OK, that's a fancy way of saying "Solar Heat Management".  But, it's relevant and descriptive because managing the tiny amount of heat you gain from spectacularly expensive and complex solar collections systems is... interesting.

Background

We've designed and built out two complete systems, both of which were successful in some ways, and lacking in others.

1187 Goodwin Circle

  • 2m2 of collector.  Evacuated tube, copper heat pipe.  Timer pump control.
  • 120 gallon stratified storage tank, dual coil (bottom: solar, top: boiler).  
  • 6 zones hydronic,  50% Prestone Ethylene Glycol.
  • Standard thermostatic controls.
  • Custom built header and pump control.
  • Dallas Semiconductor 1-wire control, custom built solid-state relay assembly.
  • Dallas Semiconductor 1-wire DS18B20 sensors (~40) on supply, return and tank (stratified).

Highway 779, Parkland County

  • 12m2 of collector.  Evacuated tube, glass heat pipe.  Computer controlled pump.
  • 75,000BTU geothermal heat pump.  Computer controlled (thermostatic backup)
  • 1500 gallon stratified storage tank, single heat exchanger, valve controlled source (bottom, siphon).
  • 13 zones hydronic, 50% Caterpillar EC-1 Ethylene Glycol.
  • Fanger's Comfort Equation hydronic control.
  • Custom built 19 pump heat management system.
  • Dallas Semiconductor 1-wire control, custom built high voltage pump controls.
  • Dallas Semiconductor 1-wire DS18S20 sensors (~80) on all floors, walls, piping, refrigeration
  • 1100m of 1.25" HDPE geothermal field (horizontal). 17% Methanol.

Real-Time Thermodynamic Model & Fanger's Comfort Equation

When designing the thermal footprint of this house, instead of making estimations based on heat loss in a typical residential structure, we decided to attempt to thermally model the entire house, and then run the model in real-time and use its outputs to control the heating system.  Heat transfer through every interior and exterior surface is modelled, and all heat gain/loss is estimated.  From this, we can estimate the radiance of every surface in each space -- and we can compute Fanger's Comfort Equation in real time, and control the heating system using it.

The fundamental problem with most heating systems is that they are based on air temperature; but, less than half of the feeling of your perception of thermal "comfort" is derived from air temperature.  The most relevant factors are the average radiant temperature of the space, and the occupants' metabolic rate and clothing.  The idea of controlling comfort by thermostatic control of air temperature can only work if the basic radiant temperature is already consistent with the occupants level of activity and choice of clothing.

From the equation we can derive a target thermal temperature, and can deduce the target temperature for the surface we can control (the floor of the zone).  Then, using a PID (Proportional Integral Differential) loop, we drive the zone's pump and the available heat sources to keep the floor near the target radiant temperature.

Solar Design

6 x 2m2 panels were placed on rails and elevated by posts about 6' off the ground.  At 52 degrees lattitude, the ideal angle for solar collectors is around a 70 degree angle.  This maximizes winter collection (when you really need it), and reduces some summer collection (when you really don't want it).  A single circuit of 3/4" hard copper tubing was built, and the supply/return lines insulated and buried inside 6" PVC pipe back to the basement.  This yields about 20,000BTU/hr (or about 6kW) at peak solar intensity.  In other words, on a full solar collection day (about 5 hours), this system will collect about the same heat as a smallish furnace would put out in 1 hour.  Now, even though the house was designed with a thermal footprint of about 75,000 BTU/hr at -30C, this is still a significant shortfall, so clearly (even on sunny winter days), other means of heating is required.  

An important feature of this system is the thermal integration of all heat sources and sinks.  A main heat loop is pumped when any source/sink is running -- except for the solar loop and big tank heat exchanger, which is designed to flow when only the Solar loop is running, and thermosiphon heat into the big tank.  So, for summer operation, only a single pump is required to maintain system integrity

Discoveries

Every aspect of this system was designed and hand-built by us, to gain a deep understanding of the specific requirements of residential geothermal, solar and hydronic systems.

Geothermal systems are rated as being around 300% efficient for heating.  However, this is highly dependent on the difference between ground water and target output temperatures and the tuning of the system.  The low quality thermal expansion valves in consumer heat pumps can go out of tune, and virtually no homeowner could be expected to tune the TX valve.  Even if they did learn, the heat pump isn't instrumented with the temperature and pressure transducers required to compute "superheat" and "subcooling", required to properly tune the valve.  Even when properly tuned and working at 300% efficiency, the cost per BTU of electrical heat is typically more than 3 times as expensive as natural gas.  So, the best you can hope for is to heat your system with electricity for around the same cost as gas.