Control system for solar heaters

A solar-heating system incorporating a photosensitive sensor locatable remotely from the collector panels but similarly oriented with respect to the solar-energy source, such photosensitive sensor being biased by one or more biasing resistors of predetermined magnitudes and temperature coefficients so that the combination, in its performance with respect to ambient temperature and (optionally) with respect to ambient wind velocity, matches the thermal characteristics of the solar energy collection system so that the solar-energy collection fluid is pumped only when it can contribute heat energy to the already stored solar-heated fluid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to solar heating systems and, more particularly, to 
improved control systems for such heating systems. 
2. Prior Art 
In the past, particularly in connection with solar hot water heating 
systems, it has been the practice to switch on and off, in response to 
signals from temperature-differential thermostats, the pump which 
circulates the solar-heated fluid. Circulation is stopped when the 
temperature of the circulating fluid falls below the temperature of the 
already stored, heated water or other fluid. Such a system is subject to 
"hunting" when the temperature of the stored water and that of the 
circulating water approach each other. 
Such "hunting" or random on-off cycling of the circulation pump is caused 
by such phenomena as water temperature stratification or by improper 
sensor location. The results are wasted energy, both electrical and 
thermal. Unnecessary pump, switch and relay wear also occur. 
Therefore, it is an object of my invention to overcome the general 
disadvantages of prior art control systems in solar heating systems. 
It is a further object of my invention to provide a control system which is 
matched in its thermal characteristics to the thermal characteristics of 
the solar energy collection system it is controlling. 
It is an additional object of my invention to provide a control system for 
solar heating systems in which automatic compensation is made for ambient 
temperature variations. 
SUMMARY OF THE INVENTION 
A photosensitive sensor is utilized in combination with a biasing resistor 
of predetermined magnitude and temperature coefficient to match, in its 
performance with respect to ambient temperature and other secondary 
variables, the thermal characteristics of the solar energy collection 
system so that the solar energy collection fluid is only pumped when it 
can contribute heat energy to the already stored solar-heated fluid. Since 
this is an openloop system the problems of "hunting" and undesired cycling 
encountered with differential thermostatic systems are avoided. Further, 
the complexity of the control system is less and its reliability greater 
then those factors found in conventional control systems.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, curve 8 represents the normal diurnal variation in insolation 
for a clear day. For a given solar energy collection system operating in a 
first ambient temperature the solar-fluid pump switch-on level is 
represented by the line "A" which intercepts the "Y" axis at the first 
insolation level "A". Between levels "O" and "A" are the inherent losses 
of the collection system, e.g., glazing transmission losses, re-radiation 
losses, conductive losses and the like which prevent the collection system 
from making a positive contribution to stored thermal energy in the 
system. In the event of a reduction in ambient temperature the insolation 
level must rise to the level "B" before the collection system can make a 
positive contribution to stored thermal energy. Thus, the pumping or fan 
means is kept "off" until the insolation level "B" is reached. Thus the 
turn-on level is automatically varied from level "A" to level "B" as an 
inverse function of ambient temperature. 
The circuit to achieve this desired goal is shown in FIG. 2. In FIG. 2, 
alternating current at, say 110 volts and 60 H.sub.z is applied to 
terminals 10, 12 of primary 14 in transformer 16. The reduced voltage 
which appears across secondary 18 of transformer 16 is rectified by diode 
20 and with the help of filter capacitor 22, a d.c. voltage appears across 
terminals 24, 26. That d.c. voltage is applied to terminals 28, 30 of 
sensor module 32 by way of solenoid 34 in normally-open relay 36. 
Phototransistor 38 in sensor module 32 has its emitter 40 connected to one 
d.c. source terminal 26 through sensor module terminal 30 and its 
collector 42 connected to the other terminal 28 of module 32 and thence, 
thru solenoid 34 to the other d.c. source terminal 24. A pair of biasing 
resistors 44 and 46 is connected between emitter 40 and base 48 of 
phototransistor 38. Phototransistor 38 may be a Fairchild FPT 120, or its 
equivalent, for example. Resistor 44 is of the silicon variety having a 
predetermined and positive temperature coefficient of 0.7% per degree 
centigrade. Such a device is available from Texas Instruments Corporation 
and is sold, by them, under the trademark, "Sensistor". The available 
resistance range of such devices is, typically, from 4,000 to 20,000 ohms. 
The resistance versus temperature curve is somewhat tailored by the series 
connection of relatively temperature-insensitive resistor 46. The 
tailoring and choice of resistors 44 and 46 is made so that the overall 
temperature response of sensor module 32 corresponds to that of the 
companion solar energy collection system, both as to initial operating or 
"turn-on" point ("Y" -intercept level) and as to slope of temperature 
response curve after pump or fan operation commences. When phototransistor 
38 turns on current flows thru solenoid 34 closing contacts 52, 54 of 
relay module 36 and applying operating a.c. voltage across terminals 56, 
58 for application to a heat-transfer pump or fan, not shown. With the 
resistor 44 having a resistance-temperature characteristic of +0.7% per 
degree centigrade and the proper choice of fixed resistor 46, the desired 
"Y"-intercept and slope of any collector from an evacuated tube to a 
single-glazed flate-plate collector can be achieved. 
Wind-cooling losses can be compensated for in this system by intentionally 
dissipating in sensor module 32 a fixed amount of power, e.g. one-fourth 
watt, as by means of auxiliary resistor 50. As the wind velocity in the 
collecting and sensing area increases, the heat generated by resistor 50 
will be proportionately removed and the control of my invention will 
compensate for wind losses in a way analogous to that in which it 
compensates for changes in the ambient temperature. In actual 
installations this feature has been found useful in connection with 
unglazed collection panels. 
The sensor module 32 of my invention is easy to install. Phototransistor 38 
need only "see" the same sun as its associated collecting panels. It can 
be remotely located with no change in system performance. Module 32 need 
not be in thermal contact with the collector panels or the thermal storage 
means. This relationship is made more clear in the drawing of FIG. 3. In 
FIG. 3, solar-energy collector panel 70 receives fluid to be heated 
through pipe or hose 72 which is coupled to the lower portion of collector 
panel 70, as shown. That liquid to be heated comes from thermal energy 
storage means 74 and heated fluid from collector panel 70 is taken from 
collector panel 70 by means of exit pipe or hose 76 through pumping means 
78 which feeds the heated fluid from collector panel 70 through tube or 
pipe 80 to thermal storage means 74. Sensor module 32, described in FIG. 
2, is mounted on some convenient support means such as the roof of a house 
so as to face solar-energy source 82. The control terminals 28 and 30 of 
sensor module 32 are connected as shown in FIG. 2 to their associated 
elements. Those elements may be included in a terminal box 84, as shown in 
FIG. 3. It should be noted that collector panel 70 and sensor module 32 
are oriented to "see" solar-energy source 82 in the same fashion. It 
should be noted that sensor module 32 is remote from collector panel 70 
and from thermal storage means 74. Any appropriate support means, such as 
pole 86 may be provided to support collector panel 70 with the proper 
orientation. 
Thus I have provided for solar heating systems a control system that has a 
minimum of components for maximum reliability, which matches the 
thermal-response characteristics of its associated solar energy collection 
system and which eliminates the heat energy losses produced by prior art 
control systems. 
While a particular embodiment of my invention has been shown and described, 
it will be apparent to those skilled in the art that variations and 
modifications may be made herein without departing from the spirit or 
scope of my invention. It is the purpose of the appended claims to cover 
all such modifications and variations.