Modular Solar Workshop




Imagine a morning sun sliding through your bedroom window and gently waking you after a satisfying sleep. A sparrow lands on your wild bird food dispenser and starts singing as the scent of geranium mixed with the aroma of tomato blossoms makes its way from the solar greenhouse below. You don’t have to get up but you want to. Orders for more apple chips are filling your inbox, but you’ll deal with them later. You must have that morning sun heat shower to start your day. The collectors are not yet hot, but there’s still plenty of heat inside the heat storage vault. The mulch digester is working better now since you’ve connected it with the solar workshop vault. Extracting methane from hot wet mulch was not so difficult, but finding a practical way to move the leftovers to the compost bin was a challenge.  As you cook your breakfast of rolled oats flavored with dried apples, strawberries and bananas you wonder….

 “Was it worth the trouble”, you wonder?

AND then you smile…


The concept


A solar heated workshop may not be right for everyone but if your property receives sufficient sunlight this sustainable energy workshop might be right for you. Bigger is better when it comes to solar, but affordability is also important. MTD solar collectors can be manufactured for less than $4/ sq ft. so a 400 sq ft. array can be assembled for $1600 and the 16 plastic drums used for heat storage can often be salvaged from food processing plants at no cost. More conventional drain back systems pump water to the bottom of collectors while trickle down systems like this trickles water down through the tops of the collector array. Sixteen 2’x12’ Modified Trickle Down collectors can harvest more than 300,000 BTU’s per day in most US mainland locations. Hot water is produced as it flows through a black absorbent absorber material within the collectors. The heated water is then collected inside a 2” hand made PVC gutter. From here it flows through 2” PVC drain pipes and into the end drums of the heat storage vault. Multiple drums trap heat by separating hot water from cold water. The same drums distribute heat efficiently by preheating water. Each drum preheats water for its adjacent drum.  

This top view demonstrates the overall solar heating concept. Notice that the heat storage drums are isolated from the shop inside their own insulated storage vault built against the North wall. The proximity of stored heat to the shop facilitates forced hot air or radiant floor heating. Both methods have their advantages and disadvantages.

Choose a location with a good southern exposure that’s higher than the surrounding terrain. You will not be happy if your shop fills with water after the first rainstorm.  If you decide to use some stored workshop heat for your home DHW system plan a trench that will connect ¾” PEX from the heat exchange platform to your home hot water system.  

After settling on a workshop site you should hire an excavator to scrape off loose loom and get down to solid sub soil. After the loom has been removed level the sub soil and fill the excavated area with sand.  The top level of sand should be about level with ground level  

More time should be spent with the footing than the workshop floor because the weight of the shop rests on the 8” x16” footing. If 2x8x16 planks are used for the framing materials they may be reused when it comes time to install the shop ceiling joists. I recommend keeping the top of the footing about 3” above ground level. After the 2x8 planks for the forms are removed line the outside of the footing with 2” strips of solid insulation.

It’s best to pour the footing and the floor separately but you could still pour a 3” deep cement pad for the heat storage vault at the same time the footing is poured.

Placing solid insulation under the cement workshop floor is not entirely necessary. Dry gravel under the floor may be used as an insulator. Cover the gravel with plastic before pouring cement otherwise the insulating properties of the gravel will be lost forever.



Cement blocks are easy to lay once the footing and floor are poured. Mortar the corner cement blocks in place and then stretch a level line between the corners. Just follow the line and the blocks will look great. Be sure to install anchor bolts about every 10 feet and fill the hollow of the blocks with insulation.



Secure a bottom sill plate to the cement blocks with the anchor bolts. Now install the stud walls on top of the bottom sill. Someone could help raise the walls, but you seem to know what you’re doing and I bet you could do it without any help. After all the stud walls are in position lock them in place with overlapping top sill plates. Keep the studs plumb and the sills level. If you do this the work will be easier and your Momma will be proud.





Finish the stud wall framing before assembling the heat storage vault First install the 2x8x16 ceiling joists AND give the building inspector a piece of your famous blueberry pie if he gives you a hard time.









Almost done! Let’s frame the ridge roof with 2x6x12s and take a break for lunch. A ridge roof is less expensive than a truss roof plus it will allow for valuable loft space.












Frame out soffit extensions to leave a hollow area under the eves. This is where the drain from the gutter will pass under the top of the sill plate.












Once the shop is complete the heat storage vault may be assembled, but don’t mount the collectors yet. The heat storage vault should first be completed. Keeping the TDM moist is a good idea, so keep the collectors off the roof until the heat storage facility is ready to go. This will extend collector life expectancy. Remember the “Trickle Down Mat” collects heat and the drums inside the vault store heat. Without heat storage drums all you have is a nice looking shop.





Storage drums are placed on the heat transfer platform last. Build the outside vault stud walls so they may be easily removed. After the vault is finished the drums are connected in series with 2” rubber boots and 2” PVC pipes. You will not be able to daisy chain the drums with a vault wall in the way. OK…Let’s now focus on preparing the insulated vault platform. If a Domestic Hot Water system is needed you’ll need a concrete slab upon which the drums rest to extract heat.  There are many ways to extract heat from the drums of hot water but I believe this method is most practical. A network of copper tubes connected in parallel will transfer more heat than a serpentine PEX tube so let’s use copper for DHW heat extraction. The heat storage vault platform is built on top of the 3” cement pad that we mentioned earlier. Let’s take a closer look at the cement pad and the copper tubes that run through the heat transfer platform.   This platform is framed with 2x6 timbers and rests on cement blocks. The frame is packed with insulation (solid and fiberglass) and capped with a ½” plywood lid. 2x4 sill plates line the perimeter of the platform. Wire mesh is stapled to the plywood before the copper tubes are laid out with a 2” space between them. After the tubes are in position the cement platform is poured inside the 2x4 perimeter. Be sure to protect the copper with polyurethane before pouring cement on it. Cement can corrode copper over a period of time. A thin protective coating will extend the life expectancy of your heat storage vault into the next century. A well constructed level platform works will be worth your efforts. Keep the drums at least 3” away from the inside walls of the vault to allow air circulation. A top view of the heat transfer platform concept is illustrated below.


 DHW HEAT EXCHANGE: This heat exchange platform is only 8 feet long. It’s designed to accommodate three 60 gallon, but of course the workshop heat transfer platform is 40 feet long and it accommodates as many as 16 drums. The input and output manifolds are much wider than the heat transfer tubes so that the flow rates within each flow tube is about the same. An ideal heat exchange platform would have cold tap water enter the cold side of the platform and exit the hot side. Ideal conditions are not always possible so it is sometimes necessary to sacrifice efficiency for practicality.






HEAT STORAGE: A dip tube heat transfer system is illustrated below. Notice how heat stratification is maintained. The coldest water is taken from the bottom of a drum with the help of a dip tube. This cold water is then deposited near the top of the adjacent drum. In this manner solar heated water is trapped within the multi-drum system. The coldest water from the last drum is then returned to the collectors to gather more heat.


These 60 gallon drums have been connected through the tops, but they may also be connected through the sides. I am now recommending side drum connections with a diminished flow velocity to reduce turbulence and increase heat stratification. Water having the same flow rate will have a greater flow velocity through small diameter orifices. To mitigate high velocity turbulence large diameter orifices should be used to transport heated water through the multi-drum storage system. High velocity water causes turbulence within the drums which in turn decreases heat stratification which decreases collector efficiency. In other words large diameter connecting pipes and large orifices connecting the drums increase collector efficiency.




 As I mentioned above we may also connect sealed plastic drums through the sides by drilling 2.7” holes through sides of the drums and by installing rubber boots within these holes. To do this you should use a circular saw, 2.75”, with the outer teeth filed down. Bulk head connectors may also be used to insure a leak free junction, but I have been lucky with simple 2” rubber boots held in place with 2” PVC so I will continue to use this simple method until I have a problem.  Here is the best method of connecting drums that I have found thus far.

The drum units may be pre-assembled in this manner. Two 2.7” holes are drilled through the sides of each drum toward the center of the sides. It is very important to keep the holes at a standard location in each drum. One 2”x 28” PVC pipe may be used in each drum, but before inserting the PVC pipe through the drum a top notch and bottom notch are cut out and the center part of the PVC (black line) is plugged so that hot water entering the drum is directed to the top and cold water leaving the drum is pulled from the bottom. The large notch cut into the PVC slows down the velocity of water. By lowering the forces of turbulence the delicate forces of convection currents are encouraged and the process of separating hot water from cold water is facilitated.  Now all we have to do is finish framing and finish insulating the heat storage chamber before we retire for the day. An even better arrangements of storage tanks might look like this:




A few cross supports should be used to hold the walls together and a 3” slab of solid insulation should be used as an inner lid above the drums. A shed roof on hinges is also a good idea. We don’t want the storage vault to fill with rain water. Four tons will be quite sufficient.

 All we need for the Domestic Hot Water is a connection from out tap water supply to our heated platform solar hot water supply. We can also dry fruit on drying racks placed on top of these hot drums but how shall we heat our workshop in winter?









1.    Push or pull cold shop air through the heat storage vault with a fan.

2.    Regulate the flow of hot convection air currents from the heat storage vault with louvers.

3.    Last but not least…Radiant floor heating

A radiant floor heating system provides an ideal method for regulating shop temperature although there are concerns in cold climates where freezing may be a problem. If you decide to heat your shop floor with solar heated water be sure to insulate the outside of the footing as mentioned before and drop in a mat of solid insulation before pouring cement. Of course the sub floor should still be level and tamped down Don’t forget about the steel flooring mesh and re-rods used to hold the PEX in place while pouring the floor. Also notice the two circulator pumps. Since heat is concentrated in the end drums hot water is circulated from the end drums and returned to the coldest drums located toward the center of the storage vault.

More information about MTD solar heating is available on line, but it’s actually easier to build MTD collectors than explain how they are built. MTD collector starter kits with construction guides are available. They facilitate the hands on experience needed to become an MTD pro. 

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