Thermal System Details
The solar thermal system produces thermal power from the heat of the sun. The key
operating principle is the thermal collectors must reach a temperature higher than the
object to be warmed for the system to operate with a positive energy gain. Therefore
the colder the object to be warmed, the less solar energy required to produce an energy
gain. Once the
collectors are hot enough, a pump is turned on to circulate water through the collectors
and transfer their heat to one of two heat exchangers located in the house.
One heat exchanger is used to heat domestic hot water. The other one is used to heat the house through the radiant floor heating system. The temperature in the domestic water heater can vary from 55° F (cold well water entering the tank) to 140° (heated water on a summer day), while the temperature of the floor ranges from 60° F to 75° F.
The solar array is a pair of
panels tilted at 30 degrees and pointing at 162 degrees. The total collector area is
66 sq. ft.
The amount of heat collected is a function of three environmental parameters and one operational parameter. The environmental elements are the amount of solar radiation, the ambiant air temperature and the wind speed. A cold clear day may produce less heat than a warm cloudy day. The operational element is the temperature of the water entering the panels.
The rated thermal performance of the panels is a function of the amount of solar radiation and the difference in temperature between inlet water temperature and the ambient air temperature. These panels are rated at 1040 BTU/sq ft/day under clear skies and a temperature difference of 36° F. So on a 60° day the rating applies to an inlet water temperature of 96° which is a typically seen when heating the hot water heater. Therefore under the conditions given, these two panels producte 68,600 BTU/day or 20 kWh/day.
The two collector panels on the roof are connected to the equipment shown here which
includes the pump to circulate the water, a drain-back tank and a 3-way valve which
routes the water to one of two heat exchangers. The picture below shows the smaller
components in greater detail. A drawing showing how the system is put together is on the
Real-Time Annimated System page.
The existance of the drain-back tank defines this as a drain-back system versus a glycol-based system. In a drain-back system, everytime the pump turns on it must pump the water held in the drain-back tank up into the collector panels. When the pump turns off, gravity drains the water out of the collectors back into the drain-back tank. With this system water can never freeze or overheat when the system is not running. However a small performance penalty is paid because the water must be pumped up hill every time the system operates.
A glycol-based system is full of water at all times. The water is kept from freezing by adding glycol antifreeze. There is no performance penalty with this system, but there is the issue of the glycol breaking down under high temperatures. High temperatures can be reached when you have a household that doesn't use a lot of hot water, so the system doesn't run much, and when you live in a climate with lots of warm to hot clear days.
In our case, we don't use much hot water, and we live in Oregon where the typical day from mid-June through September is clear and warm. It is rare to have clouds on a summer day which leads to the fact that we have more solar radiation than Florida.
There are two heat exchangers. One is located inside the solar hot water heater tank, and the other is a flat plate heat exchanger on the wall (see the picture below) which is used to transfer heat to the radiant floor system. The solar tank is a Ruud Solar Servant RSPE120HE-1 120 gallon tank with a 4500 watt heater. Its internal heat exchanger is a 143 ft. coil of 5/8" copper pipe that wraps around the storage tank.
The basic solar hot water heating system was designed and installed by Solar Assist which is located in Eugene, Oregon. The modifications necessary to connect this system to the radiant floor system was designed and installed by the owner.
Close-up of the collector loop components. The collector pump is a
Grundos UPS 15-58FC.
The flat plate heat exchanger is an AIC LB31-10x with 3.4 sq. ft. of heat transfer area.
The solar thermal system is controlled by a
Steca TR 0603mc U. This controller can monitor up to six temperatures and a flow
meter, and it controls three outputs. In this application, four temperatures are used by
the controller to make decisions. These are the temperature of the collector outflow
pipe, the temperature of the solar tank heat exchanger, the temperature of the flat
plate heat exchanger and the temperature of the concrete slab. These are all shown on the
Real-Time Annimated System page.
The controller's outputs power the collector pump, switches the 3-way valve, and requests the operation of the hydronic loop pump.
The Steca controller is surrounded by the electronics needed to monitor what the controller is doing. The controller has an RS-232 interface but the manufacturer has not published a protocol document and it's not clear the American version has the monitoring protocol that is in the European version.
The red box contains hardware for 8 analog channels with 12 bit analog to digital convertion and two 8 bit parallel ports. The analog channels are used to read temperatures and the digital channels watch the outputs of the Steca controller, a turbine flow rate sensor, the hydronic heat controller and the owner-build controller.
The basic hydronic floor heating system consists of a controller, a water
heater or boiler, a small pump, a pressure tank, valves to determine where the heated water goes,
and the pipes in the floor.
This basic hydronic system was connected to the solar system by adding a couple of check valves and the piping on the right side of the picture which connects to the flat plate heat exchanger. Not shown to the right of the picture is the pump that circulates water through this piping. This pump is identical to the pump shown two pictures below.
A controller designed and built by the owner marries the Steca solar controller to the hydronic controller (pictured below). This custom controller determines which of the two pumps should run the hydronic system and operates the zone valves when heat is coming from the solar-heated flat plate heat exchanger. It also prevents the heaters in the radiant floor tank from turning on except when the radiant tank pump is operating.
Heat is transferred to the floor through PEX pipe which is
looped through the concrete slab.
The house is divided into seven pipe loops which are then grouped together into
three zones. Each zone is controlled by a thermostat.
On top of the upper manifold are the six white valve operators which open the valve its attached to and allows heated water to flow into its loop. The seventh valve is below and to the right of the lower manifold and it controls the flow to a sub-manifold located on the lower level of the house.
The loop valve operators and thermostats are all connected to a Uponor/Wirsbro hydronic heating controller which operates the valves when a thermostat requests heat. Each valve operator contains a switch that is in the closed position when the valve is open. These switches are ORed together so when any switch is closed power is allowed to flow to the hydronic pump.
The radiant floor heating system was designed and installed by Solar Assist.
The hydronic heating tank and the hydronic pump are shown here. The tank contains two 5500 watt heaters
which are wired so they can both operate at the same time. The typical domestic hot water
heater contains two 4500 watt heaters which are wired so that the bottom one can only
operate when the top one is off.
An electric water heater was chosen based on a cost-benefit analysis. We did heat loss calculations for the house and determined that our heating energy usage was low enough that a more efficient heating system, eg, a heat pump, didn't make sense. The payback period was much longer than the life expectancy of the more efficient equipment. We used the money saved here to make the house more energy efficient and to take as much advantage of solar thermal energy as practical.
The tank is plumbed so that the cold water returning from the floor enters the top of the tank to prevent the water in the tank from stratifying. This is done so the thermostats on both tank heaters see a similar temperature and will both turn on when needed.