Overview

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.

Basic Components

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.

Solar Collector

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.

Heat Exchanger

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.

Pump

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)

Backup Heat

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)

Basic Configuration

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.

Active Solar

Active solar systems use any of a variety of “active” methods to maximize the energy they gain from the sun.   These range from simple pumps or fans to motorized retractable awnings, to computerized sun-tracking solar panels or even rotating the whole house to follow the sun.  These active systems typically cost more and are prone to break downs.

Passive Solar

Passive solar systems are typically less complex and this has made them more popular.  In many cases, designing a moderately passive solar home requires no additional cost at all, it is more about putting the normal components in the optimal place and then using the home as the collector.  Using natural convection to distribute the warm air is standard but can be improved with minor (or major) adjustments to the geometry of the home.  Summer cooling can be enhanced with windows (cross breeze) or by placing openings at different heights (Bernoulli principle) or with a solar chimney.  A solar chimney work by solar heating air in the chimney so that it rises faster (boyancy) and increases the draw from the home.  Cooler air is drawn in from the perimeter or thru earth tubes to replace the air being sucked out the solar chimney.

Augmented Solar  

In many cases, a system that was designed as a passive system, may work a little better with some help to move the heat around.  For instance, a passive home owner may find that his convective loop isn’t moving the air quite as quickly as he would like, and so a ceiling fan or other ducted fan is added to the system.  I intend to use my radiant floor heating system to also distribute solar energy from the direct gain portions of my home to the darker corners.  If the power goes out, the passive portion of the system still works, the augmented system just works better.   In many cases, the small and robust fans used to augment the system are solar powered.

Casually, we might still call that “passive solar”, but I guess we could get picky and call it “augmented passive solar” to differentiate it from the other two categories.

(like many pages on my fledgling website… this one is still a work in progress.  Illustrations to come).

Introduction to By-Passive Solar

You can read a lot about Passive Solar on the web or in books.   The benefit of this arrangement is that it is economical…  nothing “extra” to build.  Just rearrange your home’s windows a little so they are mostly on the south side, add some overhangs to keep out the summer sun and tada! you have passive solar.  Adding some mass to soak up those rays (such as a cement slab floor) would be a good idea.  In the winter, the sun’s heat comes in the windows and is already in the living space where you need it.  In many cases, the air just circulates around by natural convection and floors radiate warmth back into the rooms at night.

I am sure I will use some aspects of direct-gain passive solar in my home (all solar homes build on passive solar concepts)…  perhaps even “augmented” with a fan or in-floor pex pipe to move the heat around…

but what is “By-Passive Solar”…?

It starts with the need to fix the main problem with direct-gain passive solar…  The solar collector IS the home.  This requires certain compromises.  For instance, you need to leave the curtains open to let the sunlight in.  You need too keep your floors bare (and many recommend darker colors unless you have masonry walls to capture the reflected energy).  Also, because the home is the collector, the home is often uncomfortably warm during the solar heating hours.

But what really pushed me down this path is that I am in Michigan where I can’t rely on a daily solar charge.  I can only expect one sunny day a week in January or February.   I needed my solar charge to last longer.  Therefore, I needed to heat more mass; not just my home, but a huge mass of earth around my home (PAHS Umbrella style).  Collecting that much energy requires letting the heat in during the summer, at the same time that I would like to enjoy a cool earth sheltered home…  I don’t want the heat to be building up in the home and surrounding earth all summer,   I want the heat of summer to enter my home 6 months later, when I need it…  So the key is to flip things upside down and heat the earth 10ft away/below my home all summer.  By the time that energy reaches my floors, it will have saturated the earth around my home and be ready to carry me thru the cloudy SE Michigan winter.

This sort of by-passive solar heating system would need two extra components.

• Solar Air Heater to absorb the sunlight energy and transfer it to air
• Earth Tubes to carry the heat under the earth by convection or perhaps a small solar powered fan

Solar Hot Air Collector

It is called a collector, but really it is a generator.  It takes in air at some temperature and then heats it up with solar energy.  You can find many different designs on the internet…  But the best ones seem to be a shallow box with a glazed front and an insulated back.  The glazed front lets in sunlight and heats up air, the high temps drive the air thru by convection (or augmented by a small fan on a thermostat switch).  There is an inlet and an outlet and some means of snaking the air thru the box to gather as much heat as possible, but without creating too much back pressure… that’s it 😉

I plan to make my boxes somewhat modular so I can experiment with different configurations and swap them out later.  Each module should start with a 4×8 sheet of plywood.  You could nail a 2X6 frame around the plywood such that the plywood is approximately 1/3rd of the way thru the frame.  On the back of the plywood, you glue a 2 inch thick sheet of poly iso rigid foam insulation, such as Super Tuff-R that won’t melt under the heat.

On the front of the plywood, you build the “solar absorber”.  This is where the innovation happens.  The solar heat should be transferred to internal airflow and not lost thru the frame or glazing.  Air should not be moving near the glazing as this will result in more heat lost.  The absorber should not reflect any light, so paint it flat black (not glossy or metallic).   It should have lots of surface area to exchange heat with the air. If you design with ducts across the absorber, the spaces between the ducts should be a material that can conduct heat to the back of the ducts.

You need a way for the air to enter and exit the absorber.  In some designs you also need to turn the air within the absorber.  The performance of the Solar heater is very sensitive to the plenum or manifold design.  You want to distribute the air evenly and with a minimum of back pressure or leakage.  Adding simple turning vanes can make a huge difference.  A back flow damper may also be a good idea.

The Glazing is attached to the front of the frame… Ideally, it is something cheap and easy to work with such as Suntuff or Tuff Tecs or some other UV stabilized plexiglass.  The experts warn against using double pane glass because its seals can’t handle the heat.

Controls can be as simple as letting the sun drive the flow thru buoyancy, or you could augment the flow by adding a fan on a thermostat switch ($10 snap switch).  Some fancy designs even use 2 fans with the second one kicking in at a higher temperature.  You can buy some pretty high efficiency duct fans or use something as simple as a PC fan.  Many attic fan assemblies come with solar panels so the system can operate off the grid while the sun is shining, which is the only time you will need to pump the air anyway.

For materials, I think I will start with an “aluminum downspout design”.  This design uses rows of Aluminum downspouts glued to an aluminum back plate, all painted flat black.  The air comes in at the bottom of the left side and turns around in a plenum on the right side before returning and exiting on the top left side.  I may also try a vented soffit or double layer fiberglass screen design, but I suspect they are not as efficient because they don’t keep the air in the channel and away from the glazing.

One other interesting aspect of my design (images or sketches to come) is that it will be adjustable.  I plan to pivot my assembly by the lower intake.  The output will be able to pivot. between two positions.  The lower position (~30°) is for summer use (when the sun is higher in the sky) and will pump the heated air under the house for thermal storage.  This closed earthtube loops back to the solar heater inlet where the air is reheated.   Above this earth tube is a second earth tube, an inlet opening that takes fresh air directly into the home, moderated thru the cool earth.   In winter, I tilt the heater up to the winter angle (~60°).  This connects the outlet of the heater to the inlet earthtube and pumps the hot air directly into the home.  Meanwhile the closed circuit earth tube is now open to the winter air, since this is actually the inlet for the heater, air is first drawn under the home and tempered before running thru the solar heater and then up into the house.  Get it?  Maybe not.  I made a 2D sketch, but it doesn’t do it justice, I will cad it up some time…

A Freebee Earth Tube!

I would avoid the use of corrugated drainage pipe as a fresh air Earth Tube because of the water that gets trapped in the corrugations and the potential for mold entering your home…  It is highly advisable that all earth sheltered homes be equipped with as many waterproofing measures as possible, including foundation perimeter drains made with these corrugated drainage pipes.    I have heard of earth sheltered home owners who are both glad and a little sad that their drainage pipes never get any water in them.  What if we used them for a dual use as by-passive solar earth tubes.  The loop would never enter the home, so there would be no risk of mold or radon from the corrugated pipes.   The modifications could be small, instead of branching your drain pipes, you would need to make a continuous loop that would start and end at the solar heater.   All summer these pipes would be warming the foundation under the umbrella and helping to increase the temperature of the earth under the home…   I might take it a bit farther and use the more expensive 6 inch corrugated drain pipe for easier air flow.