MTD Solar Heating  

Modified Trickle Down solar heating is a revolutionary, low cost solar heating concept that heats water directly rather than transferring heat indirectly through copper absorber plates or copper flow tubes. It’s a trickle down open loop system similar to Harry E. Thomason’s original “Trickle Down Solar Roof” with a few exceptions. Harry used the entire surface area of a corrugated metal roof covered with glass to heat water. An MTD solar roof also uses the trickle down method to harvest heat, but individual MTD collectors are pressed together in an array to form the solar roof. 

·        Instead of a black metal roof a black polyester material is used to distribute a thin sheet of water.

·        Instead of transferring heat through a metallic absorber plate heat is transferred directly to the water through a “Trickle Down Mat" or TDM

·        Instead of using a heavy, fragile, expensive glass glazing an inexpensive,  polycarbonate material is used as an exterior glazing and a thin, inner Mylar film is used to conserve heat.

 How does the inner film conserve heat?
One problem with Thomason’s trickle down roof was the heat loss through the outer glazing material. Although the heat gain of a large solar roof is more than the heat loss due to hot water condensing on the outer glazing surface I felt that an inner film would improve the net heat gain.

 Does this inner film make the collector more expensive?
Well it does increase the cost of the collector a little but it also increases collector efficiency a lot. The TDM is the heart and soul of the MTD concept. It provides a waterproof envelop that traps heat inside a water soaked polyester material. From here the solar heated water is channeled into a gutter and drain at the base of the collector array. MTD collectors may now be built for about $4/ sq ft.

How do MTD collectors compare with standard flat plate collectors?
MTD heat collection efficiencies vary between 40% and 60% depending on temperature differential, flow rate and the method of heat storage. Large, multi tank heat storage systems work best with MTD systems.  

How about mounting?  

Mounting MTD collectors is a beautiful thing. They are designed for flush mounting on a south facing roof where they may be pressed together to form an array. Gutters and Trickle Down Distributors slide into position after mounting. Soldering expensive copper pipes is not necessary. Easy to install heater hoses are used instead.

 What is the ideal orientation and pitch for MTD collectors in an area like Long Island ? I have settled on a year round pitch of 45⁰ but near vertical mounting may prove practical in snow belt areas. The ideal orientation for all  collectors is south if you live north of the equator but this does not mean you should give up if your roof faces south-east. Collector efficiency drops by about 1% for every degree variation off from perpendicular. In other words if you have a roof that faces south or south east or south west go for it. If your roof faces east or west or north sell your house and start over. If you want to get involved with solar applications you need sunlight. 

How about freezing? Is antifreeze required? 
Freezing is not a problem because there are no flow tubes.  Antifreeze is not required or desired with this open loop system. When the pump shuts off water simply drains back into the heat storage tanks.

If no flow tubes are used inside the collector how is heat transferred? I always thought copper or some other metal was needed to transfer the sun’s heat to water.

Copper and aluminum and other metals have a high thermal conductivity, but there are other ways to transfer heat. Take a moment and consider wet clothes drying on a clothes line. On a humid day clothes take longer to dry than they would on a dry day. This is because evaporation is accelerated in a dry environment. Next time you wash clothes in an Arizona Laundromat ask someone why the dryers are free. OK let’s have a contest to see who can dry their clothes faster. You can hang your wet clothes under that tree over there and I’ll hang mine in this bright sunny spot.

That’s not fair.
Why?  

Because sunlight will make your clothes hot and cause them to dry faster.  
My clothes are hot and so am I but the temperature difference between your wet clothes under that tree and my wet clothes in this bright sunlight is insignificant. Why do you think this is so?

Could it be evaporation?
It could! Evaporation has a cooling effect. My clothes will not overheat because the heated water vibrates fast enough to become airborne. What do you think would happen if we placed a clear plastic bag over my black shirt?

I think that would put a damper on the evaporation process.
True, but what about the temperature of my wet shirt? 
Take a reading with this digital thermometer and tell me what you find.

Your shirt is now 140⁰ F.
I told you my shirt is hot.  

So what does this have to do with the Trickle Down Mat? 
Well, my wet black shirt incased in a plastic bag traps solar heat the same as a TDM by preventing evaporation, but the TDM does more than trap heat it also transports heat.

How does it do that?
Water flows through the TDM like water flowing over Harry E Thompson’s corrugated solar roof, but the thin film of water through the TDM is more uniform than the water trickled on top of a corrugated roof and the heat exchange is direct so that more heat may be collected at a lower flow rate. This heated water is then collected in a gutter and drained into a solar storage tank.

Mr. Thompson used one heat storage tank. How many will you use? 
Well that all depends on the number of collectors, the size of the tanks, the heating requirements, the flow rate of the pump, and the location.  Personally I like to see about one 55 gallon recycled, plastic drum for each 2’x12’ collector

Why do you need more than one heat storage tank?
Multiple heat storage tanks are capable of storing more heat than one tank through a process known as heat stratification. Hot and cold water may be separated to some degree by differences in density but the turbulence inside an open loop system has a mixing effect. The following illustration demonstrates the entire MTD solar heating concept. as well as a method of stratifying heat.

MTD collectors are modular. In other words they may be pressed together and connected with trickle down tubes and gutters. Antifreeze is unnecessary and the plumbing is simple and inexpensive. Most of the parts needed for an MTD project may be purchased from a building supply house such as “Home Depot”       

The MTD system takes Harry’s low cost solar heating concept to a new level and places it in the hands of the do-it-yourselfer. A major expense associated with the solar thermal industry has to do with shipping, assembly and installation. These expenses can be eliminated by the invention of "Trickle Down Mat" materials that facilitates MTD construction and also keeps the expense of shipping down.

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