The Solar Water Heater
This page covers the updated Solar Water Heater system. It includes documentation of the revisions placed in service in the summer of 1998.
SOLAR WATER HEATER - The Solar Water Heater is a liquid transfer preheater used in conjunction with an electric water tank to provide domestic hot water to my residence. The system supplies water at a temperature of greater than 145 degrees F. The basic operation involves the use of a solar collector, heat exchanger, pumps, and a storage tank to preheat the cold water coming into the house prior to sending it to the electric water heater. In an ideal situation, the temperature of the water leaving the storage tank will exceed the thermostat setting of the electric tank. This results in the electric tank functioning only as additional storage capacity. In reality, these conditions occur at times, but some electric assist may also be provided. This serves to reduce electric consumption even though the solar may not be capable of supplying the entire hot water demands at all times.
COLLECTORS - The collectors are flatplate, liquid type consisting of 78 square feet of surface area. They are covered by double pane glass panels to minimize heat loss and increase the collector operating temperature. These are salvaged patio door panels which were removed due to minor fogging. The fogging is not a significant problem in this application and, since they were to be discarded by the glass companies anyway, could be obtained for the right price (free!).
The absorber plates are comprised
of 3/8" copper tubing between an
upper and a lower header pipe.
Flow restrictors are placed in each
tube to provide even distribution of
fluid through all tubes.
(Note the exploded view of a tube
assembly in front of the panel in
Aluminum sheets, obtained from the
local newspaper printer who used them in the printing process, are
pressed around the copper tubes.
The sun heats the aluminum, and the heat is conducted into the tubes, where it heats the transfer medium. The heat transfer medium is propylene glycol and water in a 50 - 50 mix. The collector box itself is made of wood, insulated with 1 inch aluminum faced panels which are pressed into it. This keeps the high temperatures away from the wooden frame as well as provides the necessary insulation to minimize heat loss. The outside of the box is covered with aluminum to protect it against the weather. Figures 2 and 3, below, show the collector box and internal construction details.
The Collector Box
Figure 2 shows the basic collector box
installation of the absorber plates.
The insulation panel has been installed on
the right side.
Shown here is a photo of the absorber plates
as they are placed in the collector box. Those
on the right have been painted, while the left
side shows the unfinished aluminum sheets
as they come from the newspaper printer.
This configuration heats the glycol to as much as 165 degrees F., or about 20 - 25 degrees hotter than the earlier system which used a slightly larger collector but had only a single sheet of fiber glass reinforced plastic as a cover. The earlier system was replaced due to degradation of the FRP sheets from UV radiation. They did, however, meet their life expectancy of 15 years. Long term evaluation of the new collectors has begun, but the results are not yet available. There has been no sign of problems to date.
Inside the house, a heat exchanger transfers the heat from the glycol to the hot water system. The photo below shows how the heat exchanger is built. A half inch copper tube is placed inside a three quarter inch tube and reducing tees are placed at each end. The tees allow the two fluids to flow on each side of the half inch tube and the heat is conducted through the wall of the tube. Note the detail of the fittings on the left in the photo below.
A description of the entire system follows. Refer to the system diagram below to locate the major components as the description is given.
Pump P-1 circulates the glycol from the collector to the heat exchanger and back. An expansion tank is included in this loop to prevent pressure build up. The tank also includes a sight glass and filler in order to check and replace the glycol as needed. An air purge provision is made in the pipe near the top of the collector. This will remove any air which accumulates in the glycol side of the water heater. The heat exchanger transfers the heat from the glycol side to the water side. Pump P-2 circulates the water from the storage tank through the exchanger where it picks up the heat and back to the tank. A system drain is provided to allow the glycol to be easily replaced without the need to disconnect any pipes.
As originally designed control is accomplished by a differential thermostat which compares storage tank temperature with the collector temperature. It uses the results of this comparison to determine the optimum pump speed; high, low, or off. The command to pump P-2 is delayed 10 seconds after pump P-1 to allow for the heat transfer lag through the heat exchanger. Provisions are also included to monitor the status of the electric tank. (on or off)
Originally, the Solar Water heater system operated completely independent of the space heat system discussed elsewhere. This system obtains 70 - 80 per cent of its output from its solar collectors. Since this system first went on-line in 1982, the solar storage tank has been replaced; a change was made from a 40 gallon to the 80 gallon tank, and the collectors have been upgraded in 1998. The collector upgrade was done, as stated earlier, due to degradation in the original FRP glazing and to increase the system performance.
Today, control of the Water Heater is accomplished by the same system which operates the primary heating system. The water heater status is displayed on the thermostat / display panel. This integrated control has simplified operation of both systems by eliminating the need for a second solar control unit. The Digital Thermostat performs the same functions as the earlier Differential Thermostat which was an analog based system. All operational displays related to all solar functions are now viewed on a central panel.
The upper left block on the thermostat displays the current status of all solar hot water operations. Collector and water tank storage temperatures can be viewed on the Time/Temperature display on the top right.
The completed Solar Water Heater Collector Unit.