Heat Sink

There is a whole book to be written on the heat sink and I intend to do that. I've got various portions underway but it will take time.

It will probably be 2 years before the ground in and around the heat sink comes up to temperature.

Here Ben and Kent of dbBrad wrestle the stubborn pex into place at the base of the heat sink, just above the bottom layer of insulation-actually 3 layers of 2" for total of r-30.

Needless to say there is a lot of proprietary information and a few huge unknowns--how well does it work or how much energy does it save and is it cost effective.

To better understand the migration of heat, I've buried 18 sensors which will record temperature, time, and date as well as control values such as ambient temperature both inside and out. I will then be able to compare this to solar production values and local weather data to generate an objective analysis as to the performance of the high mass heat storage system.

It is my hope to show that a little smart planning, a bunch of sand and some solar panels can help heat a house, even in challenging solar heating regions like the pacific north west.

The heat sink is not heated directly. The solar system was sized to accommodate heat and DHW (Domestic Hot Water) at spring and fall equinox. Collector tilt is optimized for this period.

Even so, I have a massive heat surplus in the summer and the 200k pounds of thermal mass will be heated with the surplus, only after my primary holding tank is at temperature during the non-heating season. We also have hydronic heaters so when there is solar energy available, the heaters will use this first. This is the 'primary' heat source, per WEC (Washington State Energy Code)

The catch is that my sand can only hold 10-20 million btu's and it's loosing heat all day, every day, all year long. An interesting thing here is that bigger houses have a smaller perimeter to area ratio and can hold more sand which makes them better candidates for high mass heat storage systems.

In the above picture, Ben is over at what we call the Bouquet, a lovely arrangement of tubes, hoses, pipes and wires. Anderson Electrical and Brad are placing the direct burial stainless steel sensors and mapping their exact location. Below are shots showing wing insulation, thermal breaks and the table vaults for the main living.

Heat loss is a function of insulation (resistance to heat transfer), DeltaT or change in temperature, Area of skin and Time. The equation is UA(delta)T . You would use this to calculate heat loss through your envelope in order to size a furnace for the coldest day of the year. Predicting heat loss and gain for a solar house is a bit more complicated. Predicting the movement of heat through a bed of sand, across different mediums over the course of a year is unrealistic. Somehow my dad, Barry Hankins, did it.

Doing transient heat analysis we were able to look at how the earth around the heat sink would heat up and how long it would take for a given constant temperature in the bottom of the heat sink to exceed a maximum value at the slab. (over heating is an issue which I've also solved with a heat dump dump--at Fredley's the outside slab shunts heat which in late summer will make for extended outdoor use)

As a result of studying the transfer of heat (simulating the migration of heat and varying conductive values, temperatures, and time) and looking to optimize retention and increase performance, we installed perimeter insulation (I call it wing insulation) just under the surface and used it also as a water deflector/collector.

In these pictures you can see that the slab has no insulation (should be under the re-bar) but the entry slab, covered slab and perimeter earth do.

In this case, the surplus heat we dump into the bottom will take almost 2 months to get to the slab. Projections suggest we'll be able to maintain the slab over the heat sink at about 65 degrees not dropping below 60 by spring.

But this won't heat the house when it's freezing out. What it does I refer to as thermal ballast. It provides temperature stability. The house may not stay totally warm, but it will never get cold. Ever.

It's working already and the vacuum tubes weren't hooked up this summer and so far there has been no heat added to the heat sink actively, only passively by direct solar gain this summer. At the middle of last winter the slab and concrete underneath were at 44 degrees. At the end of summer the slab was at 65 degrees. It is currently as 60 degrees. I expect going into next winter to have the slab around 75 degrees.