Notes from Windward: #69

Heart of a Heat Battery

taking another step towards steam production


     A while back we were delighted to acquire 110 gallons of Heat Transfer Fluid, the "life blood" of a solar steam system. I'm pleased to report the acquisition of the next component in that system--an oil storage tank large enough to handle a couple hundred gallons of HTF and still have room for expansion since hot oil takes up more room than cold oil.

bring home a round oil tank

     Last month we'd acquired a 250 gallon round steel tank could be put into service as a holding tank for the hot HTF. Since the tank will need to be insulated, its round shape would minimize surface area thereby minimizing the rate it would lose heat to its surroundings. The following is a discussion of why we decided to not go that route.

     One important reason for using a heat transfer fluid is to create as safe a system as possible--moving hot oil at room pressure is inherently safer than moving the same amount of heat around in the form of pressurized steam. A horizontal parabolic reflector can reach temps of 500°F, but the system will operate more efficiently around 350°F. In order to store more heat, we'd have to heat the oil to higher temperatures, something which creates a host of problems, and even then the amount of stored heat wouldn't add up to much additional run time for the steam engine.

     So instead of using a round tank and twice as much of the expensive HTF, we decided to go with a sodium sulfate system built around a 275-gallon home heating-oil tank. Scott was able to track one down in Portland that had been located indoors and was in excellent condition--very important when working with hot oil. A quick run into Portland with the work truck, and we added another key component to the project.

the "race-track" shape increases surface area

     This type of tank is shaped like a race track, a design which we'll use to our advantage when we build our thermal battery since the more surface area, the better the battery will work. The tank will eventually become the heart of an insulated vault within which the area between the tank and the external insulation will be filled with bricks cast out of sodium sulfate. We could use sand or gravel to increase thermal bulk, but the thermal capacity of any type of fill doesn't add up to much additional heat storage. What's special about sodium sulfate that's been doped with 3.9% calcium sulfate is that at around 375°F, it undergoes a transformation from one crystalline shape to another, a transformation which absorbs a large amount of energy. The heat stored in such a transformation is referred to as latent heat, and you get it back when the transformation is reversed.

     How much heat are we talking about, you might well ask? Well, the amount of energy needed to raise something's temperature by one degree Fahrenheit determines its specific heat. The specific heat of water is defined as 1.0, and it's the standard against which all other materials are compared. For example, a block of granite, although heavier than an equal volume of water, has a specific heat of only about 0.6. Even more of a problem is the relationship between surface area and volume. As the size of a cube increases the surface area of a cube increased much more slowly than its ovlume. Therefore a solid cube adsorbs and releases heat more slowly than an equal weight of small rocks. In order to get an acceptable rate of heat transfer, one would be better off using small round rocks because doing so would (1) increase the amount of surface area, and (2) provide channels through which air could move. Incorporating channels that air can pass through increases the rate of heat transfer, but it decreases the total amount of heat that can be stored. Because of the interior voids, the effective specific heat of a volume of roundish rocks falls to something closer to 0.2.

     Our target temperature for generating steam at the pressure needed to drive our steam engine at full power is 350°F. So, in order to store the heat needed to keep operating after the sun goes down, or for that matter when a cloud blocks the sun, we would want to heat our storage medium to some higher temp. One problem there is that the higher the temperature, the greater the amount of heat we lose to the surrounding area--the better insulated our heat battery is the less heat we'll lose, but there'll always be some heat lost.

our "good as new" oil tank

     So, instead of using chunks of granite, we'll use bricks we'll cast from sodium sulfate. In preparing the mix, we'll dope the sodium sulfate with 3.9% calcium sulfate. This will allow us to set the temperature at which sodium sulfate transforms itself from a rhombic crystal into a hexagonal crystal, a transformation that involves the adsorption of 128 btu/lb. To put that another way, a sodium sulfate thermal battery would store 0.2 btu's per degree of temperature rise right up to the point where it started to transform itself into the hexagonal crystal. It would then maintain that temperature, soaking up comparatively huge amounts of thermal energy until all the sodium sulfate had been transformed. From that point on, its ability to store heat would revert back to the lower rate.

     In addition to doping the sodium sulfate with calcium sulfate to set the transition point at near to 350°F, we'll also mix in short lengths of scrap aluminum and copper wire which will act as internal heat conductors thereby speeding up the movement of heat into and then back out of the heat battery. One alternative is to create spaces between the bricks so that heat can spread through the battery by convection, but that would lower the density of the heat battery which would in turn lower the amount of heat stored.

     The concept of creating sodium sulfate heat batteries is more fully described in Patent 2,808,494

Notes From Windward - Index - Vol. 69