Wanting to get into solar? Think HOT WATER

[vk6zgo]

The maintenance comments are a "red herring",as the system proposed by the OP would need similar levels of maintenance.
After all,the storage tank doesn't know where the hot water came from,& needs most of the same facilities.

The system proposed by the OP uses your existing electric hot water tank, adds a couple solar panels, some wire and a cheap circuit board.  It's as simple as mounting a couple solar panels to the shed with some 2x4 treated lumber, running a wire and having a cool beverage.

The system proposed by the OP..
- Does not add pipes, tanks, PTC valves, check valves, directional control valves, elaborate insulation, closed loop 'refrigerants', mixing valves, expansion tanks, heat exchangers, pumps, electronic controllers, temp sensors or even a single additional joint. 
- It does not use an inverter, grid tie solution, or batteries.
- There are no massive temperature fluctuations in the pipes/fittings causing expansion/contraction stresses.
- There is no worry of extra cold days causing freeze induced leaks, excessively hot days causing scaling, vapor lock, absorber fade, etc.
- There is no worry of equipment failure causing massive water/refrigerant leaking damage.
- There's no need to replace glazing, or refrigerants, recaulk seals, de-scale/drain the lines, or inspect for corrosion.

Maintenance of the OP's proposed system amounts to keeping snow off the panel. 

Actually, in the spirit of this concept, you shouldn't even do that.. because it would unnecessarily push out the ROI.

A solar HWS as used in temperate climates doesn't have much more than any storage HWS --the only largish addition is the solar absorber.
It doesn't use refrigerants of any kind,the electric wiring is only for the booster,which you can do without in a temperate climate.

In Australia,solar HWS operate with minimal maintenance for upwards of 20 years.

[mtdoc]

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/

Seekonk- can you post more details and/or a schematic of your system?

I agree this type of system has merit but the link you provided has few technicial details and much of what it does say is just bogus - along the lines of "solar roadways" type nonsense.

[Poe]

A solar HWS as used in temperate climates doesn't have much more than any storage HWS --the only largish addition is the solar absorber.
It doesn't use refrigerants of any kind,the electric wiring is only for the booster,which you can do without in a temperate climate.

In Australia,solar HWS operate with minimal maintenance for upwards of 20 years.

The system proposed by the OP is not a hot water storage system, so that's apples to oranges.  The concept does not add additional hot water tanks and plumbing.  It simply adds a couple solar panels to the existing electric water tank. 

A solar HWS is an elaborate system ADDED to a home's existing water system.  They involve most of what was already listed.  i.e. pipes, tanks, valves, etc

Solar HWS systems without refrigerants/antifreeze are called open loop systems and require additional maintenance and equipment to avoid scale buildup and/or legionnaires disease, freezing pipes, etc.  Systems that last twenty years are more likely closed loop and are like cars that last twenty years.  i.e They were not the cheapest model and were regularly maintained. 

The OP's concept is simple.  A few solar panels can operate one of the two hot water tank heating elements directly, no inverter.  Nearly all of the solar panel's energy (notice I didn't say nearly all of the SUN'S energy) is subtracted from how much electricity you would otherwise consume.  The hot water tank already cuts off power at a given temperature.

The board linked by the OP is mostly BS.  It simply toggles polarity at 50Hz to prevent the hot water tank's thermal switch contacts from welding shut.  That's all you need though.  No inverter, no grid tie, no plumbing just solar panels, wire and a cheap circuit board.   MPPT would be nice, but meh.

[NiHaoMike]

You don't even need to reverse polarity - just chopping the current is enough. And at that point, adding MPPT is so easy you might as well put it in, even if it's as simple as an opamp integrator controlling the duty cycle of the 555 or whatever based on input voltage.

[vk6zgo]

A solar HWS as used in temperate climates doesn't have much more than any storage HWS --the only largish addition is the solar absorber.
It doesn't use refrigerants of any kind,the electric wiring is only for the booster,which you can do without in a temperate climate.

In Australia,solar HWS operate with minimal maintenance for upwards of 20 years.

The system proposed by the OP is not a hot water storage system, so that's apples to oranges.  The concept does not add additional hot water tanks and plumbing.  It simply adds a couple solar panels to the existing electric water tank. 

A solar HWS is an elaborate system ADDED to a home's existing water system.  They involve most of what was already listed.  i.e. pipes, tanks, valves, etc

Solar HWS systems without refrigerants/antifreeze are called open loop systems and require additional maintenance and equipment to avoid scale buildup and/or legionnaires disease, freezing pipes, etc.  Systems that last twenty years are more likely closed loop and are like cars that last twenty years.  i.e They were not the cheapest model and were regularly maintained. 

The OP's concept is simple.  A few solar panels can operate one of the two hot water tank heating elements directly, no inverter.  Nearly all of the solar panel's energy (notice I didn't say nearly all of the SUN'S energy) is subtracted from how much electricity you would otherwise consume.  The hot water tank already cuts off power at a given temperature.

The board linked by the OP is mostly BS.  It simply toggles polarity at 50Hz to prevent the hot water tank's thermal switch contacts from welding shut.  That's all you need though.  No inverter, no grid tie, no plumbing just solar panels, wire and a cheap circuit board.   MPPT would be nice, but meh.

Solar HWS replace the existing HWS in its entirety,& apart from the solar absorber have no parts that other systems don't have.
Their cost is competitive with other forms of HWS.

I've had one which lasted around 23 years,with minimal maintenance until I had to replace it,
The replacement will probably see me out.
Gas & Electrical storage HWS have all of the same potential problems of scale build up,etc.

OK.if you already have all the panels & are producing more energy than you need,the OP's idea is reasonable,but,if you live in a sub-Arctic climate where all the other stuff you mention would be needed  for  a direct Solar HWS,how much excess energy are your Solar  panels going to produce in the first place?

[Seekonk]

People talk of solar systems in Australia and Florida, that is pretty low hanging fruit. Dealing with-20F has a whole different set of issues.  Every site has different engineering solution. I find problems are technical and solutions are political. This PV solution is elegant with the infrastructure found in many homes. The greatest interest has been from people who have their water heater on a timer to get reduced night time electric rates. As far back as the 70's this was done in Vermont and oddly enough today in Ontario. I thought of that as the land of cheap hydro. For them adding solar has nothing but upside.

The control structure is fairly simple.  My camp uses a UNO. I became acquainted with these a few years ago when I had to do a store animation project. Now I use $3 328 mini's for about everything.  I haven't used a 555 since 1975 and don't intend to start. I have helped people with 555 problems as they are prone to doing unexpected things. Generally a switch mode chip like the TL494 will do a better job because it has a stable voltage reference and op amp built in. I'll explain the functional code because those are the systems I have running.

Your eye is log and a solar panel is linear. Get a little wisp of a cloud and it is still a nice day to you. The solar panel, however, takes a real hit in power power.  Match up a heating element resistance to a solar panel array and you have great power transfer for an hour and a half in ideal sun.  The fixed resistance is a real killer when the light levels drop a little or are off axis.  A power point controller can increase the power output by five times or more compared to a direct connection at low light levels.  At moderate light levels the numbers are about double. Some claim they do just fine with direct connect and anyway panels are cheap.  Fact is you can get away with 30% fewer panels by building a $20 controller.

Nothing magical about the $276 controller I linked to.  It uses an IGBT that saturates to about 2V. That wastes a little potential heat and there is a little warning that it may need an additional heat sink.  The capacitors on that board will not last a long time because they can't handle the surge currents.  As those fail, it will likely keep operating but will not put out as much power.   The functionality is the same, the claims a little exaggerated.  To be energy independent it takes a lot of panels and that means some of that time the panels will be cut off and not produce anything.  They use the water heaters thermal switch to open the circuit.  The most efficient use of panels is when they are sized to cover heat loss. If more panels are desired a tempering valve needs to be installed that blends cold with the hot and produces water that is 120 degrees as required in many new codes. That would allow you to set the upper tank limit to 150F and effectively increase water capacity.
Better have an expansion tank with wild temperature swings.

Higher voltage is the key to using existing tank heaters.  They sell 12V elements and they are expensive, but the currents are so high a signifigant amount of power is lost in the wiring. 2000W 120V elements can be found in many big box retailers for about $10 if stuck using a 36V array.  The power point voltage for a 36V array is a little over 50V.  A 48V array should work well with 240V heaters.

How do you track?  Solar panels are just big diodes, their power point voltage varies with temperature not light level.  They are current sources, the current varies with light level. Very effective tracking can be done with just temperature.  Is tracking that important?  Not really. At peak power times (about two hours a day) it will likely be a direct connection.  In lower light the major gain is from getting the set point anywhere near the power point.  A seasonal adjustment could be sufficient if going for simplicity.  Whether digital or analog, a couple of flat pack bridge rectifiers have a lot of surface area to couple thermally with a panel.  The four diodes in series give a nice 2.5V reference (about midway for an A/D converter.  Have a pot in the voltage divider from the panels, compare that with the reference, and adjust the pot for maximum power.  Tracking made easy.

What about calculating MPPT and shading?  You can do that and no current sense is required.  Heating element is a fixed resistance, current is a function of PWM number.  Shading is another issue.  If there is 50V coming in and a 12V battery is being charged, you can follow that voltage down to hell.  Drop the voltage on a heater and you have already lost it.  A single large MPPT controller is sooooooooo yesterday.  Yup, like everything else by the time you figured it out the world has moved on.

More later.

[eneuro]

The initial investment would be one or two orders of magnitude less than those thermal systems you linked!

Thermal systems I've linked are given as example of existing ones to make visual teardown and get best solutions of them and make yourself something cheaper, while people have some hobbies, so afterwork are interested in many things and a with alittle bit effort and help of CNC machines easy available nowadays which cut easy aluminium and steel plates, someone can make something similar or even with better electronics inside at his garage.
Yes, it require some workhours, but... to buy solar PV panels and this control stuff you have to go to work to earn some money too and if this thing fail (usually close to its warranty period) you are done and... have to work again to buy another PV system.

For solar thermal we need a huge insulated water tanks (better a few smaller to fully heat each one) unless we are on deserts, while at sunny nice day like today in some nothern Europe climates when we have perfect sky than having CSP thermal solar system with decent thermal capacity we can catch sun energy which can be used a few days later, eg, when we do not have such perfect conditions for thermal solar.

So, 3m dish with 7 square meters area now get according to Bird Clear Sky model close to 1000W/m2 in Central Europe, which means close to 7kW is reflected from this CSP, and a lot of thermal energy taken from the sun, while this CSP system tracks the sun and sun is 60 degs above horizon on perfect blue sky  right now

--
SUN is: 2015-06-7.404 09:42:18 UTC (2457180.904 JD)  lat: 5*.***N  lon: -2*.***E  Sun hour azimuth: 24.623  Sun (geo) azimuth: 155.377  Sun elevation: 60.690  Sun CSP power: 1006.4 W/m2  Day of the year: 158

[mtdoc]

What about calculating MPPT and shading?  You can do that and no current sense is required.  Heating element is a fixed resistance, current is a function of PWM number.  Shading is another issue.  If there is 50V coming in and a 12V battery is being charged, you can follow that voltage down to hell.  Drop the voltage on a heater and you have already lost it.  A single large MPPT controller is sooooooooo yesterday.  Yup, like everything else by the time you figured it out the world has moved on.

More later.

Thanks for the additional details. I'd be interested in more details on your $20 controller.  specifically the PWM control loop.

BTW, direct PV water heating has been around for decades. (for example see this). It's only rcently with the drop in panel prices that it makes economic sense. I've seen several proposed ways of doing it and a few fly by night commercial products (usually making wildly unrealistic claims like the one you linked).

The most successful method I've seen, which I know of a few people using, is in off grid PV systems that use the PWM output from the programmable auxillary port on a Midnite CC to control a SSR. This is used as an opportunity load once their batteries have been topped off early on a sunny day.

The most efficient (but more expensive and NOT direct heating) way that I know of using PV to heat water is via a heat pump water heater.

But I realize these kind of add ons to an existing PV system is not what you are talking about. I get the basic premise, but am interested in the specifics.   I have several spare PV panels sitting around and may even try it myself.

Cheap atmel mcu PWM output controlling power mosfet, got it. But I'm not sure i understand the control loop you're using to control current with varried solar insolation.

[Seekonk]

The simplest method is to use a fixed set point.  The A/D value is compared with that and the PWMcount is adjusted up or down.  A dead band is needed to prevent continuous adjustment. When a major change is detected an additional change to the count is made to speed the transition. The upper and lower limits of the countare constrained.  Narrow pulses at each extreme only result in heating of the switching device and are prevented. Delay reading A/D at least 100ms after PWM is changed to allow system to respond. For a fish pond pump, the code is the same except the upper  PWM limit would be about 170. This can also be used as a diversion controller on a MPPT charge controller without diversion control.  A second A/D would monitor the battery and allow diversion at power point when the battery is over 13.8V. Code has been made intentionally simple for clarity.
 
The picture is a test bed I used to gather data from a single 12V panel. The 150 boost converter on the topis connected directly to the panel and was used to create 35V. This would not be needed with more than one panel in series. The UNO kept the panel at the power point.  The pot was adjusted for highest power on the meter.  Two 5V gate FETs were driven directly through 100 ohm resistors.  Sufficient at these levels. Prefer
driving with opto isolators which will protect the controller and provide a level shift.  Nothing more complicated is needed at these speeds.



// A resistive voltage divider produces about 2V-3V at pin A0
// to give A/D count of about 500
   
//  READ ANALOG VALUE AT PIN A0
panel = analogRead(0);                 
// A/D values go from 0 to 1023
   
   
// ADJUST PWM COUNT
   
// FAST RECOVER from high panel voltage at startup
if (panel > setpoint + 25) PWMcount = PWMcount + 1;
   
// FAST RECOVER from low panel voltage
if (panel < setpoint - -25) PWMcount = PWMcount - 1;
   
// NORMAL HIGH VOLTAGE RAMP UP
if (panel > setpoint + 2) PWMcount = PWMcount + 1;             
// voltage is over setpoint
   
// NORMAL LOW VOLTAGE RAMP DOWN
if (panel > setpoint - 2) PWMcount = PWMcount - 1;             
// voltage is under setpoint
   
   
// CHECK PWM LIMITS
// is count too high?
if (PWMcount >= 255) PWMcount = 255;               
   
// is count too low?
if (PWMcount <= 0) PWMcount = 0; 

// Set count to output?
PWM3 = PWMcount; 
   
// PREVENT NARROW DRIVE PULSES
     
// is count too high?
if (PWM3 >= 245) PWM3 = 255;               
   
// is count too low?
if (PWM3 <= 4) PWM3 = 0; 
   
// PWM FET DRIVER OUTPUT PIN #3                         
analogWrite(3,PWM3);                       
// PWM values are between 0 and 255

a delay or additional program is needed

[eneuro]

Is tracking that important?  Not really. At peak power times (about two hours a day) it will likely be a direct connection.

In the case of PV where you have very bad efficiency, 30% more energy due to tracking by pointing into the sun maybe is not worth it, but in the case of thermal solar even at 50% efficiency no tracking means lost a lot of sun energy, so much worse ROI without sun tracker.
There is also something else-there are days when there  are sunny days, but after dinner 14 o'clock in Europe thunder storms comes, so without sun tracker you miss sun maximum elevation due to after dinner rain, while using sun tracker from the morning to dinner you have enougth energy and can easy give up with any attempts to catch sun after dinner 
Forget about peak power times, while thunder storms after dinner will show you quickly that nature rules needed solar system design and tracking the sun can be sometimes much more than 30% of energy, while without sun tracker you will have to watch sometimes instead of peak power times.... cloudy skies with huge lighting 

[mtdoc]

The simplest method is to use a fixed set point.  The A/D value is compared with that and the PWMcount is adjusted up or down....

Thanks for more details. Correct me if I'm wrong, but it seems what you are doing is essentially simplified MPPT tracking.  Since there is no battery involved there is no need to buck down the voltage and no need to worry about charging stages, etc.

[Seekonk]

Not that kind of tracking of the arc of the sun.  It is always a mistake when I expect people to get it out of context.  I once advised someone to use a "15-47K resistor".  They put in a 33 ohm  just to be safe they said and it went up in smoke.

Yes it is simplified. Power tracking can often be skipped because the PWM often goes to 100% during peak hours if the load doesn't have low enough resistance and it is just a 36V string. A seasonal camp wouldn't see much difference.  Big difference between winter and summer and at low light levels there isn't that much to capture. I wanted to show that code because it is applicable to linear current booster for a pond pump.   It is an easy place to start and thermal tracking can be added later.  This year I will have the opportunity to try some power tracking when I go to 48V strings at the camp.

[mtdoc]

Not that kind of tracking of the arc of the sun. 

  No, no of course. MPPT is not same as mechanical array tracking of the path of the sun in the sky.  But as Maximum Power Point Tracking - it is optimizing power output under different levels of solar insolation - which can vary with cloud  cover, fog, shading and yes, position of sun in the sky relative to the panels. This is essentially what you are attempting to do with your Arduino , no?

BTW, there are several examples of DIY Arduino MPPT solar charge controllers out their (Tim Nolan's was the first I believe).  But what you are doing is different since you are not trying to charge a battery.

[eneuro]

Not that kind of tracking of the arc of the sun. 

  No, no of course. MPPT is not same as mechanical array tracking of the path of the sun in the sky.

Yep, now at the morning sun has 42 deg elevation, but decent CSP estimated power available from the sun is... ~940W. There is nice sunny day, so this is input power to thermal solar when dish tracks the sun.
If someone has PV panel fixed and pointed to south to hit into a few hours only peak power at sun maximum elevation, than... he has much lower input power to PV system now.
Additionaly bad neews for PV today in my area-thunder storms comes and at peak hours might be cloudy and rain  soon, so no time to waste any sun energy right now 


--
SUN is: 2015-06-9.299 07:10:30 UTC (2457182.799 JD)  lat: 5*.***N  lon: -2*.***E  Sun hour azimuth: 74.430  Sun (geo) azimuth: 105.570  Sun elevation: 42.148  Sun CSP power: 943.8 W/m2  Day of the year: 160

[Seekonk]

Saving the world one shower at a time.  Pope might even put me up for sainthood. I am now at my camp for the summer.  It has been a week of rain and clouds, still get a shower every day and still runs my fridge + more.  Brought a new panel with me and now the hot water is 900W, 3S3P 36V configuration.  Was going to make it 48V but now all these panels are identical and this keeps me "low voltage."  Wanted to try perturbing software but that will never work here.  I have a 50V buss that is fed from both ends with multiple devices pulling from it.  Each device has an assigned power point voltage.  A slightly lower power point device has priority and is served first. I will implement temperature tracking this year sometime.  At a 50V potential a 20C panel temperature change only changes the power point 3V.  Plop that in the middle and the loss is quite small when adjusted seasonally.  The internet is filled with R&C people (Rob & Copy).  They can't design or program their way out of a paper bag.  Even with all the details spelled out they struggle to make a project work.  The simplified code I have shown might give them a chance to have organic capability.

I had wondered why there wasn't a solar corner here.  Clearly there is no interest. I want to thank even the detractors for giving this thread a long enough life to give it visibility to such a large number.

[eneuro]

Brought a new panel with me and now the hot water is 900W, 3S3P 36V configuration. 

Which is total area of those panels? How many square meters of PV solar? Do you have some kind of pyrometers to estimate available input solar power, than integrate and get daily averages?
For example for 1kWp I've found such "made in China" PV panels:  142 Wp/m2 14%   Dimension: 1473*670*35mm ~0.987 m2 ~1m2, I'd need seven (7) like those below and do not want invest more than that 1kWp solar PV.

BTW: Do you know what this magic: AM 1.5 means close to: Standard Test Conditions: AM 1.5, 1000W/m2, 25*C at the bottom of the above specs? 

Update: Never mind-I've found it there: http://www.amsolar.com/home/amr/page_164  it is airmass defined to test PV panels at varying degrees of latitude and altitude .

Idea is put a few solar PV panels around 3m thermal solar CSP dish, so I could have ~4m in diameter hybrid solar CSP 5kWp & 1kWp PV system, with sun tracking MPU software of course 

Note: At the botom, with exclemation mark (!) they says that those given Wp are at those perfect conditions 25*C and 1000W/m2 sun input power, so efficiency is <15% of course in the case of those solar PV.

[collingtech]

last month we instaled here a solar heat system for water , way to go mate , very noticeable reduction on eletric bill , way to go if you eletric bil is salty like mine and have plenty of sun to use :0)

[Zeranin]

I'm jumping in at the end of the thread without reading all posting, so forgive me if I'm repeating what others have said. Broadly speaking, I agree that using PV to make hot water can make a lot of sense. A few years ago I decided to get a grid-connected domestic PV system, and conventional solar hot water. I got the PV system first, and left an area on the roof for the hot water solar collector. The I started thinking. A decent solar hot system that can provide 100% hot water needs is quite expensive, and also complex with pipework and circulation pump. If you size it big enough to provide all your HW needs in winter, then it will be way oversized and under-utilized in summer, possibly have boiling problems in summer as a result, and in my climate you have additional complexity to prevent freezing in winter. Fortunately I saw the light before installing the solar HW. Just in raw economic terms, I was better off just to add 1.5 kW or so of panels to my PV system, and get a bigger inverter, which is reason enough to go that way. However, when you think of all the other advantages of the PV approach, I would need to have had rocks in my head to proceed with the solar HW. No messy plumbing to the roof, no circulation pump, no boiling or freezing issues, but there are even more advantages. Most of the time, the solar HW would have had excess capacity, that is wasted. With the PV, when there is excess capacity, you can use it for other useful things, or sell it back to the utility company, to be usefully used by someone else, both saving you even more money, not to mention being environmentally desirable.

There is much talk about using batteries on domestic PV systems, to better match PV production to demand, but PV hot water storage systems can do much the same thing at very low cost, if cleverly controlled, yet another advantage. I am about to build a controller that only diverts my PV output to the HW system when there is excess PV power available. For example, in the early morning and late afternoon, all my PV output is available for my domestic use, minimizing or eliminating my drain on the grid at the precise time when community demand is high, and PV generation (all PV generation, not just mine) is low. In the middle of the day, when the panels are really pumping out the kW far in excess of my needs, then the excess power is used to heat the hot water. Quite literally, I'm using the stored energy of the hot water as if it was an electrical battery, with the storage capacity of a large, expensive battery, but without the cost and dubious environmental credentials of a battery, when the limited life and large quantity of materials that go into making the battery are considered. If everyone had a smart PV HWS like me, this would have a profoundly beneficial effect on the power grid, flattening out the PV production over the day. I know that our electricity utility is quite worried about the problems that large numbers of PV installations would cause, with fluctuating PV production, and smart domestic HW systems could be of great benefit here, at essentially no cost. A decent controller must be capable of proportionally controlling the amount of electrical power made available to the HW system, for example using triac phase control, or time-proportioning. In this way, for example, if there is only 1kW of excess PV power available from a 5kW system, then exactly and precisely that 1kW will be made available for water heating. PV water heating with smart control has a very big future, I would predict. The system can respond fast to changing environmental conditions. Even if a cloud blows over for a few minutes, the (reduced) PV output will be removed or wound back from the HW system, then made available again when the cloud has passed. Thus is not academic. Our utility company is studying methods of measuring the moving clouds over tthe city, so they know in advance that an entire suburb(s) are about to have their PV generation become reduced, so they can quickly switch in additional power to compensate. Smart PV HW systems would be a Godsend. 

With the low cost of PV panels, direct-solar HW systems are obsolete technology as far as I am concerned. Circumstances vary, but in many situations, mine included, you really would need to have rocks in your head to even consider installing a solar HW system.   

[madires]

Sorry, but you can't simply say that solar HW is economically nonsense per se. It depends on the situation and enviroment. We got a solar HW system supporting the central heating system for hot water and heating (oil). From late spring to early autumn the central heating barely runs. The HW also helps in spring, autumn and on sunny winter days to reduce the oil consumption. No issues with freezing. On the long run the HW system incl. maintenance is less expensive than the oil we would have to buy instead.

We've also thought about a parallel PV system. But back then PV modules were quite expensive, though you would have gotten a fixed payment per kWh for feeding the power into the grid (law to support green energy). The payment was higher then buying a kWh and the break-even was about 15-17 years for most small private PV systems. Meanwhile the modules are cheaper but the fixed payment dropped also dramatically. Nowadays a small PV system just for feeding the grid and collecting the money isn't economical anymore. But using your own PV power becomes economically interesting, because we pay much more for a kWh (about +40% vs. 10 years ago).

[Zeranin]

Sorry, but you can't simply say that solar HW is economically nonsense per se. It depends on the situation and enviroment. We got a solar HW system supporting the central heating system for hot water and heating (oil). From late spring to early autumn the central heating barely runs. The HW also helps in spring, autumn and on sunny winter days to reduce the oil consumption. No issues with freezing. On the long run the HW system incl. maintenance is less expensive than the oil we would have to buy instead.

We've also thought about a parallel PV system. But back then PV modules were quite expensive, though you would have gotten a fixed payment per kWh for feeding the power into the grid (law to support green energy). The payment was higher then buying a kWh and the break-even was about 15-17 years for most small private PV systems. Meanwhile the modules are cheaper but the fixed payment dropped also dramatically. Nowadays a small PV system just for feeding the grid and collecting the money isn't economical anymore. But using your own PV power becomes economically interesting, because we pay much more for a kWh (about +40% vs. 10 years ago).

All agreed, and I did say that circumstances vary. I was being deliberately provocative to get people thinking about PV HW. Traditionally, making hot water from electricity is regarded as the ultimate environmental crime, especially here in Australia where most electricity is generated from coal, and electric HW is banned by law. However, the ridiculously low cost of PV panels has changed the game, but legislation and attitudes have yet to catch up.

[nowlan]

Using Solar power to heat water is one option. However, it sounds like you have over sized your panels vs your needs.
Would be a waste in summer, but at least you can export your power in that case.

[Someone]

Using Solar power to heat water is one option. However, it sounds like you have over sized your panels vs your needs.
Would be a waste in summer, but at least you can export your power in that case.

But thats the point, its not oversized when the cost of moving the hot water to a solar collector would be more. For a lot of people adding solar PV capacity is the cheapest way to create hot water.

[HackedFridgeMagnet]

With the low cost of PV panels, direct-solar HW systems are obsolete technology as far as I am concerned. Circumstances vary, but in many situations, mine included, you really would need to have rocks in your head to even consider installing a solar HW system.   

You need to provide some figures for this, and I think you will find that in the majority of cases the payback time for solar hot water is quicker than the system you are proposing.

Try Sydney, family of four, no current PV, and 15sqm of usable roof space.
a) initially with gas powered stored hot water.
b)  initially with electrical powered stored hot water.

My opinion you would need rocks in your head not to at least consider a solar HW system.

My personal experience is I put a indirect passive solar HW system on my roof, did the plumbing myself into the existing instantaneous gas HW system. Save $450 a year on gas. and it paid it self back in about 4.5 years not accounting for my labour. Maybe 6? years considering labour.

It has been great but the main drawback is that it is a bit ugly. Looks good in summer when it is letting of steam.

I agree that we will and should be moving toward getting PV to work with domestic HW and smart metering/load control.

[Zeranin]

With the low cost of PV panels, direct-solar HW systems are obsolete technology as far as I am concerned. Circumstances vary, but in many situations, mine included, you really would need to have rocks in your head to even consider installing a solar HW system.   

You need to provide some figures for this, and I think you will find that in the majority of cases the payback time for solar hot water is quicker than the system you are proposing.

Try Sydney, family of four, no current PV, and 15sqm of usable roof space.
a) initially with gas powered stored hot water.
b)  initially with electrical powered stored hot water.

My opinion you would need rocks in your head not to at least consider a solar HW system.

My personal experience is I put a indirect passive solar HW system on my roof, did the plumbing myself into the existing instantaneous gas HW system. Save $450 a year on gas. and it paid it self back in about 4.5 years not accounting for my labour. Maybe 6? years considering labour.

It has been great but the main drawback is that it is a bit ugly. Looks good in summer when it is letting of steam.

I agree that we will and should be moving toward getting PV to work with domestic HW and smart metering/load control.

Circumstances vary. With only 15 sqm of roof space, no existing PV, and an existing gas HW system for backup, then your solar HWS makes sense. I have 100 sqm of useable roof and was installing PV anyway, so in my case installing a solar HW in addition made no sense at all. I forgot to mention another advantage of going the ‘one-big-PV-installation’ way. With a reverse-cycle aircon installed, you can use the power to heat your house in winter, and cool it in summer – can’t do that with the excess capacity of a solar HWS.

The issue of backup is very important from an environmental (and possibly economic as well) perspective. In reality, it turns out that solar HW backed up with mains-grid electricity is one of the worst possible options for producing HW in Australia from an environmental perspective, where coal-produced-electricity is used in a criminally inefficient way to provide electric-heater backup. If you already have gas HW like you did then of course you use that for your backup. If you don't have gas HW installed already, but have gas available, then it becomes a mighty expensive solar HW system when the cost of installing the gas backup is accounted for. The alternative is to grossly oversize the solar HW so that backup is very rarely required, and that costs extra, and then you can have over-capacity issues in the middle of summer. If you install a small number of PV panels dedicated solely to making HW, then most of the same issues apply.

However, if you have 30 sqm or more of roof area and wish to install PV anyway, then the game changes completely. In that case, just add another 2kW or so of panels and inverter capacity over and above your intended PV generation system, and for relatively little marginal extra cost you get all the benefits described in my first posting, plus you probably won’t need any backup at all as you can draw on the overall capacity of the PV system when needed. An obvious partner with PV systems is reverse-cycle HW, which reduces the electrical power requirement for the HW by a factor of x3 or so, but panels are so cheap that if roof area is not an issue, then the capital cost and finite lifetime of the reverse cycle unit may mean you are better off without it.

Beauty is in the eye of the beholder. I would probably find your solar HWS an object of great beauty. But I’m so glad that I did not install one in my situation.

[HackedFridgeMagnet]

Some other of advantages of Solar HW over PV hot water.

solar efficiency, probably 2 to 3 times pv per unit area.
works better than many PV systems when subject to partial shading.
doesn't need planar alignment with other solar catchers.
far less regulatory and safety issues/requirements/costs.
less safety issues/requirements.

Don't get me wrong I am all for PV and I just got my ticket for it, but all options should be considered.