Reply #20 on: June 02, 2015, 05:29:48 am »
It seems like a more efficient way to heat water with a PV system would be to run the water under the panels, solar panels get less efficient as their temperature goes up, keeping them cool and pre-warming water on its way to your heater seems like a win-win situation. You get solar heating and solar electricity at the same time.

Online IanB

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Re: Wanting to get into solar? Think HOT WATER
« Reply #21 on: June 02, 2015, 05:39:01 am »
I thought it was due to the expansion of a gas and usually they use water for it, I guess I have to read up on them.

I do understand they don't need a boiler but I thought it was steam powered but not in the conventional way.

Stirling engines usually use air as the working fluid. Here's a very nice one that someone built. It uses heat from a wood burning stove to circulate warm air around the room:

https://youtu.be/XzE7pkIknmQ
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Online Someone

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Re: Wanting to get into solar? Think HOT WATER
« Reply #22 on: June 02, 2015, 05:46:05 am »
Sorry I wasn't clear in my definition of "grid tie". Many people just have solar panels that push back to the grid, without any useful connection to their house. This is what I meant and is not really what should be called a "grid tie system". My error, sorry.
It all depends on the metering scheme, net metered you can use the power from the grid tied system as you wish. Its the gross metering systems where you have no control and always pay full tariff, removing any incentive to time shift loads.
 

Offline mtdoc

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Re: Wanting to get into solar? Think HOT WATER
« Reply #23 on: June 02, 2015, 06:08:13 am »
Anything that has water in it is susceptible to corrosion.  The other thing is that adding an electrical system to heat water is going to need permits and certified installers or an inspection afterward. I see your point about the dropping prices making PV production lower I still think that direct heating i the most economical and easiest way. Without hard numbers from a a properly conducted study, I am just going by experience. If you have a something with hard numbers I am very interested.

I can't give you "hard numbers from a properly conducted study". I can give a quick analysis of what it looks like for me.  I installed a Nyle Geyser heat pump water heater and the measured power it uses to keep our home (2 adults and 2 small kids) in hot water is 2 to 3 kWh per day with normal use. Obviously the amount of PV needed to provide this will depend on the solar insolation but of course so would the area of direct solar water heating panel needed.

For my location, 1,000 watts of PV can produce this on most days. Since i already have a PV system, the cost of adding the additional PV is about $1/watt (I paid $0.72/watt for my panels). The Nyle Geyser cost me about $700 IIRC. So about $1700 total (I did my own install). BTW Permit fee here in WA state for this is less than $100 and in my case I was already expanding my system so no extra cost.

To get the equivalent with direct solar hot water heating, i would have to install a 40 sq ft closed loop system (it freezes here) for about $4000 (assuming i could install it myself which is questionable!).

Also since i have grid tied PV system (with battery back up), i can sell any excess PV back to the grid on sunny days.

I realize my situatuon is different from someone who doesn't already have a PV system in place and i'm not arguing that using PV to heat water always makes more sense than direct solar water heating. It's just that the previously held common wisdom that heating water with PV never makes sense is no longer true given the low cost of PV now.
« Last Edit: June 02, 2015, 06:15:16 am by mtdoc »
 

Online NiHaoMike

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Re: Wanting to get into solar? Think HOT WATER
« Reply #24 on: June 02, 2015, 06:19:24 am »
What I would like to see is an open hardware inverter specifically designed to drive a common split phase refrigeration or A/C compressor direct off solar to make a heat pump. It could fall back to resistive when there's not enough power to operate the compressor.
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