Solar Heating - Low-Tech Active System

This is an interesting COMBINATION of concepts regarding solar energy. It seems to have many advantages over existing attempts at collecting and using solar energy. The performance is FAR greater than any previous solar heating system (except our NorthWarm Version 1 system) while also having occupant comfort that is indistinguishable from conventional (fossil-fuel) furnace heating.

(Glucose is the basic material that all plants create, and which later can become cordwood for a fireplace or woodstove, or even coal or petroleum or natural gas, if given many years underground!)

Photovoltaic (PV) cells convert solar energy directly to electricity, but the best efficiency in practical products is only around 7%, and the products are rather expensive. Higher performance PV cells exist (up to about double that efficiency) but they are even more expensive.

(This 7% figure is for the most economical technology of solar cells, which is based on Cadmium Sulfide. There ARE higher efficient technologies which exist, such as those based on Gallium Arsinide, but they are far more expensive and not within the price range of most people. There are even more expensive technologies that are based on silicon semiconductor technologies, which require a [metal] silicon ingot to be sliced so thin that sunlight can pass through it, which is extremely expensive to do! So higher efficiencies exist in solar cells, which are reported in media stories, but they are currently far too expensive for broad use. This all results in MOST commonly available solar cells being Cadmium Sulfide, and therefore around 7% efficient.) Solar Cells Photovoltaic Cells, PV, Electricity from Sunlight

There have been efforts at using Stirling engine technology to convert solar energy into mechanical energy, but again, the overall efficiency is only a few percent. There are some ways to build more efficient hot air engines, like the Stirling, but they have been immensely expensive to build. Even extremely expensive ones have only been able to produce 6 or 30 horsepower of energy flow, not nearly enough considering the high cost of them.

This Concept

The tank just described has a top surface area of 43,560 square feet. If it is sunny, each of those square feet can receive over 300 Btu/hr of heat from sunlight. Depending on the local climate, maybe half the time the sky is overcast, and so the tank could receive around 65 million Btus of solar heat energy per day. Even in a cold climate, most houses only lose around 40,000 Btu/hr when the outdoor temperature is 0°F, so on the coldest day of the year, that is around ONE million Btus of building heat loss (24 hours at 40,000 Btu/hr).

So we have a system which CAN receive around 65 million Btus per day, while the building heat loss is only around 1 million Btus per day or less! This is "over-design" for a number of reasons. First, in the event that there had been ten consecutive days which were totally overcast, where the Sub-Basement storage chamber had been depleted, this great heat collection capability can replenish the Sub-Basement heat storage in a single sunny day, or even less! Second, with such over-design, the need for thermal insulation under and around the heat collection chamber is less important, and ditto regarding putting glass above it. By NOT insulating the chamber at all, it WILL lose a LOT of that 65 million Btus of daily heat! But it WILL still have millions of Btus left which get carried to the Sub-Basement heat storage chamber. Over-kill on size can mean that great cost savings can be had regarding insulation and glass.

For some applications where mechanical power is desired to operate machinery or to generate electricity, there can be an identical-sized tank on the adjacent acre, but this one has NO insulation underneath it and has R-10 insulation on top of it and around its sides.

We will describe these higher performance versions of this system shortly, including a large air pipe connecting the two tanks. But first, we will describe the simplest version of this system, one exclusively meant to provide space heating for a building and for its hot water.

In fact, we will simplify some of the description above to eliminate all of the insulation and the glass sections and even the rigid structure of the tank! So we have devolved this down to a simple INFLATABLE CHAMBER, somewhat resembling a hot-air-balloon, but where it is entirely painted a black color that has very high emissivity.

Just like the way the interior of a closed black car in a parking lot on a hot summer day can get to above 140°F in an hour or two, this inflatable chamber will do the same. Where the bottom of the hot-air-balloon would be, there are TWO 24" diameter (metal or PVC) air tubes are connected. Both of these SHOULD be well insulated. One of them would have a large fan or blower inside to cause air to be blown into the chamber and then through the tube. The other tube is simply a return tube for air to recirculate, which also keeps the inflatable chamber filled with air.

The 120°F or 140°F (or hotter) air that is inside the inflatable chamber is therefore sent through the (insulated) tube into a heat storage chamber. We feel that the most logical version of such a (low-grade) heat storage is what we call the Heat and Cool a House (Hidden) Sub-basement heat storage chamber The Sub-Basement heat storage is entirely underground, and therefore, completely invisible, but it is quite large. We like the idea of the Sub-Basement storage for a normal house being large enough to be able to easily store at least 25,000,000 Btus or 30,000,000 Btus of heat, in storage medium which is never above around 120°F or a maximum of 140°F.

Since a medium-sized house in a climate like Chicago's is likely to have an entire winter heat loss of around 40,000,000 Btus we have just described a storage which can fairly easily hold enough heat for an entire half winter! A larger Sub-Basement storage could even be built, to ensure containing an ENTIRE winter's heating needs for such a house, which would then not even require any other winter heating source at all, simply using heat that had been captured and collected from the previous Summer's weather! The basic design described here is not that extensive, so it is much smaller and less expensive, but it then requires a PARTIAL source of heat during the winter. One or another version of this solar heating collection system is felt as being the best and cheapest way to go.

Conventional fossil-fuel-fueled furnaces create warm air for a house which is usually in the 110°F to 125°F temperature range, which gives excellent comfort for the occupants. This system has no trouble providing warm air of those temperatures to the house as desired.

Consider a relatively large hot-air-balloon-like bag that can be trapped to being a rather flattened pancake shape, a few feet tall and 100 feet in diameter. That area of surface can receive a lot of solar energy! Around noon on a sunny day, even in winter, at moderate Latitude, around 300 Btus of solar energy arrives at each square foot of area. We have just described around 31,000 square feet of surface, so that would be a maximum of around 9,000,000 Btus of solar energy hitting the chamber in that hour! The emissivity of the black coating is not perfect, and since we have described intentionally leaving out most insulation and glass, and due to a number of other energy losses, it is realistic to expect that a maximum of around 3,000,000 Btus per hour could actually be transferred into the Sub-Basement storage. NOTE: ONE decently sunny winter day near Chicago, could therefore transfer around three hours worth of the above performance to the Sub-Basement storage, that is, around 10,000,000 Btus of heat into storage. That single day's heat collection then would provide AN ENTIRE MONTH of heating for the house!

We recognize that we have just described what many people might call overkill, but we are very conservative and insist that any such system have enough collection capability and enough heat storage capacity to be able to deal with an ENTIRE MONTH OF HEAVY CLOUDS while also enduring temperatures of below 0°F! Anything less, and we consider it unacceptable!

There are two problems in the situation we just described. One is that anything resembling a hot-air-balloon is known to be extremely unstable in anything over around a 3 mile per hour wind, and so when a severe storm would occur with 40 mph wind gusts, it all might blow away! Because of this issue, the entire chamber would have to be strapped down, much like mobile homes are for the very same reason. In this case, we also see value in FLATTENING the air chamber down as low as possible to reduce the area that might be subject to such strong winds. Such a chamber is also then not nearly as big and ugly to neighbors!

The second problem is somewhat related. We have been describing an inflatable chamber which is not likely to be very durable. Either a lot of repairs would have to be expected or the heat collection chamber could be the metal structure we first mentioned above, or a wide variety of other approaches which are more durable could be used.

The concept just described and discussed would be exclusively for providing heating of a home or other building and its hot water. We have intentionally economized by leaving out a lot of insulation and other structures, simply because the basic system is so very able to collect (and then store) far more heat than any normal building could ever need.

We will now add some references to some more sophisticated versions of this same concept. The general theme here is that we now will look toward creating mechanical power from the solar, to either directly power machinery or of producing electricity for more broad later usage.

So now we need to go back to the acre-sized rigid metal tank structure with the insulation, along with a second identical tank structure with different insulation characteristics. No Sub-Basement heat storage is necessary or involved from here on.

Here is what is to happen. During a night, the second tank loses any heat of contained air, after any pressure had already been released, into the ground underneath it (because of no insulation) and that tank's internal air drops to nearly the deep soil temperature by the morning. Say that is around 50°F. The first tank is therefore warmer to start with. As sunlight begins to fall on the first tank, the low-emissivity black paint absorbs a large fraction (around 90%) of the incoming solar energy, but that energy was slightly reduced by losses through the layers of glass. Depending on a number of selectable variables, we can expect that around 80% of the incoming solar energy gets absorbed by the metal top of the first tank. (The second tank is covered by good insulation so it does not significantly heat up.)

In the same way that a closed car's interior can heat up to over 140°F on a sunny summer day, the same is true of this first tank. We can do some calculations. Each square foot of area receives around 300 Btu/hour of solar energy on a sunny day, but that amount drops off in the morning and afternoon when the sun is lower in the sky. It is realistic and conservative to assume that a total of 1,500 Btu of heat will be received by each square foot during a full sunny day. Our tank has a one-acre top, or 43,560 square feet, so we have around 65 million Btus of solar energy hitting the top of our structure. Due to our 80% efficiency, we have around 52 million Btus of heat actually absorbed by the tank's metal top. Therefore, the air inside that tank would be able to gain that 52 million Btus.

This is a significant amount of energy, enough to entirely heat a medium sized house for an entire Chicago winter! One obvious and simple application of this is to simply pipe some of that heated air, by a blower, to an adjacent house or building to entirely heat it easily!

Our tank contains 218,000 cubic feet of air in it (at standard atmospheric pressure), which weighs around 17,000 pounds. We know that the thermal capacity of air is around 0.24 Btu/pound/°F. Therefore, 4,020 Btus of heat will raise the average temperature of the air inside that first tank by one degree Fahrenheit. Therefore, just 400,000 Btus of added heat will raise the air inside that tank by 100°F, up to 170°F if the ambient temperature is 70°F.

This indicates that in just the first few minutes of the morning, after we have collected only 0.4 million Btus of the 52 million we can expect to collect during the whole day, we already have a huge supply of 170°F heated air and we also have our 50°F other tank.

The Ideal Gas Law PV = nRT tells us that for a constant Volume tank, the air pressure inside the tank is directly proportional to the (absolute) temperature. We have raised the air temperature from 70°F (or 530°R) to 170°F (or 630°R), in other words, by a factor of 630/530 or 1.19. Therefore, the normal atmospheric pressure of 14.7 PSI rises to 17.5 PSI.

Noting that we only needed 400,000 Btus for this, which is absorbed in as little as about two minutes, we therefore have a continuous supply of large amounts of slightly pressurized air (3 PSIG, but 218,000 cubic feet each two minutes or so). If we send this slightly pressurized air into an exit pipe, there are a variety of things we could do with it.

Please note that the 52 million Btus of solar energy actually absorbed by our tank is equal to 15.2 million watt-hours or 15,200 kilowatt-hours of energy. The maximum rate of this energy absorption is around 3,000 kilowatt-hours per hour, or 3,000 kilowatts or 3 megawatts. This again is describing an attractive amount of energy!

Now, this energy exists as rather low pressure air at relatively low temperature. That is not directly very compatible with modern industrial uses! However, there are many existing technologies to improve these characteristics, although each introduces efficiency losses. Our point here is that we are discussing really huge amounts of energy, available with relatively simple tank construction and relatively inexpensively.

For example, we could consider installing double-ended pneumatic cylinders where the end exposed to the air of our tank has large area (20 square feet) while the opposite end has a cylinder area of one square foot. Due to the way fluid pressures act, the cylinder would be able to compress air at 20 times our 3 PSIG. Given our extremely large supplies of 3 PSIG air, this could provide useful quantities of 60 PSIG compressed air. Higher pressures are easily obtained by selecting the dimensions necessary.

We could also make a Pelton Turbine-styled device where the 3 PSIG pressure would cause the device to rotate (with the back side at ambient air pressure). We could produce high-horsepower mechanical motion in this way, on the order of 4,000 horsepower, although at very slow rotational speed, where significant gear-trains would be needed for useful speeds of equipment.

A variety of types of Stirling engines might also be effective.

One obvious use of this could be to drive such turbines and gear them up to drive alternators to generate electricity. If we look at all the efficiency factors along the way, this can produce around five times as much electricity from a given amount of incoming solar than the best of the very expensive photovoltaic cells can now produce. Far cheaper, far simpler, and all the components can be home built! And this approach does not have all the failure factors that PVs have, because not much can ever go wrong!

Enhancements

We have described a tank that really did not need to be very strong, as it never had to deal with greater than around 3 PSI difference between the inside air and the ambient air outside. But say we made the tank extremely sturdy and strong, particularly by installing many vertical struts between the floor and top of that tank. We could then use the two tanks together. We might put a check-valve in the pipe between the two tanks, which only allowed the pressurized air from the first tank to flow into the second tank. This would slightly pressurize the second tank. That night, as that second tank cooled, its pressure would reduce, allowing even more air from the first tank to enter it. Once this happened, different air valves could be opened to allow cool night air to enter the first tank. As that new air was heated during the following day, additional air could be compressed into the second tank. Gradually, over a number of days, the pressure inside the second tank could be increased. At some point this process of connecting to night air would end, with the result being that both tanks were then at higher pressure than before. (This could also be achieved by conventional air compressors.)

For discussion, we might say that the pressure inside the first tank is now triple ambient, or 44 PSIA or 29 PSIG, still a relatively low pressure. However, now when the sun heats that tank, and causes the 1.19 increase in temperature, the proportionate pressure increase is now over 8 PSI instead of the 3 PSI that we have been working with before. Not much else is changed, but we now have a greater differential air pressure available to drive turbines or pneumatic cylinders or other devices.

The tank or tanks could be made out of quite a variety of materials. Say the first tank was made where its top was simple polished aluminum, which is fairly low priced. Polished aluminum has relatively low thermal absorptance of solar energy (around 30%) but has even far lower radiation in the far infrared (around 5%) and so such a tank could probably be made without even having insulation!

There are thousands of existing technologies which can be applied to this basic concept to achieve production of mechanical energy or electricity or compressed air or even hydraulic pressure.

Smaller Experimental Size

This at least is a more realistic approach if that is your goal. Since you can build this tank yourself, and rig up a standard car alternator and associated items, you would not be spending all that money on PV cells. Better, it is likely to generate around five times as much electricity as PVs for the same area, and since it is so simple and cheap to go far bigger, producing a useful amount of electricity is actually credible!

Say you build a chamber/tank that is 20 feet square, and essentially as described above. Such a tank of 400 square feet has the maximum ability of absorbing around 600,000 Btu/day or 180 kWh/day or 28 kilowatts-equivalent of heat energy flow. If you can even make 10% use of that energy, that would result in around 2,800 watts of power. If you bought that much in PV cells, just the 400 square feet of PV cells would cost you around $4,000 plus a lot of required additional equipment. This tank would cost you far less to build! The PV setup would have a maximum output ability of around 2,800 watts, at noon on a perfectly sunny day, and more commonly 1,000 watts or less. We are talking about potentially having ten times that much available (the 28 kilowatt rate), but where realistic losses in mechanisms and alternators would drop that down to around half that, 14 kilowatt absolute maximum, but on days of average weather, more likely 5,000 watts. For much less money investment, as much as five times as much electricity. Make sense?

Electricity cannot be easily stored, except in small quantities in batteries, which is a problem. However this tank concept can be combined with a large insulated water tank (possibly even inside the first tank) where large amounts of water could be heated to the 170°F that we expect the air in the tank to get to. Inside this smaller (20-ft-square) tank, an 18-foot round, four-foot-high, above ground swimming pool could be put. Such a pool holds around 7500 gallons of water, or 64,000 pounds of water. At 170°F, each pound of water has 100 Btus of heat energy above the ambient of 70°F. This means that we might be able to provide a simple storage of 6.4 million Btus or 19,000 kWh of heat energy. This might represent a simple, economical and practical way of storing the energy still as heat, to be converted to electricity as the demand occurs. This amount of heat storage might even be sufficient to still be providing electricity after several days of overcast weather.

The point of much of this presentation is that a thousand alternatives exist regarding what is described above. It is intended as a motivation for people who like to experiment and try new ideas. The different parts are all pretty simple and easy to build, and if you do something wrong, it should be pretty easy to fix almost anything. Very little you could do could cause any real problems!

Larger Industrial Size

Unfortunately, there were a number of political complications in India which arose in the Summer of 2008, and those plans got shelved, at least for now. Apparently, there are some leaders there who expect to soon use these methods to entirely heat and cool, either that Airport or some other Airport in India, but apparently such things must wait until politicians resolve differences about other matters first!

The only reason that is mentioned here is that the basic concepts of the Sub-Basement heat storage and the Low-Tech Solar Heat Capture and Collection, are easily scalable for very large applications. It seems likely that some large commercial or industrial building, somewhere in the world, will install this pair of systems, in order (1) to become GREEN; (2) to save really massive amounts of heating and/or cooling utility bills; and (3) to attract media attention for being an socially-responsible company (or country).

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago