Thermal Energy Storage Heating System for a Tiny House

I designed several Thermal Energy Storage (TES) systems in my previous life. The basic idea is that you generate heat or coolth when it is most thermodynamically convenient, store that energy in tank, and then deploy that energy when needed.

• A TES in heating mode might absorb sunlight during the day, store it in an insulated tank, and then transfer that heat to the building via in-slab radiant at night when it is cold.

• A TES in cooling mode might run water through night-sky radiant panels at night, store the cooled water in a tank, and deploy that coolth into the building during the day. There are many ways to approach energy collection and delivery.

My studio is an eight foot by sixteen foot insulated box with windows on the North and South. It is almost always sunny here and even on cold days the south windows let in enough solar radiation for it to be tolerable by about midday. But during our 14 hour nights and in the early mornings it can be bitter cold — too cold to type, which is what I like to do in the early mornings.

Sunny and frosty.

I decided to put my old skills to work and build my very own miniature TES. My first instinct was to build a solar thermal panel, like I did in Portugal. However:

1. Solar thermal panels require much care to ensure the pipes don't freeze and burst,

2. I’d have to penetrate my studio with pipes, creating a potential for rodent intrusion, and

3. Not ten feet away from my studio is a shade structure built from salvaged photovoltaic panels. Time to stack another function.

I had an extra 14 gallon stainless steel water tank from my Serenity build, as well as some salvaged 6awg wire. I bought an electric resistance heating element, a temperature controller, a relay, and some plumbing odds and ends. It was a couple afternoons of work to assemble the prototype system.

Operation is simple: when the sun shines, the PV panels send energy to the heating element. As long as the tank isn't above max safe temperature, the relay lets the energy through and stores energy in the water in the tank.

Trust me, I’m a professional

At night, the stored energy radiates to my studio. I'm not using any other devices to transfer the heat out of the tank - the tank itself is my radiator. Here is the first few day’s performance data:

The dip in the middle is a day when I left the system off because I was away all day and couldn’t keep an eye on it. Never leave unproven experimental hillbilly electrical-hydronic heating systems unattended. They teach you that in engineering school. That’s rule number two.

So Does it work?

Yes! After charging the tank a few days in a row the studio was noticeably warmer at night and in the mornings. I expanded my window of use considerably.

However, it shouldn’t take several days to charge. My array can nominally put out 480 watts, and I was only getting 170 watts into my tank. So, two things:

1. I get noticeably improved indoor temps even at 1/3 power. Nice. Concept proven.

2. I need to figure out how to get closer to nominal power output. See below.

What about if it’s cloudy for several days in a row?

Then it’ll get cold AF inside my studio. This doesn’t happen often here, but it does happen several times over the winter. I’ll need to either:

1. Build more thermal storage capacity (a bigger water tank),

2. Build a separate system, such as a tiny woodstove, or

3. Just suck it up and deal with it.

Serenity has a woodstove, so I can just conduct strategic retreats to her those few times a year when the studio will be too cold to do anything in.

But I estimate that even my one-third nominal power system has expanded my days of use by a couple of months. Not bad.

I Need More Power

So as I said above, I measured the incoming power as about 150-170 watts. This is about a third of what the panels should be able of generating. What's the deal?

Long story short, I am not an electrical engineer otherwise I would have expected this behavior. Since I am a dumb mechanical engineer, I had to go back to Ohms laws and work it out for myself. Here's what's happening:

• My panels are capable of putting out a maximum of ~18amps. At that amperage they can put out about 30v.

• The heating element has a resistance of 0.64 ohms.

• Because V=IR, the maximum voltage drop across the heating element at 18amps is 10v, which is a wattage of... about 170. Which is what I’m seeing.

• If I want more power, I need to send more amps.

My options to get more power are to reconfigure the panels or use an adjustable DC/DC buck converter. I'll look into that.

Also, the relay isn't doing what I expected it to. No matter what signal my temp controller sends to it, the relay remains closed. I'm not sure if I got a bad unit or if I'm making another dumb mech engineer error. As it is, I keep an eye on the system to make sure I'm not about to boil off my water. At only 170W I haven't gotten above 105F yet.

So I have some more tweaking to do but, for a first pass, I'm encouraged by the operation.

To Do:

• Get the relay working (new unit? figure out what I did wrong?)

• Either reconfigure the panels or get a buck converter to send more amps to the heater so I can get closer to my nominal max power of 480 watts.

• Improve the cap to keep moisture inside the tank, without creating a dangerous pressure vessel.

• Clean up the wiring.