Control system for solar heater

Disclosed is a solar heating system including solar collectors through which a heat transfer medium is circulated by a pump between the solar collectors and a reservoir. Temperature sensors monitor the temperature of the heat transfer medium and the temperature of the environment of the system. The electronic output signals from the sensors represent these temperatures, but these signals "drift." A sunlight sensor monitors the intensity of the sunlight and provides electronic output signals representing sunlight intensity. The signal from the sunlight sensor also "drifts." To compensate for signal drift an equilibrium curve for the collectors is employed. A controller for the system has a memory element in which is stored data representing the equilibrium curve for the collector means. This curve enables the controller based on the electronic output from the sensors to determine if the collectors can gain or lose heat. The controller operates the pump in accordance with a predetermined program which relates pump operation to the ability of the collectors to either gain or lose heat as predicted by the equilibrium curve. The curve is adjusted periodically based on temperature and sunlight measurements.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a solar heating system, and particularly to one 
including a controller having an equilibrium curve for the system that 
enables the controller based on temperature and sunlight conditions to 
determine if the system can gain or lose heat. 
2. Background Discussion 
In solar heating, a pump circulates a heat transfer medium in a reservoir 
through a solar collector exposed to sunlight. Temperature sensors sense 
the temperature of the heat transfer medium, commonly water. The problem 
is that these temperature sensors are inaccurate. Consequently, the pump 
is frequently not activated even though the system can gain heat. 
Conversely, the pump often is activated when the system shall lose heat. 
The pump operation is based on the measured temperature difference between 
the water in the collector and the water in the reservoir. When the water 
in the collector is at a higher temperature than the water in the 
reservoir, a controller for the system turns the pump on and circulation 
commences. When a positive measured temperature difference can no longer 
be maintained, the controller turns the pump off. This approach does not 
work well if the measured temperatures are not the same as the actual 
temperatures. 
It is common for the output of the temperature sensors to drift due to 
electronic errors, thermal cycling, and environmental degradation. The 
problems with conventional solar heating systems are discussed in a number 
of publications by the Solar Energy Research Institute of the Department 
of Energy, including: 
1. Evaluation and Laboratory Testing of Solar Domestic Hot Water Control 
Systems (SERI/TR-254-1805, February 1983), and 
2. Reliability Testing of Active SDHW Components, Part I: Test Results of 
Sensors Used in Control Systems (SERI/TR-253-2602). 
Thus, an uncertainty exists in the accuracy of the measured temperatures. 
To compensate for this uncertainty, the industry introduced bias terms (TB1 
& TB2) used in programming the operation of the controller. Typical values 
for TB1 and TB2 are between 2 and 15 degrees Fahrenheit, for example, TB1 
equals 8.degree. F. and TB2 equals 3.degree. F. The measured temperature 
difference must be greater than TB1 before the pump is turned on. When the 
measured temperature difference is less than TB2, the pump is turned off. 
This technique assumes that the drift of the sensors never exceeds the 
value of TB1 or TB2. The problem with using bias terms is that pump 
cycling occurs and that over time the drift in the sensors sometimes 
exceed the bias terms. 
Some performance factors inherent in control systems which use measured 
temperature differences are worth noting. During start-up conditions, the 
system waits until the required value of TB1 is reached, at which time it 
turns the pump on. The temperature of the collector at this time was 
reached under stagnant conditions (no flow). It had taken from sunrise 
until that time to reach that temperature difference. When the pump turns 
on, the sunlight intensity will not be high enough to maintain the 
temperature difference of TB2 under non-zero flow conditions and so the 
pump will turn off. The hot water that was in the collector is now waiting 
in the return line between the collector and the reservoir where its heat 
losses are high due to the high surface to volume ratio of the return 
line. Cold water that was in the supply line between the reservoir and the 
collector is now in the collector gaining heat. Hot water that was in the 
reservoir is now cooling down in the supply line. On/off pump cycling 
occurs until a measured temperature difference greater than TB2 can be 
maintained or exceeded. A similar situation occurs in late afternoon when 
the sunlight intensity is decreasing and a measured temperature difference 
greater than TB2 can not be maintained, even though there is still a net 
heat gain potential. Under certain conditions, this mode of operation can 
actually lose heat when it should be gaining heat. 
On/off cycling induces unnecessary duty cycles on the pump, the electronics 
which turn the pump on and off, and any valves in the system. Each time 
the pump turns on, high electrical current transients pass through the 
windings in the pump motor due to its natural inductance. This increases 
operational costs significantly. Transients of this nature shorten the 
pump life and the life of the electronic components which deliver the 
power. Valves with plastic or rubber seats suffer excessive wear, 
shortening their useful life. To diminish this problem, the industry 
introduced proportional control which varies the water flow rate with the 
measured temperature difference. However, a non-zero value of TB1 and TB2 
must still be used due to the sensor errors. 
It is important to maintain turbulent flow throughout the collectors so 
that heat transfer efficiency is optimized. Although proportional control 
diminishes the on/off cycling problem, it does not eliminate it and the 
system may operate with laminar flow much of the day instead of fully 
turbulent flow. Laminar flow can not remove heat from the collector as 
efficiently as turbulent flow, causing the system to operate at an 
efficiency level lower than it could. Proportional control systems can 
only be used on small fractional horsepower pumps. Pool and commercial 
solar systems use large pumps (1 to 10 horsepower). 
Government tests and surveys as reported in Evaluation and Laboratory 
Testing of Solar Domestic Hot Water Control Systems (SERI/TR-254-1805, 
February 1983) indicate that the mean time between failure for control 
systems, which use measured temperature differences, is 4.1 years. The 
causes are: 
1) If the drift of the collector temperature measurement is positive and/or 
the drift of the reservoir temperature measurement is negative, the 
measured temperature difference will be greater than the actual, causing 
the system to turn on too early in the morning and turn off too late in 
the afternoon. Eventually, the pump may remain on even after sunset. 
Conversely, if the drift of the collector temperature measurement is 
negative and/or the drift of the reservoir temperature measurement is 
positive, the measured temperature difference will be smaller than the 
actual. The system will then turn on too late in the morning and turn off 
too early in the afternoon. Eventually, the system may not even be able to 
maintain a steady state measured temperature difference greater than TB2 
and the pump will than cycle on and off repeatedly during daylight hours, 
defining another type of failure. Regardless of the direction of the drift 
of the measured temperatures, the performance of a controller which uses 
measured temperature differences is not stable nor is it optimal. 
2) Another failure mode, but more serious, can occur under freezing 
conditions. Some control systems circulate the water through the 
collectors to keep them from freezing. If the drift of the measured 
collector temperature is positive, the water in the collector can actually 
freeze before the measured temperature triggers the control system to 
begin circulation. Once frozen solid, no circulation can occur, even if 
the pump is turned on. Many of the pumps used in the industry employ the 
circulating water to keep them from overheating. Since no circulation can 
occur, the pump can overheat and burnout. Additionally, the collectors can 
be ruptured by the freezing water. To compound the problem, the 
temperature sensors for the collectors have the greatest drift potential 
since their environment is much harsher than that for the reservoir 
temperature sensors. The most reliable freeze protection system, aside 
from using glycol as the heat transfer medium, is to use the pump to 
circulate the water when freezing temperatures are approached within the 
collector. For this approach to be effective, sensor accuracy is crucial. 
3) Sensor malfunctions present another problem to conventional controllers. 
If a water temperature sensor in a system fails and the control system is 
using measured temperature differences, the controller can do one of two 
things. It can turn the pump on continuously, regardless of temperature 
conditions, or it simply leaves the pump off regardless of conditions. In 
the first case, the heat gained during daylight hours is lost during 
twilight hours, resulting in significant heat losses at night. In the 
later case, no heat is gained. This subjects the collectors to extremely 
high temperatures during daylight hours, and risks potential freezing 
conditions at night. 
SUMMARY OF THE INVENTION 
The objective of this invention is to provide a solar heating system which 
operates more efficiently and optimizes the usage of pumps and valves in 
the system to prolong their operational life. 
The solar heating system of this invention has several features, no single 
one of which is solely responsible for its desirable attributes. Without 
limiting the scope of this invention as expressed by the claims below, its 
more prominent features will now be discussed briefly. After considering 
this discussion, and particularly after reading the section of this 
application entitled, "DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS," 
one will understand how the features of this invention provide its 
advantages, which include near optimal system performance, a single 
controller which is user friendly, automatic sensor calibration, monitors 
check valve seal integrity, and back-up modes of operation in case of 
sensor malfunction. 
The first feature of this invention is that stored within the memory of the 
controller means is an equilibrium curve employed to determine if the 
system can gain or lose heat. A solar collector has associated with it an 
equilibrium curve which predicts the conditions under which the collector 
can gain or lose heat. The system is at equilibrium when the rate of heat 
gain equals the rate of heat loss. This curve is defined by the following 
equation: 
EQU Q=k(T.sub.w -T.sub.a) 
where 
Q is the rate at which heat is being gained from sunlight, 
T.sub.w is the temperature of the water, 
T.sub.a is the temperature of the air, and 
k is a factor which is a function of the momentary environmental conditions 
in which the system operates. 
The system of this invention utilizes successive approximations to 
determine the values for k under different conditions. Errors in 
measurements of conditions are automatically compensated for by the values 
of k. When environmental conditions approach equilibrium, the heat 
gains/losses approach zero. By flowing water through the collectors when 
reaching the predicted equilibrium point and then stopping the flow, a 
determination of the accuracy of the equilibrium curve can be made. If the 
temperature of the collectors start to increase after the flow is stopped, 
the collectors are capable of gaining heat at that point and the value of 
k should be decreased for these conditions. Conversely, if the temperature 
in the collectors start to decrease, the collectors are losing heat at 
that point and the value of k should be increased for these conditions. 
The second feature is that it automatically calibrates sensors. Near 
equilibrium conditions, the measured temperature values of all the water 
temperature sensors will be approximately equal with water circulating 
through the system. Any difference in measured values at this time between 
a reference water temperature sensor and the remaining water temperature 
sensors can be automatically corrected. Sunlight sensor output is also 
corrected to compensate for drift of its output. Its voltage offset can be 
corrected on a daily basis while its gain can be compensated for in the 
equilibrium curve on a daily basis. 
The third feature is that it automatically uses multiple back-up modes of 
operation in case of either single or multiple sensor failures. The system 
of this invention includes back up modes in case of sensor failure. 
Sensors are continually monitored to determine if they are functional. 
Upon detection of a non-functional sensor, the controller of the system 
automatically goes into a back-up mode of operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As illustrated in FIG. 1, the solar heating system 10 of this invention 
includes a tank or reservoir 12 containing a heat transfer medium 14 such 
as, for example, water or a glycol solution. This heat transfer medium 14 
is heated by a pump 15 circulating it through an array of solar energy 
collectors 16 which are exposed to solar energy from the sun 18. There is 
an inlet line 20 extending from the reservoir 12 to the inlet end 16a of 
the solar collectors 16 and an outlet line 22 extending from the outlet 
end 16b of the collectors to the reservoir. A check valve 17 in the outlet 
line 22 only allows flow in one direction, from the collectors 16 to the 
reservoir 12. 
The collectors 16 lose heat at a rate based on the temperature differential 
between the heat transfer medium 14 and the surrounding environment 
(temperature of the air). The collectors 16 gain heat at a rate based on 
the intensity of the sunlight striking the collectors. Several electronic 
temperature sensors 24 through 27 are utilized to measure the temperature 
of the heat transfer medium 14 as it circulates through the collectors 16 
and the reservoir 12. There is also an electronic temperature sensor 28 
located in the environment surrounding the collectors 16, namely, the 
atmosphere, which measures the temperature of the air in the vicinity of 
the collectors. The air temperature sensor 28 is positioned in the shade 
and in a location which is best representative of the temperature of the 
environment in which the system is located. Mounted as illustrated in FIG. 
3 is a sunlight sensor 30 which measures the relative intensity of the 
sunlight energy striking the collectors 16. 
It is important that the sunlight sensor 30 be positioned correctly with 
respect to the path of travel of the sun 18. Failure to properly position 
the sunlight sensor 30 will result in degraded measurement of the 
intensity of light striking the collectors 16. The ideal position of the 
sunlight sensor 30 is on the surface 16c where it is exposed to the same 
intensity of sunlight as any other point on the surface 16c. This ideal 
position is dependent on the season and the position of the collectors 16. 
As illustrated in FIG. 3, the sun 18 travels along a course through a plane 
32 to expose the surface 16c of the collector 16 to sunlight. Preferably 
the collectors 16 are titled at an angle .beta. and face in a southern 
direction. The plane 32 tilts to intersect a plane 34 which is at a right 
angle to the surface 16c of the collectors 16. This plane 34 seasonally 
defines the angle of the plane 32 with respect to the surface 16c. This 
plane 34 should be at a right angle with respect to the plane 32. As the 
winter approaches, the sun 18 descends in the sky in the northern 
hemisphere, and the plane 32 tilts, moving into one of the dotted line 
positions 36 which indicated the sun's path of travel through the sky 
during the winter months. The plane 32 moves into the dotted line position 
38 during the summer months as the sun changes its course of travel with 
the change of seasons. The tilt angle .mu. between the surface 16c and the 
plane 32 is determined by the season and the angle .beta.. The ideal 
position for the sunlight sensor 30 is on the surface 16c at the 
intersection between the planes 32 and 34. 
Ideal positioning of the sunlight sensor 30 may also be ascertained by 
measuring the sensor's output, which is maximum at solar noontime as 
depicted in FIG. 3. On a trial and error basis, the sunlight sensor 30 can 
be positioned and repositioned until at solar noontime its output is 
maximum. This will then be the best position for the sensor 30. 
The operation of the solar heating system 10 is controlled by a controller 
40 illustrated in FIG. 5. The central control element of this controller 
40 is a MC68HC705C8 microcontroller 42 sold by Motorola Corporation. This 
microcontroller 42 is programmed in accordance with the flow diagrams and 
directions set forth in Appendix A attached hereto. "What Every Engineer 
Should Know About MicroComputers," authors Bennett and Evert, "Using 
Microprocessors and Microcomputers, 2nd Edition" authors Greenfield and 
Wray, which may be purchased from the Motorola Corporation, explain how to 
program its MC68705 microcontroller. Assembly language is used for 
programming. Specific information for the MC68HC705C8 is available from 
the Motorola Corporation in its MC68HC05C4 Advanced Information Data Sheet 
(AD1991R2) and its appendices J and K. 
The signals from the sensors 24, 25, 26, 27, 28, and 30 are analog current 
signals which are converted by an A/D converter 44 to digital signals for 
use by the microcontroller 42. Random access memory (RAM) 46 is used to 
store, for example, the equilibrium curve data, sensor calibration data, 
etc. There are outputs 48 from the microcontroller 42 to a TTL chip 50 
which controls the pump 15. A display 51 can be used to show sensor 
malfunctions, check valve malfunction, and/or sensor readings. 
The controller 40 turns on the pump 15 when the collectors 16 can gain heat 
and turns off the pump when this condition no longer prevails. When the 
collectors 16 lose heat as fast as they gain heat they are at equilibrium. 
If the whether does not change drastically during the course of a day, 
equilibrium conditions occur usually twice a day in the American Sun Belt: 
usually between 6:00 and 10:00 A.M., and 3:00 and 7:00 P.M. 
When the pump 15 is turned on, it pumps the heat transfer medium 14 from 
the reservoir 12 through the inlet line 20 into the lower, inlet end 16a 
of the collectors 16. The heat transfer medium 14 flows through the 
collectors 16 and its temperature is elevated. This heated heat transfer 
medium 14 exits the upper, outlet end 16b of the collectors 16 and then 
flows through the outlet line 22 and check valve 17 into the reservoir 12. 
Except when testing to determine if equilibrium is occurring in the 
collectors 16, the pump 15 circulates continually the heat transfer medium 
14 through the system 10 as long as conditions prevail that result in a 
reasonable amount of heat being gained by the heat transfer medium. 
The check valve 17 closes when flow stops. This prevents circulation of the 
heat transfer medium 14 when the system 10 can no longer gain heat. At 
night when the heat transfer medium 14 in the collectors cools, it drains 
due to gravity from the collectors 16 through the inlet line 20 and forces 
water from the reservoir 12 upwardly through the outlet line 22. The check 
valve stops this flow through outlet line 22, thereby preventing lose of 
heat from of the system 10 at night. Alternately, the check valve 17 could 
be in inlet line 20. 
The system 10 may have the heat transfer medium 14 in the collectors 16 on 
a continuous basis. In this case it is important to have the check valve 
17 in the system 10 in order to prevent the heat transfer medium 14 from 
circulating at night. A system such as system 10 has the sunlight sensor 
30 so that it can be determined when nightfall has arrived. It also has 
calibrated temperature sensors 24, 25, 26, and 27. With these features it 
then can establish with certainty if the check valve 17 is leaking. 
When the check valve 17 leaks, warm water from the reservoir 12 convects up 
the outlet line 22 to the collectors 16 and leads the controller 40 to 
believe that the system can gain heat by the measured temperature 
difference between the collector outlet temperature sensor 27 and the 
reservoir temperature sensor 24. If the controller 40 turns the pump 15 on 
to retrieve this heat, it will turn off shortly thereafter since the 
sensor 27 will drop in temperature as soon as the cold water in the 
collectors 16 flows by it. Hence the pump 15 would then turn back off. If 
this occurs several times during the night, then the check valve 17 needs 
to be replaced or repaired. As specified in routine M7 in Appendix A, the 
controller 40 is programmed to detect this condition of the check valve 17 
and provide a warning signal that is shown on the display 51 that the 
check valve 17 is malfunctioning. 
In accordance with this invention, the controller 40 turns the pump 15 on 
and off based on the predicted ability of the heat transfer medium 14 to 
gain or lose heat. When it is predicted that the heat transfer medium 14 
is expected to gain heat, the pump 15 is turned on. When it is predicted 
that the collectors 16 cannot transfer heat to the heat transfer medium, 
the pump 15 is turned off. Accurate control is provided even though the 
temperature and sunlight intensity measurements are not accurate. This 
result is achieved because derivatives of the temperature and sunlight 
intensity measurements are used to determine an equilibrium curve. 
Moreover, the equilibrium curve is continually redefined to compensate for 
measurement errors. 
FIGS. 2A, 2B and 2C depict exemplary equilibrium curves for the system 10. 
In FIGS. 2A, 2B and 2C, the difference (.DELTA.T) between the temperatures 
of the heat transfer medium and the environment in degrees Fahrenheit are 
plotted along the abscissa, and the values of the sunlight intensity in 
British Thermal Units per hour per square foot are plotted along the 
ordinate. FIGS. 2A, 2B and 2C each show a dotted line curve A and a solid 
line curve B. The dotted line curve A is an estimation of the equilibrium 
curve for the system 10. It is not expected to be initially accurate and 
it shall be redefined to improve its accuracy based on actual measurements 
taken during the course of the operation of the system 10. The solid line 
B represents the actual equilibrium curve for the system 10. This solid 
line curve B has factored into it the errors in the sensor measurements. 
It shall change shape with changing seasons. 
FIGS. 2A, 2B and 2C illustrate how the estimated equilibrium curve A is 
modified over a three day period based actual temperature and sunlight 
intensity measurements. For example, according to curve A in FIG. 2A, when 
the .DELTA.T is 100.degree. F., the light intensity must be the equivalent 
of at least 100 BTU's/ square foot-hour, otherwise the heat transfer 
medium 14 can not upon passing through the collectors 16 gain any heat. 
Thus, the pump 15 does not turn on until this condition prevails. 
While there are an infinite number of ways to define the estimated curve A 
so that it approximates the actual equilibrium curve B, a very simple and 
effective approach is to approximate it using a piece-wise linear curve. 
In other words, the nonlinear curve B can be approximated with a finite 
number of linear segments 54a-54i which approach its shape. This technique 
is illustrated by the following EXAMPLE. 
EXAMPLE 
1. Define the useful operating .DELTA.T temperature range of the system 10, 
for example, 0.degree. to 180.degree. F., and break this range up into a 
finite number of segments 54a through 54i, for example. Define the end 
points of each segment so that they are common to the neighboring 
segments. In FIG. 2B an improved estimated curve A is illustrated. Hence, 
an adjustment to a particular segment results in an adjustment to its two 
neighboring segments as well. More segments will improve curve fitting 
accuracy but will increase the time required for the system 10 to 
accurately approximate the actual equilibrium curve B and will result in 
slower shifts due to seasonal variations. 
2. As conditions for the system 10 approach the estimated equilibrium curve 
A in the morning and afternoon, the temperatures will fall within one of 
the segments. If, as determined by calculation of temperature derivatives 
as discussed below, the system 10 is losing heat for a particular segment, 
then this particular segment needs to be adjusted so that higher values of 
sunlight intensity are used to redefine this segment. Conversely, if the 
system 10 is gaining heat, the segment needs to be redefined for lower 
values of sunlight intensity. 
3. Occasionally, an adjustment will result in a sunlight intensity value 
higher than what exists at an adjacent segment. Since higher operating 
temperatures require higher values of sunlight intensity, curve smoothing 
techniques can be used to adjust points which are out of line with the 
most recent adjustment. This is exemplified on the curve A, FIG. 2B for 
day two, where the point 56 at 160.degree. F. for the segment 54h would 
have been below the newly adjusted endpoint 58 at 140.degree. F. for the 
segment 54g. 
4. Repeating steps 2 and 3 every day redefines the piece-wise linear curve 
A to approximate closely the actual equilibrium curve B. Large adjustments 
will change the curve faster but will reduce the accuracy of the final 
curve fit. 
The preceding EXAMPLE arbitrarily defined temperature as the independent 
variable along the abscissa, resulting in adjustments to the sunlight 
intensity being arbitrarily defined as the dependent variable along the 
ordinate. It is important to note that sunlight intensity could have been 
defined as the independent variable which would have resulted in 
adjustments being made to the temperature values when determining the 
estimated equilibrium curve A. 
As depicted in FIG. 2A, on day one, the system 10 turns the pump 15 on when 
the water temperature minus the air temperature is between 20.degree. and 
40.degree. F., segment 54b, and turns the pump off between 120.degree. and 
140.degree. F., segment 54g. Since the system 10 according to the actual 
equilibrium curve B was capable of gaining heat when turned on, the 
sunlight intensity values in segment 54b are reduced as shown in the curve 
A in FIG. 2B for day two. In the afternoon when the pump 15 is turned off, 
the system 10 is not capable of gaining heat. Hence, the values of the 
sunlight intensity were increased as shown in the curve A in FIG. 2B for 
day two in the segment 54g. 
On day two, the pump 15 is shown to turn on between temperatures of 
60.degree. and 80.degree. F., segment 54d, while turning off between 
temperatures of 140.degree. and 160.degree. F., segment 54h. The results 
are shown on the adjusted curve A in FIG. 2C for day three. 
The way to determine if the collectors 16 are gaining or losing heat near 
equilibrium conditions, after thermal transients have subsided after 
discontinuing full flow, is to calculate temperature derivatives of the 
collectors 16 under stagnation conditions. After thermal transients have 
subsided, the heat transfer medium 14 in the collectors 16 are at nearly 
the same temperature as the heat transfer medium in the reservoir 12. 
Derivatives from this point forward under stagnation conditions indicate 
whether the collectors 16 can gain or lose heat for sunlight intensity, 
temperature of the ambient air, and temperature of the heat transfer 
medium at particular measured conditions. (It is assumed that heat gains 
and losses from the inlet line 20 and outlet line 22 are not significant.) 
Temperature derivatives can be approximated by calculating finite 
temperature differences over a finite time. The collectors 16 have a 
positive derivative if they are gaining heat and a negative derivative if 
they are losing heat. An example of how one could determine the 
temperature derivative is as follows: 
a) Create a set of moving time lines which store sensor outputs as a 
function of time. The size of the time line must be large enough for the 
environmental conditions to change sufficiently to detect a measurable 
difference, yet be small enough to reach a decision before system 
performance is significantly degraded. 
b) Collect periodically new data and shift the time lines to include this 
new data and exclude the oldest data. 
c) Calculate the average change in temperature per change in time for the 
sets of data in each time line. 
During the course of a typical day the system 10 passes through equilibrium 
twice. In the morning, the light intensity will be such that the system 10 
will neither gain nor lose heat. That is, the temperature difference 
between the temperature sensors 24, 25, 26 and 27 will be near zero. This 
condition will occur also later in the day when the system 10 will no 
longer either gain or lose heat. For example, if one of the sensors 
indicates a significant difference between the other sensors, this sensor 
can be compensated for or adjusted by the controller 40 offsetting or 
correcting this sensor reading. 
It may be desirable to calibrate manually one of the temperature sensors 
measuring the temperature of the heat transfer medium 14, for example the 
temperature sensor 25 at the exit of the reservoir 12. This manual 
calibration can be accomplished by simply immersing the sensor 25 in ice 
water and setting the output at 32 degrees Fahrenheit and the immersing 
the sensor in boiling water and setting the output at 212 degrees 
Fahrenheit. This sensor 25 may now be used as a reference in conducting 
the automatic calibration discussed above. 
Derivatives are also useful for determining if sensors are operating 
properly or are malfunctioning. Environmental conditions are not constant. 
Hence the derivatives of the measurements of these conditions can not be 
zero since there will always be some measurable change. Continuously 
monitoring the derivatives of the sensor output signals provide 
notification if the sensors are still functioning. It is important to 
separate variations in sensed conditions from electronic noise. It is 
comon practice in design of electronics to establish a "noise floor." This 
noise floor can be used to establish the maximum contribution to a 
derivative that the noise floor can make. This value then becomes the 
minimum value allowable for derivatives of sensed conditions. The values 
of the derivatives as contributed by the noise floor must be smaller than 
the derivatives of the sensed conditions. 
While derivatives can be used very effectively to confirm temperature 
sensor operation they are less effective in confirming sunlight sensor 
operation. At night the sunlight sensor will only measure the changes in 
light intensity coming from the night sky (stars, moon and street lights). 
Hence the derivative of the sunlight intensity may be smaller than the 
derivative due to the noise floor. In this case additional information is 
needed. When sunlight derivatives are small, the sunlight intensity must 
be near zero or near its maximum value. If the sunlight is near the 
minimum value and the collectors 16 are significantly warmer than the 
resevoir 12, then the sunlight sensor 30 must have failed. If the measured 
sunlight is near its maximum value when its derivative is zero, the 
derivative can not stay near zero for very long. Initiating a time delay 
counter during this condition allows the sunlight intensity to decrease 
enough to change the derivative significantly enough for it to rise above 
the minimum threshold. 
An alternate embodiment of this invention, the solar heating system 70, is 
depicted in FIG. 4. The solar heating system 70 is for a swimming pool 
with the pool 72 constituting a large reservoir of water 14, the heat 
transfer medium. Water is preferably drawn from both the top and bottom 
levels of the pool 72 through the lines 66 and 68, each of these lines 
including gate valves 64. This water 14 is pumped by a pump 76 to an array 
of solar collectors 16. This pump 76 is operational for a minimum time 
period each day to assure that the water 14 is circulated through a filter 
78, a line 80 including a check valve 82, through a motorized diverter 
valve 84, a line 86 into a heater 88. From the heater 88 the water flows 
through a magnetic water conditioner and purification device 89 and then 
either through line 90 or line 92 back into the pool 72. A manually 
operated gate valve 94 is in the line 90, and a solenoid operated valve 
96, feeder 98 for feeding sanitizers like chlorine into the water, and 
check valve 100 are in line 92. 
The diverter valve 84 directs the flow of water either to the solar 
collectors 16 or bypasses these collectors to direct the water through the 
water purification and conditioner device 89 back to the pool 72, either 
through line 90 or line 92, depending on the position of the solenoid 
valve 96. The diverter valve 84 and pump 76 work together to conserve 
energy. (The two working in conjunction with each other are considered 
pump means.) It is impractical to turn the pump 76 on and off because it 
must remain on for the minimum time period each day. So instead, when 
stagnation conditions are desired to adjust the equilibrium curve, the 
diverter valve 84 is moved to a position so the water bypasses the 
collectors 16. When the collectors 16 can gain heat and the equilibrium 
curve is not being adjusted, the diverter valve 84 is opened, and the pump 
76 pumps the water from the pool 72 through the diverter valve 84 and line 
102 and then through the solar collectors 16 and line 104 back through 
lines 86 and 90 or 92 into the pool 72. 
Similar to the system 10, a sunlight sensor 30 positioned as shown in FIG. 
3 is employed to measure the intensity of light striking the collectors 16 
in system 70. There is a temperature sensor 104 in the pool 72, a 
reference temperature sensor 106 which may be manually calibrated as 
discussed above in the inlet line to the filter 78, a collector inlet 
temperature sensor 108 at the inlet of the solar collectors 16, a 
collector outlet temperature sensor 110 at the outlet of the solar 
collectors, and an air temperature sensor 112, preferably in a shaded 
environment. 
This solar heating system 70 operates generally in the same manner as 
system 10, except the diverter valve 84 is used to achieve stagnation 
conditions when adjusting the equilibrium curve. This avoids cycling the 
pump 76 in the morning since pool pumps are relatively large and 
equilibrium curve adjustment periods are short, typically 5-10 minutes. 
SCOPE OF THE INVENTION 
The above presents a description of the best mode contemplated of carrying 
out the present invention, and of the manner and process of making and 
using it, in such full, clear, concise, and exact terms as to enable any 
person skilled in the art to which it pertains to make and use this 
invention. This invention is, however, susceptible to modifications and 
alternate constructions from that discussed above which are fully 
equivalent. Consequently, it is not the intention to limit this invention 
to the particular embodiment disclosed. On the contrary, the intention is 
to cover all modifications and alternate constructions coming within the 
spirit and scope of the invention as generally expressed by the following 
claims: