- Email: [email protected]
Microprocessor control of distributed storage, active solar heating systems
Jourmzl
ofMicrocomputer
Applications (1987) 10, 119-I 26
Microprocessor control of distributed active solar heating systems
storage,
C. C. Lefas Nuclear Research Center ’ ‘Democritus “, 153.10, Aghiu Paraskevi, Attiki, Greece
The present paper describes the controller design of a distributed storage solar system. A single-chip computer is used as the central unit. Four types of temperature sensor are considered and IC semiconductor sensors are shown to suit this application best. The pump and auxiliary heat supply is controlled in an on/off fashion by optically isolated relays.
1.
Introduction
Solar water heating for domestic applications is steadily gaining acceptance over the last few years and many solar energy collection systems are available in the market today. Small systems, usually intended for a single user, comprise a single collector with 2-3 m* active area and 100-l 50 1 storage capacity. For larger systems a wider range of options exists where multiple collectors, distributed storage and other techniques are used. A typical situation arises in the use of solar energy for water heating for domestic use in multistorey apartments. The situation today is the use of independent electric boilers for every apartment and, for example, over 10,000 boilers are sold in Greece every year (Merritt, 1982). The introduction of individual solar heating for each apartment is confronted with practical problems such as considerable roof-apartment distance, limited roof space etc. A more feasible approach in this case is to use a larger central solar energy collector which is used to feed every apartment with hot water separately. Local apartment boilers are then used as local storage tanks with auxiliary electric heating (Figures 1 and 2). In such a system user demands are made to a central controller which has to decide whether they can be met by solar heating or auxiliary heating. Temperature of individual tanks has to be monitored and since tanks are local, sensors have to be located at considerable distance from the main controller. Care should therefore be exercised in the choice of the appropriate sensors, the design of the signal conditioning circuits, the design of the controller front end etc., to screen noise interference. The present paper discusses design aspects of controllers used for distributed storage system control. Single-chip computers with considerable computing power are today commercially available. The Intel 8051, Motorola 6801, Zilog 28 and Rockwell 6501 are just a few examples of the products available. The controller described in the present paper is built around the EPROM-fitted 68701 single-chip computer. As will be seen, relatively simple controller configurations satisfy quite demanding control requirements. However. the 119 0745-7 138/87/020119 + 08 $03.00/O
0
1987 Academic
Press Inc. (London)
Limited
120
C. C. Lefas
control strategy differs according to the particular application and only very simple situations are described in the present paper. Circulation in every thermal circuit is controlled by a pump which in turn has to be controlled in an on/off fashion (e.g. Rorres, Orbach & Fischl, 1980).
2.
The system
Figure 1 shows the system configuration. A central solar energy collector is used and the collected energy is distributed to individual users via a primary heat exchanger and secondary thermal circuits. Local boilers are used as storage tanks and a central controller is used for system management. The collector liquid is circulated through the primary heat exchanger forming the primary thermal circuit. Secondary thermal circuits are formed between the primary heat exchanger and local units. The central collector liquid is recirculated until the desired temperature is reached, when one or more sec$ndary circuits are switched on according to current requirements. With this configuration a wide range of solar energy collectors can be used, providing the necessary system flexibility. Collector characteristics, such as the optimum operating conditions, are programmed into the controller. Heat transfer can be in both directions, i.e. from the primary heat exchanger to the storage tanks and vice versa. In this way unused tanks are utilized by the controller to store energy in the case where excess energy is received. This is subsequently retrieved and distributed to other users. An additional factor is that by using separate secondary thermal circuits faults in one of them do not put the entire system out of action but just one loop.
3.
The controller
Figure 2 shows a general block diagram of the controller. The central module is the heart of the controller containing the central processing unit (CPU), the necessary memory
Secondary circuits T-a-2
r
I Storage element
If (Primary
Storage element Pump
T’kZanger circuit
Figure 1.
Multistorage
system configuration.
Multiple primary loops may be used.
Control
of solar heating systems
121
1
Pump control module
Central l
module
Auxiliary heat supply 8 control module
.
I8
ROM
Ig
RAM
Ix tg
I/O
Temperature sensing module
a ’ ‘5
CPU
Counter Serial line interface
1 1
‘5 1.f
Watchdog I$
I
timer
Figure 2.
Controller
Temperature sensing module
1 ’ I block diagram.
(both read only memory (ROM) for program storage and random access memory (RAM) for data storage, a real-time clock, a serial line interface for controller/operator communication, and suitable input/output (I/O) with front end circuitry to interface the sensing modules to the CPU. Most of the above is contained within a single component i.e. the single-chip computer, as will be later outlined. The temperature sensing modules contain the temperature sensors and accompanying signal conditioning circuits and are located near to the sensor. The pump and auxiliary heat supply control units are in practice CPU operated relays providing on/off control signals (and mains power) to the pumps and the auxiliary heaters. Individual users make requests to the controller who then turns electric heaters on if insufficient solar energy is available. The main modules are now described in some detail.
4.
The temperature
sensing
modules
(TSM)
Four different types of sensors have been considered for this application: platinum resistors, thermistors, thermocouples and semiconductor integrated circuit (IC) sensors. Platinum resistors are high quality, highly linear sensors used for precision measurements. They are usually employed in a bridge configuration where resistance changes are converted to bridge output voltage changes. Besides their considerable cost, these sensors have a relatively low temperature coefficient so that the bridge output is relatively low. This requires the use of high quality, very low drift operational amplifiers which tend to increase further total system cost and reduce long term stability. Thermistors offer a low cost alternative. These sensors too convert temperature to resistance and therefore are incorporated in a resistance bridge. The main disadvantage of thermistors is that they are highly non-linear sensors, almost logarithmic. This in turn means that logarithmic amplifiers have to be used, which tend to reduce long term stability of the circuit. However reasonably compensated monolithic log amp components are today commercially available but the cost of these tends to offset the low cost advantage of thermistor sensors. Thermocouples are popular sensors offering a wide measurement range and small thermal capacitance. However, neither of these factors is of major importance for the
122
C. C. Lefas
class of applications considered in the present paper. Measurement range is fairly limited (e.g. 0°C to 100°C) and usually the thermal capacitance of the component whose temperature is being monitored (as in thermal storage tanks) is many orders of magnitude higher than that of the sensor, so that the thermocouple low thermal capacitance advantage is not utilized. As disadvantages of thermocouples are considered their non-linearity, although it is not so severe as that of thermistors, and their low output voltage. Also the thermocouple output is a measure of the temperature difference between the hot and cold junction so that a special signal conditioning circuit has to be used to convert the thermocouple output to absolute temperature measurements. Such circuits, electronic ice points, use their own absolute temperature sensors. However, their cost offsets the low cost advantage of thermocouples and the use of the additional temperature sensor tends to reduce measurement accuracy. Long term drifts in the signal conditioning circuit also reduce measurement quality so that thermocouples have been considered a poor choice for the class of applications considered here. The last alternative investigated in the present paper is the use of semiconductor IC sensors. These sensors exploit the temperature dependence of the transistor base-emitter voltage dependence. Through suitable IC manufacture and trimming, highly linear and stable IC sensors are produced (Winn & Hull, 1978; McDonald, 1978; Proctor, 1984; Merritt, 1982). The signal conditioning circuit is fabricated on the same wafer resulting in low cost, easily interfaceable sensors. Typical IC sensors (e.g. the National LM 35) provide a measurement accuracy of 0.25”C and sensitivity 10 mV/“C. The cost of these sensors is in the order of high quality thermistors. The main disadvantage of these sensors is their limited measurement range which is usually 0°C to 70°C for commercial grade types and - 55°C to + 125°C for military grade types. This however is satisfactory for solar heating applications. Also IC sensors have considerable wafer-ambient heat resistance so that considerable self-heating takes place if they are continuously powered. This problem is overcome by powering the sensor intermittently and by dipping the sensor into the hot liquid thus reducing considerably the can-ambient thermal resistance. On the other hand IC sensors settle rapidly to a steady-state value after power up (in the order of 2 ps) so that the duty cycle can be made very small to avoid self-heating. Since long sampling intervals, in the order of 1 s, are easily tolerated in solar heating applications the duty cycle does not have to be greater than 10m4(100 ps every second). The above reasons show that IC temperature sensors are an appropriate choice for solar heating applications. Figure 3 shows the diagram of the temperature sensing module designed for this application. The sensor is an LM 35 IC acting as a temperature dependent voltage source (reference) giving a 10 mV/K output. The temperature range in which the sensor is set to operate in the present application is 270K to 370K. Measurements are converted to frequency modulated pulse sequence for the present application. Datel’s VFQ-2 monolithic voltage-to-frequency converter is used in the circuit shown in Figure 3. The output frequency varies from l-9.2 kHz and total V/F conversion non-linearity is less than 0.2%. Output pulses are transferred to the main controller via a &20 mA current loop. This provides some protection against noise picked up by the transmission line. In cases where this protection is insufficient low cost fibre links are used. The temperature sensing module is only intermittently powered under CPU control. Total current consumption is approximately 30 mA. The link to the central module is a simple twisted-pair cable. Two of these cables are required, one for measurement transmission and one for the power supply.
Control of solar heating systems 4.7k
-15
v 330k
123
7’5”
10; ref
1.5 k
1OOk
+
.
VFQI
-
1M
Figure3.
5.
The central module
Temperature
sensing module.
and the pump switching module
The central module is based on a single-chip computer, the 68701 which is the EPROM version of the 6801 mentioned earlier. The central module block diagram is shown in Figure 4. The 68701 contains 2 k ROM where the control programs are being stored, 128 bytes RAM for measurement storage, three parallel I/O ports, a serial interface and a timer/counter. Thus the 68701 contains within a single package all logic necessary for this application. A watchdog timer is included for system operation monitoring. Pump control and auxiliary power control is achieved by relays in an on/off fashion. The pump control module and the auxiliary heat supply control module are controlled by the CPU by writing suitable words to the relevant output ports. Since any combination of pumps and auxiliary heat supplies is possible to be on, relays are controlled in parallel. Output pins are electrically isolated from the relays, via optoisolators. Real-time information is essential for this application and is being used, to estimate the amount of energy to be received until nightfall. A real-time clock monolithic device is included in the present controller. The real-time clock operates directly as a CPU peripheral and is interfaced to the CPU in a manner similar to that of the PIA. The CPU operates in the expanded non-multiplexer mode being capable of addressing 256 external locations. The real-time clock occupies 64 locations and the PIAs 16 locations. The rest remains unused for controller expansion. The temperature sensing modules (TSM) are driven through port 1 of the 68701. Data on port 1 are decoded by a 6 to 32 decoder block and transistor switches are driven supplying 15 V power to a single TSM (in the TSM power control unit). The same data are used in the input module to drive a digital 32 to 1 multiplexer block. Current pulses from the TSM arrive in optoisolators whose output is directed via the multiplexer to the timer/counter input of the 68701. Obviously only the TSM whose output is directed to the timer input needs to be powered and this is the reason why only one TSM may be powered at a time.
124
C. C. Lefas
I
Centralmodule
Pump control module
I I
I,
TSM power control unit
-I-+15v,
I
PIA 6821
I
I I I
.
jSW\TCH
I
I
I
I I I
1 I
.
I
I
I T+15Vl
;,
.
I I __-----
l Auxiliary heat supply l control module
I I
.
I
.
J I
.
I , Electric. heater
’ on/off
L_-~~~_----_-~~__~--__I~~~-----~_-
--__
Vvatchdog timer
Figure4.
Central
Serial interface
--_a
Input module
module and pump control module block diagram.
One additional feature of the present design that is worth discussing is the implementation of the watchdog timer. The watchdog timer is a single monostable multivibrator which is triggered at system RESET. It then has to be retriggered by the CPU in a certain time or it assumes that the CPU has failed. In the present design a second monostable flip-hop is cascaded to the first one attempting a cold RESTART 10 s after a CPU failure has been detected. Ten seconds is a fairly large time interval to let transient phenomena die out. In this way recovery from transient failures is made possible. If cold RESTART fails then the controller is assumed to have statistically
Control of solar heating systems
125
failed and a flag requesting service is raised. The real-time clock provides the CPU with interrupts every 50 ms and the CPU executes the watchdog timer retrigger routine. The CPU is then forced to return to a predetermined address so that it practically undergoes a hot RESTART when the watchdog timer is retriggered. This has been considered to be necessary to ensure normal CPU operation if retriggering normally takes place. Usual causes of transient failures are spikes that disrupt program execution at a time where the interrupt mask is set and therefore no interrupts are acknowledged. Other causes are partial power failures, atmospheric EM1 etc. Figure 5 shows the main control algorithm. Routines for taking measurements and driving the relays are not shown. A minimum acceptable storage temperature of 40°C is set. If the primary circuit temperature is below that, all secondary circuits are turned off
start t
t Turn off primary circuit
Sampleall requests
Y
.
Turn on primary circuit. Store to thelowesttemperature store
-
Measuretime and estimatethe amount of energyto be received until nightfall
Measure storage
temperatures
storeto requesting “SerS
ri
-
Figure 5
Operational
flowchart.
126
C. C. Lefas
and the primary circuit is on until the minimum acceptable temperature is reached. If the primary circuit temperature is above the minimum acceptable level secondary circuits are turned on until temperature at all stores is raised to that level. After that the primary circuit temperature is permitted to rise to 45°C and all stores are brought one by one to this level. This process is repeated until the maximum acceptable temperature is reached. Users use their own tank. If this is not sufficient, they are serviced as is shown in Figure 5. In practice this system has covered about 60% of the total hot water load. This could be higher but hot water is more in demand during the evening, when insolation is low, or during the night. An additional problem is that in several cases of high demand and/or low insolation the primary circuit temperature drops below the minimum acceptable temperature and the secondary circuits are turned off. This results in delays in the service of hot water requests, which some users find unacceptable and switch on manually the auxiliary heating system. It seems that this problem is eased by the use of larger local storage tanks. Users with tanks larger than 200 1 appeared to have less problems than users with smaller tanks. 2OWOO 1 seems to be the optimum, although this depends strongly on the average demand of each individual user.
6.
Conclusions
Active solar heating systems with distributed storage require a controller with some intelligence to incorporate the operational flowcharts. However solar heating systems are fairly slow and their requirements can be met by g-bit single chip computers so that the cost of the controller is a very small fraction of the cost of the total system.
References McDonald, T. 1978.Energy conservation through adaptive optimal control for a solar heated and cooled building. Proceedings of the First Workshop on Control of Solar Energy Systems, Hyannis, Massachussetts. Merritt, R. 1982. New trends in temperature technology. Instruments and Control Systems, June. Proctor, J. 1984. Temperature transducer IC is linear over wide range. Electronic Design, 5 April. Rorres, C., Orbach, A. & Fischl, R. 1980. Optimal and suboptimal control policies in solar collector systems. IEEE Transactions on Automatic Control, AC-25 (6). Winn, C. & Hull, D. 1978. Optimal control of active solar systems. Proceedings of the First Workshop on Control of Solar Energy Systems, May, Hyannis, Massachussetts, pp. 15 l-l 60. Chris Lefas was born in Athens, Greece, in 1953. He received a degree in physics from the University of Patras, Greece, in 1977 and the MSc degree in automatic control, and the PhD degree in electrical engineering from the University of Manchester, UK. From 1978 to 1980 he worked on the design of improved algorithm exploiting the SSR mode S data-link capabilities. During 1980 and 1981 he worked with the Microprocessor Engineering Unit of the University of Manchester Institute of Science & Technology on the design of advanced automatic test equipment suitable for single board computers. During his national service he joined the Greek Air Force Academy and the National Centre for Space Research, where he remained until 1982. From 1982 to 1984 he was a professor with the Technical and Vocational Teachers School (SELETE) and with the Greek Air Force Academy. He is currently with the Nuclear Research Center “Democritos”, Department of Electronics. ‘Dr Lefas is a member of the Technical Chamber of Greece.
ofMicrocomputer
Applications (1987) 10, 119-I 26
Microprocessor control of distributed active solar heating systems
storage,
C. C. Lefas Nuclear Research Center ’ ‘Democritus “, 153.10, Aghiu Paraskevi, Attiki, Greece
The present paper describes the controller design of a distributed storage solar system. A single-chip computer is used as the central unit. Four types of temperature sensor are considered and IC semiconductor sensors are shown to suit this application best. The pump and auxiliary heat supply is controlled in an on/off fashion by optically isolated relays.
1.
Introduction
Solar water heating for domestic applications is steadily gaining acceptance over the last few years and many solar energy collection systems are available in the market today. Small systems, usually intended for a single user, comprise a single collector with 2-3 m* active area and 100-l 50 1 storage capacity. For larger systems a wider range of options exists where multiple collectors, distributed storage and other techniques are used. A typical situation arises in the use of solar energy for water heating for domestic use in multistorey apartments. The situation today is the use of independent electric boilers for every apartment and, for example, over 10,000 boilers are sold in Greece every year (Merritt, 1982). The introduction of individual solar heating for each apartment is confronted with practical problems such as considerable roof-apartment distance, limited roof space etc. A more feasible approach in this case is to use a larger central solar energy collector which is used to feed every apartment with hot water separately. Local apartment boilers are then used as local storage tanks with auxiliary electric heating (Figures 1 and 2). In such a system user demands are made to a central controller which has to decide whether they can be met by solar heating or auxiliary heating. Temperature of individual tanks has to be monitored and since tanks are local, sensors have to be located at considerable distance from the main controller. Care should therefore be exercised in the choice of the appropriate sensors, the design of the signal conditioning circuits, the design of the controller front end etc., to screen noise interference. The present paper discusses design aspects of controllers used for distributed storage system control. Single-chip computers with considerable computing power are today commercially available. The Intel 8051, Motorola 6801, Zilog 28 and Rockwell 6501 are just a few examples of the products available. The controller described in the present paper is built around the EPROM-fitted 68701 single-chip computer. As will be seen, relatively simple controller configurations satisfy quite demanding control requirements. However. the 119 0745-7 138/87/020119 + 08 $03.00/O
0
1987 Academic
Press Inc. (London)
Limited
120
C. C. Lefas
control strategy differs according to the particular application and only very simple situations are described in the present paper. Circulation in every thermal circuit is controlled by a pump which in turn has to be controlled in an on/off fashion (e.g. Rorres, Orbach & Fischl, 1980).
2.
The system
Figure 1 shows the system configuration. A central solar energy collector is used and the collected energy is distributed to individual users via a primary heat exchanger and secondary thermal circuits. Local boilers are used as storage tanks and a central controller is used for system management. The collector liquid is circulated through the primary heat exchanger forming the primary thermal circuit. Secondary thermal circuits are formed between the primary heat exchanger and local units. The central collector liquid is recirculated until the desired temperature is reached, when one or more sec$ndary circuits are switched on according to current requirements. With this configuration a wide range of solar energy collectors can be used, providing the necessary system flexibility. Collector characteristics, such as the optimum operating conditions, are programmed into the controller. Heat transfer can be in both directions, i.e. from the primary heat exchanger to the storage tanks and vice versa. In this way unused tanks are utilized by the controller to store energy in the case where excess energy is received. This is subsequently retrieved and distributed to other users. An additional factor is that by using separate secondary thermal circuits faults in one of them do not put the entire system out of action but just one loop.
3.
The controller
Figure 2 shows a general block diagram of the controller. The central module is the heart of the controller containing the central processing unit (CPU), the necessary memory
Secondary circuits T-a-2
r
I Storage element
If (Primary
Storage element Pump
T’kZanger circuit
Figure 1.
Multistorage
system configuration.
Multiple primary loops may be used.
Control
of solar heating systems
121
1
Pump control module
Central l
module
Auxiliary heat supply 8 control module
.
I8
ROM
Ig
RAM
Ix tg
I/O
Temperature sensing module
a ’ ‘5
CPU
Counter Serial line interface
1 1
‘5 1.f
Watchdog I$
I
timer
Figure 2.
Controller
Temperature sensing module
1 ’ I block diagram.
(both read only memory (ROM) for program storage and random access memory (RAM) for data storage, a real-time clock, a serial line interface for controller/operator communication, and suitable input/output (I/O) with front end circuitry to interface the sensing modules to the CPU. Most of the above is contained within a single component i.e. the single-chip computer, as will be later outlined. The temperature sensing modules contain the temperature sensors and accompanying signal conditioning circuits and are located near to the sensor. The pump and auxiliary heat supply control units are in practice CPU operated relays providing on/off control signals (and mains power) to the pumps and the auxiliary heaters. Individual users make requests to the controller who then turns electric heaters on if insufficient solar energy is available. The main modules are now described in some detail.
4.
The temperature
sensing
modules
(TSM)
Four different types of sensors have been considered for this application: platinum resistors, thermistors, thermocouples and semiconductor integrated circuit (IC) sensors. Platinum resistors are high quality, highly linear sensors used for precision measurements. They are usually employed in a bridge configuration where resistance changes are converted to bridge output voltage changes. Besides their considerable cost, these sensors have a relatively low temperature coefficient so that the bridge output is relatively low. This requires the use of high quality, very low drift operational amplifiers which tend to increase further total system cost and reduce long term stability. Thermistors offer a low cost alternative. These sensors too convert temperature to resistance and therefore are incorporated in a resistance bridge. The main disadvantage of thermistors is that they are highly non-linear sensors, almost logarithmic. This in turn means that logarithmic amplifiers have to be used, which tend to reduce long term stability of the circuit. However reasonably compensated monolithic log amp components are today commercially available but the cost of these tends to offset the low cost advantage of thermistor sensors. Thermocouples are popular sensors offering a wide measurement range and small thermal capacitance. However, neither of these factors is of major importance for the
122
C. C. Lefas
class of applications considered in the present paper. Measurement range is fairly limited (e.g. 0°C to 100°C) and usually the thermal capacitance of the component whose temperature is being monitored (as in thermal storage tanks) is many orders of magnitude higher than that of the sensor, so that the thermocouple low thermal capacitance advantage is not utilized. As disadvantages of thermocouples are considered their non-linearity, although it is not so severe as that of thermistors, and their low output voltage. Also the thermocouple output is a measure of the temperature difference between the hot and cold junction so that a special signal conditioning circuit has to be used to convert the thermocouple output to absolute temperature measurements. Such circuits, electronic ice points, use their own absolute temperature sensors. However, their cost offsets the low cost advantage of thermocouples and the use of the additional temperature sensor tends to reduce measurement accuracy. Long term drifts in the signal conditioning circuit also reduce measurement quality so that thermocouples have been considered a poor choice for the class of applications considered here. The last alternative investigated in the present paper is the use of semiconductor IC sensors. These sensors exploit the temperature dependence of the transistor base-emitter voltage dependence. Through suitable IC manufacture and trimming, highly linear and stable IC sensors are produced (Winn & Hull, 1978; McDonald, 1978; Proctor, 1984; Merritt, 1982). The signal conditioning circuit is fabricated on the same wafer resulting in low cost, easily interfaceable sensors. Typical IC sensors (e.g. the National LM 35) provide a measurement accuracy of 0.25”C and sensitivity 10 mV/“C. The cost of these sensors is in the order of high quality thermistors. The main disadvantage of these sensors is their limited measurement range which is usually 0°C to 70°C for commercial grade types and - 55°C to + 125°C for military grade types. This however is satisfactory for solar heating applications. Also IC sensors have considerable wafer-ambient heat resistance so that considerable self-heating takes place if they are continuously powered. This problem is overcome by powering the sensor intermittently and by dipping the sensor into the hot liquid thus reducing considerably the can-ambient thermal resistance. On the other hand IC sensors settle rapidly to a steady-state value after power up (in the order of 2 ps) so that the duty cycle can be made very small to avoid self-heating. Since long sampling intervals, in the order of 1 s, are easily tolerated in solar heating applications the duty cycle does not have to be greater than 10m4(100 ps every second). The above reasons show that IC temperature sensors are an appropriate choice for solar heating applications. Figure 3 shows the diagram of the temperature sensing module designed for this application. The sensor is an LM 35 IC acting as a temperature dependent voltage source (reference) giving a 10 mV/K output. The temperature range in which the sensor is set to operate in the present application is 270K to 370K. Measurements are converted to frequency modulated pulse sequence for the present application. Datel’s VFQ-2 monolithic voltage-to-frequency converter is used in the circuit shown in Figure 3. The output frequency varies from l-9.2 kHz and total V/F conversion non-linearity is less than 0.2%. Output pulses are transferred to the main controller via a &20 mA current loop. This provides some protection against noise picked up by the transmission line. In cases where this protection is insufficient low cost fibre links are used. The temperature sensing module is only intermittently powered under CPU control. Total current consumption is approximately 30 mA. The link to the central module is a simple twisted-pair cable. Two of these cables are required, one for measurement transmission and one for the power supply.
Control of solar heating systems 4.7k
-15
v 330k
123
7’5”
10; ref
1.5 k
1OOk
+
.
VFQI
-
1M
Figure3.
5.
The central module
Temperature
sensing module.
and the pump switching module
The central module is based on a single-chip computer, the 68701 which is the EPROM version of the 6801 mentioned earlier. The central module block diagram is shown in Figure 4. The 68701 contains 2 k ROM where the control programs are being stored, 128 bytes RAM for measurement storage, three parallel I/O ports, a serial interface and a timer/counter. Thus the 68701 contains within a single package all logic necessary for this application. A watchdog timer is included for system operation monitoring. Pump control and auxiliary power control is achieved by relays in an on/off fashion. The pump control module and the auxiliary heat supply control module are controlled by the CPU by writing suitable words to the relevant output ports. Since any combination of pumps and auxiliary heat supplies is possible to be on, relays are controlled in parallel. Output pins are electrically isolated from the relays, via optoisolators. Real-time information is essential for this application and is being used, to estimate the amount of energy to be received until nightfall. A real-time clock monolithic device is included in the present controller. The real-time clock operates directly as a CPU peripheral and is interfaced to the CPU in a manner similar to that of the PIA. The CPU operates in the expanded non-multiplexer mode being capable of addressing 256 external locations. The real-time clock occupies 64 locations and the PIAs 16 locations. The rest remains unused for controller expansion. The temperature sensing modules (TSM) are driven through port 1 of the 68701. Data on port 1 are decoded by a 6 to 32 decoder block and transistor switches are driven supplying 15 V power to a single TSM (in the TSM power control unit). The same data are used in the input module to drive a digital 32 to 1 multiplexer block. Current pulses from the TSM arrive in optoisolators whose output is directed via the multiplexer to the timer/counter input of the 68701. Obviously only the TSM whose output is directed to the timer input needs to be powered and this is the reason why only one TSM may be powered at a time.
124
C. C. Lefas
I
Centralmodule
Pump control module
I I
I,
TSM power control unit
-I-+15v,
I
PIA 6821
I
I I I
.
jSW\TCH
I
I
I
I I I
1 I
.
I
I
I T+15Vl
;,
.
I I __-----
l Auxiliary heat supply l control module
I I
.
I
.
J I
.
I , Electric. heater
’ on/off
L_-~~~_----_-~~__~--__I~~~-----~_-
--__
Vvatchdog timer
Figure4.
Central
Serial interface
--_a
Input module
module and pump control module block diagram.
One additional feature of the present design that is worth discussing is the implementation of the watchdog timer. The watchdog timer is a single monostable multivibrator which is triggered at system RESET. It then has to be retriggered by the CPU in a certain time or it assumes that the CPU has failed. In the present design a second monostable flip-hop is cascaded to the first one attempting a cold RESTART 10 s after a CPU failure has been detected. Ten seconds is a fairly large time interval to let transient phenomena die out. In this way recovery from transient failures is made possible. If cold RESTART fails then the controller is assumed to have statistically
Control of solar heating systems
125
failed and a flag requesting service is raised. The real-time clock provides the CPU with interrupts every 50 ms and the CPU executes the watchdog timer retrigger routine. The CPU is then forced to return to a predetermined address so that it practically undergoes a hot RESTART when the watchdog timer is retriggered. This has been considered to be necessary to ensure normal CPU operation if retriggering normally takes place. Usual causes of transient failures are spikes that disrupt program execution at a time where the interrupt mask is set and therefore no interrupts are acknowledged. Other causes are partial power failures, atmospheric EM1 etc. Figure 5 shows the main control algorithm. Routines for taking measurements and driving the relays are not shown. A minimum acceptable storage temperature of 40°C is set. If the primary circuit temperature is below that, all secondary circuits are turned off
start t
t Turn off primary circuit
Sampleall requests
Y
.
Turn on primary circuit. Store to thelowesttemperature store
-
Measuretime and estimatethe amount of energyto be received until nightfall
Measure storage
temperatures
storeto requesting “SerS
ri
-
Figure 5
Operational
flowchart.
126
C. C. Lefas
and the primary circuit is on until the minimum acceptable temperature is reached. If the primary circuit temperature is above the minimum acceptable level secondary circuits are turned on until temperature at all stores is raised to that level. After that the primary circuit temperature is permitted to rise to 45°C and all stores are brought one by one to this level. This process is repeated until the maximum acceptable temperature is reached. Users use their own tank. If this is not sufficient, they are serviced as is shown in Figure 5. In practice this system has covered about 60% of the total hot water load. This could be higher but hot water is more in demand during the evening, when insolation is low, or during the night. An additional problem is that in several cases of high demand and/or low insolation the primary circuit temperature drops below the minimum acceptable temperature and the secondary circuits are turned off. This results in delays in the service of hot water requests, which some users find unacceptable and switch on manually the auxiliary heating system. It seems that this problem is eased by the use of larger local storage tanks. Users with tanks larger than 200 1 appeared to have less problems than users with smaller tanks. 2OWOO 1 seems to be the optimum, although this depends strongly on the average demand of each individual user.
6.
Conclusions
Active solar heating systems with distributed storage require a controller with some intelligence to incorporate the operational flowcharts. However solar heating systems are fairly slow and their requirements can be met by g-bit single chip computers so that the cost of the controller is a very small fraction of the cost of the total system.
References McDonald, T. 1978.Energy conservation through adaptive optimal control for a solar heated and cooled building. Proceedings of the First Workshop on Control of Solar Energy Systems, Hyannis, Massachussetts. Merritt, R. 1982. New trends in temperature technology. Instruments and Control Systems, June. Proctor, J. 1984. Temperature transducer IC is linear over wide range. Electronic Design, 5 April. Rorres, C., Orbach, A. & Fischl, R. 1980. Optimal and suboptimal control policies in solar collector systems. IEEE Transactions on Automatic Control, AC-25 (6). Winn, C. & Hull, D. 1978. Optimal control of active solar systems. Proceedings of the First Workshop on Control of Solar Energy Systems, May, Hyannis, Massachussetts, pp. 15 l-l 60. Chris Lefas was born in Athens, Greece, in 1953. He received a degree in physics from the University of Patras, Greece, in 1977 and the MSc degree in automatic control, and the PhD degree in electrical engineering from the University of Manchester, UK. From 1978 to 1980 he worked on the design of improved algorithm exploiting the SSR mode S data-link capabilities. During 1980 and 1981 he worked with the Microprocessor Engineering Unit of the University of Manchester Institute of Science & Technology on the design of advanced automatic test equipment suitable for single board computers. During his national service he joined the Greek Air Force Academy and the National Centre for Space Research, where he remained until 1982. From 1982 to 1984 he was a professor with the Technical and Vocational Teachers School (SELETE) and with the Greek Air Force Academy. He is currently with the Nuclear Research Center “Democritos”, Department of Electronics. ‘Dr Lefas is a member of the Technical Chamber of Greece.