Solar water heater

[RaymondR]

9.8.1 Hybrid PV/T applications
The hybrid PV/T systems are considered an alternative to plain PV modules in several applications. They can be used effectively for converting the absorbed solar radiation into electricity and heat, therefore increasing their total energy output. In these systems, PV modules are coupled to heat extraction devices, in which water or air is heated and at the same time the PV module temperature is reduced to keep electrical efficiency at a sufficient level. Water-cooled PV/T systems are practical systems for water heating. These new solar energy systems are of practical interest for many applications, as they can effectively contribute to cover both the electrical and thermal loads.

It should be noted that the cost of the thermal unit remains the same irrespective of the type of PV material used, but the ratio of the additional cost of the thermal unit per PV module cost is almost double when amorphous silicon modules are used rather than the crystalline silicon ones. In addition, amorphous silicon PV modules present lower electrical efficiency, although the total energy output (electrical plus thermal) is almost equal to that of crystalline silicon PV modules.

The additional thermal output provided from the PV/T systems makes them cost-effective compared to separate PV and thermal units of the same total aperture surface area. In PV/T system applications, the production of electricity is the main priority; therefore, it is more effective to operate the PV modules at low temperature to keep a PV cell electrical efficiency at a sufficient level.

The daily and monthly performance of a hybrid PV/T system is investigated through modeling and simulation using the TRNSYS program (see Chapter 11, Section 11.5.1). Such a system provides more electrical energy than a standard photovoltaic system because it operates at a lower temperature; in addition, thermal energy is obtained, which can be used for water heating. As shown in Figure 9.25, the system consists of a series of PV panels, a battery bank, and an inverter, whereas the thermal system consists of a hot water storage cylinder, a pump, and a differential thermostat (Kalogirou, 2001). In each case, the TRNSYS-type number used is indicated.


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FIGURE 9.25. Hybrid PV/T system schematic.

A copper heat exchanger is installed at the back of the photovoltaic panel, and the whole system is enclosed in a casing in which insulation is installed at the back and sides and a single low-iron glass is installed at the front to reduce the thermal losses (see Figure 9.23). Water is used as a heat transfer medium. The system also employs eight batteries connected in a 4 × 2 mode, i.e., four batteries in parallel and two in series.

The findings from this investigation are very promising. In addition to the increased electrical energy produced by the system, almost 50% of the hot water needs of a four-person family are satisfied with such a system, and because of the heat removal, the PV cells’ annual efficiency increased considerably.

In another case the application of water PV/T systems in industry has been studied. Water-type PV/T systems were also considered for this application. The PV/T systems can be used in several industrial applications, but the most suitable are applications that need heat in low (60–80 °C) and mainly very low (<50 °C) temperatures, since in these cases, both the electrical and the thermal efficiency of the PV/T system can be kept at an acceptable level. It should be noted that the fraction of heat demand at low temperatures is high, especially in the food, brewery and beverage industries and in the paper and textile industries, where its shares could be up to 80% of the overall thermal energy needs. For example, water-cooled PV/T systems could heat water for washing or cleaning processes. The PV/T collectors could be installed on the ground or on either flat or saw-tooth roofs, or on the façade of a factory (Kalogirou and Tripanagnostopoulos, 2007).

Finally, the performance and financial improvement of the PV/T systems was compared to the standard PV systems for building applications and proved very beneficial (Kalogirou and Tripanagnostopoulos, 2006). Additionally, it was proven that PV/T systems could be beneficial to the greater diffusion of PV units. This is especially important for countries with good penetration of solar water heaters, where it is a habit to produce hot water with solar energy. In these cases, it would be difficult to convince potential customers to install a PV system, whereas a hybrid system producing both electricity and hot water has better chances of success.

[noelex 77]

Originally Posted by PirateGuy

I’ve never seen that sort of white stuff on the palms and cacti down here. Is it conducive to bikini wearing?

Amazingly, some of the locals are still swimming (no wet suit). As Australians, the occasional sound of sheets of ice hitting our hull is an indication at least that the weather is more conducive to a hot shower .

The idea of generating hot water from the underside of solar panels has the intriguing possibility of providing hot water without any extra energy input and without taking up more deck space. This is achieved while cooling the solar panels and therefore increasing their performance.

I hope someone can make this work on a boat. However, I doubt these goals are achievable in practice. I suspect you are going to end up with a complicated and fragile system that produces lukewarm shower water most of the time and have solar panels that are overall hotter and therefore have lower output.

Anyway, we are extremely happy with our existing shower system. It always produces a perfect shower temperature, is very simple, and as it can take advantage of hot water produced from multiple sources such as electric, propane, diesel and of course solar either from water heated on a bag on deck or indirectly by using our solar panels to produce electricity which is used to heat the water.

This latter option is used for most of summer and while this energy conversion is not very efficient, it is using energy that would otherwise be wasted so this poor efficiency is not a concern. We currently do not bother to heat water directly from solar radiation (using a black bag on deck for example). However, if we do wish to use this energy source it is quite easy to add water heated this way to our shower tank, allowing a normal pressurised and indoor shower if desired. A little more fuss, but reliable, simple and versatile which are great attributes for any marine system.

[Pete7]

Originally Posted by noelex 77

The shower would be a little too cold for my liking if we used water from our solar panels at the moment .

True, but circulating a good Scotch through the system would allow you syphon a little off for both medicinal purposes and to make Whiskey Ice cream

[PirateGuy]

Originally Posted by Pete7

True, but circulating a good Scotch through the system would allow you syphon a little off for both medicinal purposes and to make Whiskey Ice cream

Whiskey ice cream? 👍🏼

[poiu]

Originally Posted by noelex 77

.... I suspect you are going to end up with a complicated and fragile system that produces lukewarm shower water most of the time and have solar panels that are overall hotter and therefore have lower output.

I think the opposite would be true. In hot places where solar panels usually get hotter than you want your water to be. The heated water flowing behind them will work to cool the panels and a lower panel temperature will increase efficiency.



In cold places there will still be heating, but I suspect the temperature will be so low anyway that efficiency is not impacted. The issue then is more likely to be that there is not enough heat, but you can still top it up the usual way.

[noelex 77]

Originally Posted by Pete7

True, but circulating a good Scotch through the system would allow you syphon a little off for both medicinal purposes and to make Whiskey Ice cream

Whiskey ice cream!

I am not sure if that is sacrilegious waste of whiskey or a valuable new culinary idea.

Some experimentation is obviously needed .

[noelex 77]

Originally Posted by poiu

I think the opposite would be true. In hot places where solar panels usually get hotter than you want your water to be. The heated water flowing behind them will work to cool the panels and a lower panel temperature will increase efficiency.
In cold places there will still be heating, but I suspect the temperature will be so low anyway that efficiency is not impacted. The issue then is more likely to be that there is not enough heat, but you can still top it up the usual way.

That is the idea and I think it can work in this way some of the time, but there are some practical problems.

Typical peak solar panel temperatures in a hot climate at solar noon is only slightly above shower temperature especially after the inevitable heat loss transporting and storing the water. So many of these systems rely on insulation underneath the panels to work in more average conditions which is going to result in hotter panels the rest of the time. The temperature/power coefficient of solar panels is linear across normal temperatures so heating up a cold panel is just as bad as heating up a hot panel. The power consumed circulating the water, while not high, also needs to be subtracted from the solar panel output.

The solar panel method of heating water can be combined with other water heating methods, but this ends up becoming a complex system, especially given the difficulty of transporting water up to the solar panels and then hot water from the solar panels down to a storage tank.

I think it could be made to work for some climates, but I think there are better systems for most boats, but it would great to hear some real world experiences.

[s/v Jedi]

Yes I’m afraid I have to agree that underneath PV panels isn’t the best option. So now I’m looking at making a custom size/shape collector that fits leftover space. Or maybe that can be deployed from underneath PV panels where they store during sailing. Flip up from the lifeline/handrail is another possibility.

Much has to do with the distance and route of the plumbing run. What to use for plumbing... diameter and material? 3/8” or 1/2” PEX needs UV protection and isn’t flexible for a moving collector.

Also, how to fabricate the collector. All the YouTube videos show redneck backyard options that aren’t usable aboard.

[MikeHoncho]

Originally Posted by PirateGuy

I think you are missing the point of a combo electric/hydronic panel. Most monihulls are severely limited in real estate to mount any sort of panel energy collector. Electrical generation takes a far, far higher priority IMO for the little usable space available. The hot water just is icing on the cake.

I know the point all too well owning a Hunter 30 with two panels mounted on my davits. I’m not buying the cause and effect of the idea. The return would be minimal. Add the additional complexity of the water system, added weight of the system to the panel structure and the return isn’t enough. You’d be better off designing a stand alone, portable system that takes advantage of both radiant and ambient heat. Plumb the internals plumbing to quick disconnects and put the system out on the deck during the heat of the day. You’d have to have a circulation pump and consider that as part of your energy plan. For me, I normally motor some part of the day that heats up my tank within minutes and holds the heat all day. I’m assuming this isnt an option?

[Jamme]

Originally Posted by PirateGuy

I’ve never seen that sort of white stuff on the palms and cacti down here. Is it conducive to bikini wearing?

Yes, albeit inversely.

[alaskanviking]

Originally Posted by Captain Bill

There is nothing difficult about building a stand alone solar thermal collector. Basically a box with a bit of insulation with a any number of types of collector on top of it, and a transparent cover will serve as a solar thermal collector. The collector can be metal or a coil of plastic tubing painted black. Sure using IR selective coatings and other techniques can increase the efficiency but they aren't necessary to get adequate performance. The glazing does not even have to be perfectly clear. When I was building solar water heaters 40 years ago we used a fiberglass glazing product that was 90% transparent because it was more resistant to damage than glass and because we were in Florida a loss of 10% efficiency didn't matter much. In more northern climates it would have, especially in winter. One can use a circulating pump hooked to a small PV panel. Because ideally the flow rate is proportional to the insolation, connecting the pump directly to the PV, early in the day when and late in the day the flow rate is low and high at noon, and automatically off at dark.

i am working on my new fresh & hot water systems. was thinking about solar water heaters. I have always thought this is a good idea on boats, not positive on how feasible it is in practice though.

In an application where you have an arch & bimini frame where you mount solar panels, I believe there is enough room to put one of these style solar collector in between the solar panels. maybe 2" on either side of the backstay, split for the backstay. foam core construction may help insulate the inside to keep it hotter, or simple 4-6mm marine ply that's fiberglass encapsulated. realistically you could make it out of stainless from a sheet metal fabricator for a decent price. on top of the davits would be a good space too but that's prime solar panel territory. paint white on the outside with a good quality paint so it looks decent. run as many PEX coils as you can get in there. paint the inside including the PEX with black engine or BBQ paint. seal and cover with some plexiglass. fairly simple devices.

the part that im not sure about is how to make the hoses going from the deck to the bimini/solar collector look nice. I am assuming you need a flexible hose instead of PEX because of how stiff the pex is. i am assuming it would need insulation on that run as well to keep the heat in or the cold out at night. that would make for a pretty bulky thing going up. probably really only need 3/8" here to help collect heat so maybe not as bad as I'm thinking. maybe a sunbrella sleeve then ziptie it to the bimini poles, not sure how i feel about that.

it would need a pump and a controller to ensure the system only operated while it was warm and turned off at night when things got cold. simple arduino or PIC project, but I'm sure they have off the shelf controllers for this purpose. Laing D5 or E1 pumps are only 10w pumps. there are other similar brands. i think i have seen DC solar circulation pumps before that were in the 5watt range. controller would turn the pump on and off to regulate heat.

best practice on a boat is to use an open system connected to your hot water system. probably t-ed at the water heat supply and cold/return. you would have to have drain/purge valves for the winter and drain it. otherwise you would have stagnant water sitting in there for several months and then connect it back to your water supply the next summer. i do know these can collect a lot of heat into the winter if the sun is regular. system would have to be drained or the controller would have to circulate water just in the solar collector to prevent freezing. PEX doesn't burst when frozen, but i suspect regular freeze/thaw cycles wouldn't be good either.

Probably have to have a hot water loop with circulator pump type of system, or some good relief valves. i know these can get really hot in the summer sun. the water may require a faster flow through the whole system to keep things under control at a 140-180degree temp. you could also use a water heater with twin exchanger coils and use one for the solar collector in a closed system, but id worry about the ability to ditch heat into the water heater fast enough on a hot summer day.

Bottom line is this is very possible but there is a lot of design work you have to think about how to fit it and make everything work. at anchor in a sunny area this would be awesome, and low power. 10w at 12v is ~5-8ah (10 hours) running constantly. if the controller is turning off and on or controlling speed to maintain temperature it would use less. not sure how to estimate that without seeing one in use. likewise I'm not sure if the juice is worth the squeeze to do everything required to install and run this setup compared to just turning on your water heater wen your batteries are topped off. However here in Virginia by 0800-0900 my black interior car is unbearably hot when i get in it. i have seen these work in a house, but i don't think anyone has done it on a boat. competes with solar panel SQFT. worth more time looking into i think.

food for thought