Solar hot water is widely considered to be the best “bang for buck” in solar. Instead of trying to get photons to push electrons around, they just need to heat something up, which is something solar is very good (efficient) at. Solar hot water can even be used for radiant heating. While it works best on sunny days, it can provide a lot of hot water, even on cloudy days.
This page will look at the pros and cons of various solar hot water configurations, particularly as I consider them for heating the floors in my own earth sheltered home.
Passive Solar Hot Water systems generally have the tank at the top of the panel. Gravity feeds the liquid into the solar collector where energy from the sun is added, causing the liquid to heat up. In some cases, there is phase change to a gas which heads back up to the tank. In other designs, the increased energy reduces density (increases buoyancy) and the warmer fluid “floats” back up to the tank. It is a nice idea, but doesn’t tend to work very well, particularly in very cold areas like mine where it may not be a good idea to have your hot water tank exposed on your roof in winter. My neighbors probably wouldn’t like the industrial look of it much either.
Passive systems usually rely on the fact that the density of water reduces with increased temperature. One cubic ft of water weighs 62.41 lbs at 32°F, but reduces to only 59.82lbs at 212°F. This is the principle of thermal circulation used in boiler designs for a very long time.
Active systems keep the hot water tank safely concealed in the home, but requires water pressure, typically provided by a pump, to actively push the water up to the roof and thru the solar collector. If it is a closed system, the weight of the water coming down again counter balances (siphons) the water going up and the pump only needs to be big enough to overcome frictional and dynamic losses.
There are several basic components which can be assembled in various different ways. If purchased as set, the components can easily add up to between $5,000 and $7500 or more. When I mentioned radiant heat to one company, the quote jumped up to over $12,000 when they more than doubled the collector area. Of course, installation cost is extra, but much of it can be done yourself. Many studies show that these systems will pay for themselves very quickly. They can also be leased or financed directly from the system retailers. It is also possible to manufacture many of these components or systems yourself, if you have the time and the know how.
The collector panel goes up in the sunshine and actually collects the energy transfers it to some liquid. There are flat panel collectors and evacuated tube collectors.
Flat panel collectors are pretty basic technology. They usually involve a series of conductive tubes (such as copper pipe) thermally connected to a collection plate (such as copper fins) and painted a dark color. Insulation is applied to the back and Glass is added to the front. Fluid picks up solar heat as it is pumped thru the collector. These can be purchased (roughly 20% less per BTU than evacuated tubes ) or even built as a “do it yourself” project. However, they are not as efficient, particularly in winter when heat losses thru the panel can meet or exceed solar collection. In warm climates, these can overheat. When the glycol overheats, it breaks down (chemically) into an acid which damages the pipes and pump, but there are a variety of ways to avoid this problem, including dumping the heat in the earth, draining the glycol when it gets too hot (or cold) or simply shading the collectors.
Evacuated tube collectors are much more advanced and definitely not a “do it yourself” project. Their solar collection in done across a row of double wall evacuated glass tubes. Each tube is a separate collector and can be replaced separately if damaged. The glass tubes let light enter the inner tube and heat up the collector, but the vacuum (between the inner and outer tube) isolates the inner tube, preventing conduction and convection and significantly reducing heat loss (at least from the collector tubes themselves). The ability to collect and retain heat, even in very cold weather, improves the efficiency of these collectors and enables their use in northern climates (for temperatures down to -60°F). The tubes themselves are “passive” solar and usually have a phase change liquid that literally boils in sunlight and transfers the energy to the antifreeze fluid passing thru the top of the unit. If these units overheat, the steam can no longer condense and the cycle just stops without damage to the unit. There are gaps between the tubes that can allow snow and wind to pass thru the collector. The price is fairly reasonable at less than 1000$ for a typical installation of about 75 sqft.
Of course, orientation of the solar collector is important, as with any solar installation and follows the same basic rules.
Hot Water Tank
The collectors only gather energy while the sun is shining. Assuming you will want hot water after dark, you will need a way to store that energy and most solar hot water systems use a hot water tank. Because the heat generation period is typically only a small fraction of the day, these tanks must be larger than you would find for a gas or electric hot water heater. Most systems have a hot water tank that is at least double the size of a gas or electric hot-water heater. These tanks can be hooked up a variety of ways (see the systems below), including with external or internal heat exchangers, with additional heating coils, etc. these tanks are not cheap and can easily run you $2500 for a 120 gallon tank. Extra heating elements, etc. are extra.
Some tanks have more than one heat exchanger. The bottom one is intended for the solar heat transfer fluid, the other is for some auxiliary heat source (such as a boiler). I recommend you use both for solar and rig up an auxiliary heat source down stream of the hot water tank.
In freezing cold weather climates, running an “open” system where the homes water passes directly thru the solar collectors is just not a good idea. Instead some heat transfer fluid, such as a water-glycol antifreeze mixture is used instead. In that dual fluid setup, a heat exchanger is needed to transfer heat from the heat transfer fluid to the household water supply. Most hot-water tanks include an internal heat exchanger, but some layouts include a separate heat exchanger between the solar panel and the hot water tank. The commercial grade ones look like a small box, maybe 6″ x 6″ x 30″, mounted on the wall. Pipes from the solar collector bring hot glycol thru one side of the exchanger to exchange heat with cool water from the bottom of the hot water tank. Heat is exchanged, but the fluids are kept separate.
I suppose the advantage is that you never risk glycol entering your hot water tank. You may also be able to save some money by purchasing a cheaper tank without an integrated heat exchanger. However, you still need to purchase the separate heat exchanger and the plumbing will be more complicated, so it is hard to imagine that you get very far ahead. There is also still a risk of cross contamination within the heat exchanger.
It is reasonable to assume that a separate heat exchanger probably loses more heat to the environment that an internal heat exchanger within a well insulated hot water tank. These losses would happen along the extra tubing and along the surface of the exchanger its self. The external heat exchanger also requires a number of extra connections and an additional pump for the separate loop.
As you can probably tell, I don’t recommend an external heat exchanger… but I am interested in hearing more “pros” or “cons” if anyone wants to comment on this post.
An open system, where the actual supply water is lifted and pushed thru the solar collector, may not need a separate pump since the water supply is already under pressure. However, that supply pressure may not be sufficient to lift the water and either a larger well pump or additional auxiliary pump may be needed.
Cold weather designs typically push an antifreeze liquid thru the collector. This requires a separate pump for the separate loop. However, since these loops are closed, the weight of the column of liquid falling back to the pump counter balances the weight of the liquid being lifted and the pump only needs to overcome frictional and dynamic losses.
If your system has an external heat exchanger between the antifreeze loop and the hot water tank, an extra pump may be needed for that loop.
(More on this to come)
You can’t always count on solar, particularly in areas with cold and cloudy winters, so there are a variety of methods that can be used to provide heat to the system when solar isn’t enough. These include wood, oil or gas boilers, electrical resistance heaters, heat exchangers, etc. Some of these can be built right into the hot water tank. On demand hot water heaters are also a popular component.
As mentioned in other sections of this earth sheltered site, I am concerned about burning natural gas or oil in an underground home, and wood heat just seems like too much work, so I will focus on electrical systems.
Some solar storage tanks have more than one heat exchanger. The solar supply heat exchanger is usually at the bottom of the tank where the coolest water is. Higher up in the tank there may be an electronic heating element and or a second heat exchanger (auxiliary supply). Some layouts suggest hooking up a boiler to this second exchanger as a backup heat source. Some layouts even show a closed loop hookup with an “On demand” instant hot water heater. Many of these show natural gas boilers or instant hot water heaters, but electric on demand heaters are closing the efficiency gap and provide a lot of convenience.
Personally, I can’t imagine using an instant hot water heater to heat up my solar hot water tank. As you can see in just about any thermodynamic transfer equation, ΔT (the temperature difference) is a key driver. If you pay to heat up your hot water tank over night, then it will be less able to harness solar energy in the morning when the sun comes up. The two heat exchangers are monitored by separate temperature sensors. Proponents of this secondary heat source say that because the auxiliary sensor is higher in the tank, it will reach the required temperature and turn off while the temperature at the bottom of the tank is still cool enough to gather heat from the solar collectors.
I see the heat sources as competing rather than cooperating. As water enters the tank, the water in the tank is mixed and the thermal stratification is disrupted. I would not expect buoyancy to be able to maintain more than a few degrees difference between the high and low sensors. If your tank has a second heat exchanger coil, I suggest you hook it up to the solar supply also (in series or parallel). This will just give more surface for heat exchange and may improve your efficiency.
For the same reason, I don’t suggest using a electrical resistance heating element built into your solar storage tank. Try to get a lower cost tank without the built in heating element, but if you tank comes with one, don’t wire it it up. Use an on demand “instant” hot water heater instead. Plumb it so the water comes out of the hot water tank and then passes thru the instant hot water heater only when you need it. If it is late on a sunny day and the water in the tank is hot, the “instant” hot water heater can check the incoming temperature and is programmed to add little or no heat. However, if it is a cold winter morning and the tank has cooled off over night, the instant hot water heater can sense this also and add heat as needed, and only when needed.
Controls, piping, etc.
And a bunch of other stuff that I will cover
(More on this to come)
Searching the web, I found many different ways to assemble the components of a solar hot water system. With even basic thermodynamics knowledge, it is clear that some configurations are better than others. I can only speculate that the poor configurations are primarily beneficial to the individuals who are selling them, but you can decide for yourself.
(More on this to come)
Interesting Earth Sheltered Ideas
Wind and solar energy are not “on demand” as we would like. Sometimes they are not there when you need them, and some times they are over producing and the equipment can actually be damaged if you don’t draw off enough energy. In an ideal world, you could store that energy for when you need it later. This is the primary reason for the extra large and well insulated hot water tanks. But some times, even that is not enough. If collectors overheat, the glycol can become acidic and damage the system. Many collectors are able to shut themselves down to prevent damage, but then you are just not collecting on your investment.
On idea that appeals to earth sheltered home owners is to sink some of that excess heat into the ground. Windmills or waterwheels can dump excess electricity into buried electrical resistance heater coils. Solar hot water can dump it in a similar way via a heat exchanger buried in the earth.
My particular design includes a small green house. In the winter, it will likely loose more heat thru the glass than enters and may not support plant life as well as I would like. However, If I ran some hot water pipe from my solar tank, by-passing the instant hot water heater, and down 6 to 10 ft under my green house floor, I could use it to warm a large volume of earth. On very sunny days when the tank temperature exceeded some specified level, the valve would open automatically. A small auxilary pump would push the free hot water down under my green house where it would give up much of its heat and then re-enter the tank via the cold water inlet. That heat should slowly conduct up thru the earth and hopefully keep that green house floor warm during the winter. Optionally, I could also run a second branch at a shallower depth with a shunt so I could dump heat at that shallower level as winter approached.