Modular Heat Storage


Large home made solar heated storage tanks can be made from 2x4's and plywood and they may be waterproofed with an EDPM pond liner. The construction of a tank like this is fairly easy, and this kind of large storage tank may be just right for you but before you run to the Home Depot I have a few more ideas that you may like to consider.


Modular Heat Storage

As you know heat storage tanks work by trapping hot water from collectors and by returning cold water to gather more heat. Heat may only be collected when there is a difference in temperature to allow heat transfer by conduction or convection. Three factors that determine heat transfer rate are: differential temperature, flow rate and surface area. Increasing the flow rate of a pump increases the heat collection rate, but there are points of diminishing return that vary with application and location. Generally speaking a practical flow rate is about .03 gallons per min. per sq. ft.  of collector surface area. l will assume you understand the importance of  differential temperature, surface area, flow rate so we will move on to the concept of modular heat storage. 

What is a modular?

A modular design uses standardized units for easy assembly, repair and  flexible arrangements. Modular panels and modular collectors are flush mounted on roofs. This facilitates the installation process, lowers the cost of installation, utilizes roofs for more than water sheds, prevents attics from overheating and improves the resale value of a house without increasing the property tax. 

This house may not be modular but the interlocking collectors on the South facing roof are. These are the things we see from the outside of the house, but remember an array of solar collectors is just the tip of the iceberg when it comes to heating a house. Without a place to store the intermittent energy from sunlight collectors have little value. Generally speaking about 3 gallons of solar heated water should be stored for every square foot of glazing. Since this array has a surface area over 400 sq. ft. a storage vault containing about 1200 gallons of water could be a practical investment unfortunately a dozen 100 gallon solar hot water tanks could easily cost about $12,000. 
Is their a less expensive heat storage alternative?


 We could build our own 1200 gallon heat storage chamber out of 2x4s and plywood and EDPM and we could allow the natural convection currents to stratify layers of water according to temperature. This method could be used to trap hot water, but remember convection currents are fragile and disturbances caused by water entering or leaving the tank has a mixing effect.

The water temperature inside  undisturbed drums will eventually reach equilibrium. In other words the temperature of water throughout the drums will eventually be the same, but layers of hot and cold water form for awhile.

While layers of hot and cold water are forming we can effectively separate them with a series of modular tanks. All drums are connected in series through the sides. Hot water entering a drum is directed to the top of the adjacent drum and the coldest water from the adjacent drum is directed to the top of the next drum and so on.  This is how modular drums trap hot water and return cold water.


 Plastic drums may also be connected through the top lids , but it's impossible to install DHW heat transfer tubes inside the drums when the lids are on. If drums are connected through the top lids domestic hot water may be extracted from tubes imbedded in a cement platform upon which the drums rest. Unfortunately it would require a large number of drums to make this method of DHW heat extraction practical.


 Heat Stratification Pipes provide another method of connecting drums. To simplify the connection process  I modified a PVC pipe that passes right through rubber boots installed in the drums. The large holes drilled on the tops and bottoms of the PVC pipe were designed to minimize turbulence by minimizing the flow speed of the hot water entering the drum. It was my hope that slow moving water would maximize heat stratification by allowing the natural convection current to carry hot water to the tops of the drums.

In this experiment I used an 8 sq. ft. collector to provide heat for a 5 gallon drum. Three probes were used to measure the three temperatures between 12:20 PM and 1:20 PM on a sunny day.  From this graph you can see that the differential temperature between the water in and the water out is about 10*F and the differential temperature of the water at the top of the collector is about 5* F . This is a small drum that uses a small collector, but it still demonstrates how hot water stratification may be increased with a pipe like this.

My main concern about the heat stratification pipe has to do with the distance between the top of the pipe and the top of the drum. A tall drum means more room for heat stratification to take place, but it also means more mixing while the water travels the distance. More testing would be needed to properly evaluate the heat stratification pipe concept. I do like the idea of connecting drums through the sides, but I also believe pipes pointed up and pipes pointed down inside storage drums provides the best method of isolating hot water from cold.

By enclosing these drums inside an insulated storage vault heat may be taped as needed by simply blowing air across the outer surfaces of the drums. A DHW heat extraction system with PEX tubes inside the drums could also be included as part of the overall system. I am not suggesting that everyone recycle plastic drums to store hot water. At temperatures above 160* F these drums become soft and start to lose their shape. What I am suggesting is that heat loses at temperatures above 160* F are great and excessively high temperatures are not necessary for home heating or DHW requirements.

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