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Cold In

Ten standard 4 by 8 foot solar water collectors: 320 square feet of collector surface area; 240 linear feet of frame

Multiple Standard Collectors

Hot Out economy of the larger custom-built design, along with the standardization and savings of a shop or factory environment.

The retail cost of a standard 4 by 8 foot collector (net area about 30 square feet; 2.8 m2) is US$700 to $850, including crating and freight. The maxi-collector cost for this job (equal to ten collectors) was under US$500 per 4 by 8 collector section. Ten individual collectors would have been much more expensive to install. Because of their size, maxi-collectors are very difficult to ship long distances, and probably only have significant cost savings within a half day to a day's drive to the job site from the shop. That's why the custom-built maxi-collectors made good sense for a job like this.

SMC ordered the custom absorber plates from AAA Solar Supply in Albuquerque. The painted plates they make are not quite as efficient as black chrome absorbers, but the cost is much lower. Richard figured that the extra expense of selective surface absorbers couldn't be justified for this lower temperature job. The advantages of selective surface absorbers are covered in HP84.

The size of low iron tempered (LIT) glass used in solar collectors is usually what determines the sizes of collectors available. Tempered glass cannot be cut by any conventional methods, and you are stuck with the standard sizes the glass factory produces unless you want to order a crate (4,000 pounds; 1,800 kg). LIT glass transmits more light than regular window glass, and improves glass performance by 8 to 10 percent.

Popular sizes of low iron glass are 34 by 76 inches, 34 by 96 inches, 46 by 96 inches, and 46 by 120 inches. In nominal terms, these sizes equate to the standard collector sizes of 3

Custom Maxi-Collector System

One 38.3 by 8 foot maxi-collector: 306.4 square feet of collector surface area; 93 linear feet of frame

Expansion Chamber

One 38.3 by 8 foot maxi-collector: 306.4 square feet of collector surface area; 93 linear feet of frame

Expansion Chamber

Pressure & Temperature Ç fj! Gauge

Heat Exchanger:

200 gallon stainless steel tank

Differential Controller:

Independent Energy C-100

To floor sensor

Expansion Tank

Fill Valve

Expansion Tank

To Greenhouse floor

Pressure & Temperature Ç fj! Gauge

Heat Exchanger:

200 gallon stainless steel tank

Differential Controller:

Independent Energy C-100

To floor sensor

Fill Valve

To Greenhouse floor

Backup Heater:

50 gallon propane water heater

Backup Heater:

50 gallon propane water heater solar hydronic greenhouse by 6 feet, 3 by 8 feet, 4 by 8 feet, and 4 by 10 feet (0.9 x 1.8 m, 1.2 x 2.4 m, 1.2 x 3 m). SMC purchased the LIT glass from AAA Solar—ten pieces of 46 by 96 inch glass, the equivalent of ten 4 by 8 collectors. The outer enclosure and absorber plates were built to accommodate the glass size, as are almost all collectors.

Experienced-Based Solar Engineering

The absorber plates and interior plumbing of the collector were designed with experience Richard gained when operating the large Packerland solar heating system. The Packerland system was originally set up with fifteen collectors in each row. Leaks developed from the expansion and contraction of the cumulative length of the copper tubing. The piping was modified to have only ten collectors in each row. This solved the expansion/contraction problem, and is the maximum length of solar collector that SMC will build.

Richard measured the collector temperatures constantly when operating the Packerland system. He noticed that the middle collectors in each row had different temperatures than the outside collectors. The difference of 10 to 15°F (6-8°C) in the middle collectors indicated a difference in the flow of the antifreeze solution through the riser tubes in the collectors. Any system is more efficient with even flow through all the collector absorbers.

The drawings show the traditional way of manifolding ten collectors in a row, and the way SMC constructed the maxi-collector for the Oneida greenhouse. The collector has 2 inch copper inlets and outlets, and the tubing narrows one size every two absorbers to 3/4 inch tubing at the opposite ends. Temperature sensors were built into the collector at numerous locations. The configuration was measured after the installation, and the temperatures were equal throughout, indicating an even, efficient flow through all the absorber plates.

Jill Martus-Ninham, Agricultural Food Production Supervisor, is shown with the 200 gallon storage tank with immersed coil heat exchangers and the controller and pumping equipment mounted on the tank.

Jill Martus-Ninham, Agricultural Food Production Supervisor, is shown with the 200 gallon storage tank with immersed coil heat exchangers and the controller and pumping equipment mounted on the tank.

Close-up view of the tank, with the pump, valves, and controls for the solar heating system.

A single large collector has an added benefit in efficiency over many small ones. The total outside surface area of the sides is decreased, and results in slightly less heat loss. A set of ten traditional 4 by 8 foot (1.2 x 2.4 m) collectors has a total outside area of 240 lineal feet (73 m), while the maxi-collector has a total of 93 lineal feet (28 m). Every foot less is a little less heat lost.

The outer enclosure for a collector of this size needs to be made with heavier duty materials than the standard collector. SMC used 5 inch (13 cm) steel angle iron to form the skeleton of the collector; it had to be strong to be moved solar hydronic greenhouse

A crane lifts the 2,500 pound, 8 by 40 foot long maxi-collector to its mounts in front of the greenhouse for the Oneida Nation in Wisconsin.

to the job site. The rest was standard stuff: a 24 gauge sheet metal back to withstand the weather, 1 inch (25 mm) of high temperature foam insulation on the back and sides, with the absorbers and the LIT glass front facing south.

Putting the Balance of System Together

A 200 gallon (760 l) stainless steel storage tank, pumps, controls, piping, and valves were all available locally to complete the system. The storage tank is unpressurized, and would be considered undersized if this was not a radiant floor heating system.

A 15°F (8°C) rise in the concrete floor of the greenhouse (66,800 pounds; 30,300 kg) has the same thermal storage capability as the 200 gallon (760 l) tank of water (1,600 pounds; 725 kg) with a 90°F (50°C) rise in temperature. Two, 100 foot (30 m) coils of 3/4 inch copper tubing immersed in the tank provide the heat exchanger to transfer the heat from the antifreeze solution to the water in the floor. Immersed coil exchangers are covered in HP92.

A 120 VAC Taco 008 hot water circulating pump was used for the closed antifreeze loop. The pump is controlled with an Independent Energy C-100 differential controller (no longer available, but Goldline makes a suitable replacement). When the controller senses that the collectors are warmer than the greenhouse floor, the pump turns on and heats the tank. The Grundfos 15-42 SF radiant floor pump is controlled by a traditional room thermostat that is set to its lowest setting in the winter months.

The radiant floor system in the greenhouse runs water through the tubing in the slab. The water is heated with a 50 gallon (190 l) propane water heater when the sun doesn't provide enough energy. The tubing in the floor is PEX (cross linked polyethylene) tubing—see HP49 and HP79. The system is a simple, efficient design.

The other valves and system components shown in the diagram are standard for antifreeze solar heating systems, and have been the subject of past articles on closed loop SDHW systems, notably "Closed Loop Antifreeze Solar Domestic Water Heating Systems" in HP85. The expansion chamber serves the same purpose as an expansion tank except that it is larger to account for the maxi-collector, and does not have a rubber bladder like a normal expansion tank. Expansion chambers (bladderless tanks) must always be plumbed on the bottom of the chamber, and need the airspace above the pipe connection to allow room for the fluid expansion.

Installation—Crane or an NFL Team?

A crane or NFL football team is needed to set a maxi-collector in place. This is where prior planning really paid off. The crane was a better fit in the budget. The maxi-collector was hauled to the job site on a 40 foot (12 m) flatbed semi-trailer. The crane picked it up and placed it on the ground mounts, the collector was secured to the mounts, and the inlet and outlet piping was soldered to the collector. The system was filled with a 50/50 solution of propylene glycol and water, the sensors were hooked up, and the system was running in a couple of hours.

The wiring harness for all the interior sensors to determine the collector performance was then connected to a datalogger on site. The datalogger records all the temperatures at the various locations and stores the data for later retrieval. Richard was able to verify even temperatures throughout, which indicated even flow.

George Zachariasen fills the maxi-collector with the propylene glycol antifreeze mixture.

George Zachariasen fills the maxi-collector with the propylene glycol antifreeze mixture.

solar hydronic greenhouse

Custom Maxi-Collector System Costs

Collectors

Collectors

Absorbers; black aluminum, copper waterways

$1,989.00

$6.50

Fabrication labor

1,597.32

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