AU669293B2 - System to control a solar water heater - Google Patents

Description

1100/0 1 Io

AUSTRALIA

6 b Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT 4nvention Title: f r following statement is a full description of this invention, including the best method of performing it known to me:- 0:00 System to control a solar water heater Thermal solar systems can be classified to be active or passive systems. In a passive system compared to an active system no pumps or equivalent are used to transport thermal energy from the collector to the storage tank. The heat is transferred by natural convection, conduction or radiation. Both active and passive systems can be filrth;er divided into mains pressure and low pressure systems. In a mains pressure system the tank is directly connected to the mains water supply and the pressure in the tank is in general equal or greater than mains pressure. Cold water enters the tank as hot water is used from the tank. In general the volume of water in the tank is constant. In a low pressure system the pressure in the tank is independent cf he mains pressure and depends mainly on the level of water in the tank.

99 9 9 9 9 Solar hot water systems primarily use the energy radiated by the sun to heat up water.

15 Although a large percentage of the required energy can be gained this way, it is not economical nor technical sensible to design hot water systems to rely completely on solar energy. They woud need to use very large tanks and very good insulation to supply hot water during periods of low solar radiation. So at present, systems are 9*° designed to store enough hot water equivalent to an average consumption of 1 or 2 days. During longer periods of low solar radiation or high consumption of hot water a so called booster is used to heat up the water in addition to the solar collectors. This booster can be a gas or oil burner or an electrical heating element. In the following it is assumed the booster is an electrical heating element, even though it can be applied to gas and oil burners too. The booster is conmmonly controlled by a thermostat and a manual switch connected in series with the thermostat. The hooster can be switched on by the user. If the temperature is below the set point of the thermostat the booster will heat up the water until the set temperatur e. g. 60degrees Celsius, is reached. Then I -the thermostat switches the booster off. If the temperature drops below another set point, e.g. 55degrees Celsius, the thermostat would switch the booster on automatically.

The problem with this configuration is, that the user, in most cases, will switch on the booster, when the temperature of the water is already unacceptable low. I-e then has to wait until the booster has heated up the water sufficiently. This takes usually more than an hour. In many cases the manual switch is then left on, because the user has forgotten to turn it off again. In this case mainly electricity is used to heat the water.

To solve the problem, that the booster is left on unintentionally, a one shot switch is known to be used. This switch, once activated by the user, turns off itself after a time, sufficient to heat up the water once. But it still needs to be activated in time by the user, 5 Another known configuration is to use a timer to switch the booster on or off at fixed times of the day. While this configuration ensures, that hot water is available at o 15 specified times, it does not allow to use solar energy efficiently.

It is an olbect of the invention to provide a system to control the booster of a solar water heater in a way to achieve convenient, automatic operation and at the same time use solar energy efficiently. Another benefit is, that the system \will make it possible to reduce the size of the water tank, which will lead to a reduction in initial cost, energy loss. weight and space requirements of the hot water system.

The system described by the invention is a system to control a passie or active mains pressure solar hot water system. The control system consists of a temperature sensor to measure the water temperature in the solar hot water tank, an electronic controller, which is able to switch the booster of the solar hot water system on or olf and performing a special control method to activate the booster. The control method li^ having all of the following characteristics from a to c: a The control method regulates the tank water temperature by switching the booster on or off.

b The temperature of the water in the tank is adjusted by the controller urng closed loop control. wherein the value the temperature is adjusted to, depends on the time of day. This value of temperature versus time of day is further referred to as the temperature profile.

c The temperature profile is set in a way to make efficient use of solar energy.

The control method is based on the fact, that the solar collector efficiency of known collectors decreases with the temperature of water fed into the collector and that energy lossc. in the hot water system increase with water temperature. Thus the hot water system is most efficient at low water temperatures. On the other hand the user of the hot water system general expects it to supply water at an acceptable temperature at all times. Although the amount of water required varies depending on the time of 15 day. In most cases the main demand will be in the morning and then later in the evening. The user in general adjusts the temperature at the water outlet e.g. the shower by mixing hot water from the tank with cold "w.'ater to re.eive water of the temperature he desires. Hie will consequently use less water out of :ie hot water tank. if the tank water temperature is higher and ,ice versa. If the trnk water temperature is low. for example 35degrees Celsius. he will possibly only use water out of the tank and no cold water.

It is therefore proposed by the invention to vary the temperature in the tank according to the expected demand for hot water. It is possible to calculate the required temperature profile for an expected demand profile using known physical principles and making an assumption for the cold water temperature.

Simple example how to calculate the temperature profle: Consumption pattern: Time of day Consumptiou 7:00h to 8:00h lOOfitrcs W 4Odegrees C' 20:00h to 21:00h 250litres 0a, 4Odeg'ees C Assumptions for the exaniple: Tank size: 1 Cold water temperature: 25degrees C Minimum acceptable temperature: 35clcgrees C Under ideal conditions: no ecrgy loss, ideal stratification, no error in the tcmnperature measuremnent, specific heat independent of temperature. specific maiss independent of temperature etc. the following equations ire valid: ml -mr2*c*('l2-'l3) rquired hot (tank) water temperature T'2 I 'l'3-m I *11 m2*T3) n12 m3 =ru I -I2 ,3~v -v2 m2 -mass of hot water nm3 mass of warmi water VI volume of cold water Q volumec of hot water v0 volume o1' warmi water SRAO~ TI cold water temperature NT0 T2 hot (tank) water temperature T3 =resulting warm water temperature c specific heat of water cv specific miss of wvater Time of day O:OGit to 7:00h 7:00h to 8:00h 8:0011 to 20:00h 20:00h to 21:00h 21:00h to 0:001i Consumption pattern: Consumption Olhtres lO0litres 4 4Odegrecs C Olitres 250l-itres 1il 4Odlegrees C Oliters Temperature profile: min. Tank temperature 35degrees C 40degrees C 35dlegrees C 5Odlegrecs C 35degrees C V) B~etween 7:00h and 8:0011 the lo0litres of water 40clegrces C call be delivered Out or the tank without an irrrceasc in the temperature value. Depending on the position of the temp~erature sensor and the actual water temperature inside the tank the boostor mnay be switched on by the controller to maintain the temperature at 40Odegrees C.

From 8:0011 to 20:00h no Nvater is taken firom thle tank. Thcl controller Tnaintains the mninimium temperature of 35degreos C, D~uring this period on a sunny day the wvater be heated by solar enero' up to 65dlcgreces C. In this case thle booster will not be sv\ itcheci onl by thle controller' when between 20:00h iand 21 :00h 2501iitrcs at 4Odegroes C are required, because the tank holds more thermal cen than requlired. If' thle sun (lid not shine the booster wvill be switched on so that the tank holds I 50litre,., at .s C before they are required. I 5Olitres at 5fldegrees firom the tank are added to I O0litres of co!l w-otL' at 25dlegrees C to provide 250litres at 40degrCS' C'.

RA4/ T~N o$ The above method is only one of many methods to generate the temperature profile, other methods e. g. an empirical adjustment by a user or the controller itself are also possible.

The temperature profile can be selected out of a number of different profiles stored in the mer--ry of the controller. These profiles represent typical consumption pattern for differe,, ,ituations, for example normal working days or holiday periods.

The controller selects a temperature profile based on the tank water temperature changes measured during the day. Another way to select a profile is, that the user selects a profile most suitable to his specific situation. One method to select a profile by the user is by means of a switch.

Another method to establish a temperature profile is that the controller measures the hot water consumption for example on an hourly basis for one day and then calculates 15 the temperature profile based on the measurements. This prolile will be stored in the controllers memory and can be used for the following day. The consumption can be measured using a flow sensor, which generates an output signal proportional to the amount of water flowing out of the tank. Another solution is to measure the consumption based on the temperature drop when hot water is taken out ofl the tank and cold water enters the tank fiom the mains water supply.

To further improve the quality of the consumplion pattern and the resulting temperature profile it is proposed by the invention to use a filter algorithnm, e.g. a low pass filter for the consumption on an hourly basis. This filtler algorithm is applied to every pattern value of the pattern. Assuming there is a pattern value for every1 hour of' the day kept in the memory of the controller, then there would be 24 separate pattern values kept in this pattern.

S The 9 o'clock consumption would be calculated as follows: new Pattern value(9h)=(7*old Pattern value(9h)/8)+(new Consumption(9t" Wherein: new Pattern value(9h) is the new value to be stored in the Pattern for nine o'clock in the morning, old Pattern value(9h) is the pattern value for nine o'clock of the day before and new Consumption(9h) is the consumption for nine o'clock of the current day. The factor and denominator in the above equation are typical example values only.

In general the controller switches the booster on or off depending on the result of the comparison between the actual tank water temperature and the current value out of the temperature profile. For example, when the tank water temperature is lower than requested by the temperature profile the controller will activate the booster. The specific fonvard transfer characteristic of the controller in the closed loop (temperature 15 sensor controller booster tank water temperature sensor) is not critical.

as long as it controls the tank water temperature to be close to the temperature values given by the temperature profile for any given time of day. The actual temperature of the tank water can be higher than the value given by the temperature profile, because solar energy can heat the water up independent of the booster being switched off by the controller in this case. Examples of transfer characteristics are proportional (IP).

l)proportional derivative (PD) and hysteresis. To prevent a rapid switching between on and off a hysteresis function can be used.

A further development of the invention is to incorporate the first derivative of the tank temperature and the temperature prolile as a function of time into the decision to switch the booster on or off. For example, it will turn on the booster, if the temperature is below a given threshold and the consumption lattern shows, that most likely water will be needed soon and the temperature gradient indicates, that the temperature rise is to slow using solar energy only to meet the demand.

Another extension of the invention is a function, which switches the booster on, if the temperature at a specific time is below a defined value. After an upper temperature limit "Tm" is reached the booster is switched off again. The upper temperature limit is variable and depends on the time of day. This additional function can be uswd in advance to an expected consumption peak.

According to the invention the heating element itself can be used as a temperature sensor, if it will be made from a material, which has a temperature coefficient of its electrical resistance large enough to give a usable temperature information by measuring the electrical resistance of the heating element.

A further development is to use the thermostat set point to calibrate the temperature sensor. The set point of the thermostat represents a defined temperature, which is used as a reference point for the temperature sensor. For example the set point is Celsius, when the thermostat switches off at the same time the temperature sensor has a electrical resistance of 1000ohm. This measurement can be used to compensate for tolerances in the sensor or the measuring circuit, and to detect failures in the measurement circuit or sensor.

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One way to gain a time of day information i' ;o synchronise the internal clock of the controller to the solar activity detected by :mperature rise caused by solar activity.

The time of day information is generated oy the internal clock of the controller e.g.

based on a quartz oscillator. This internal clock needs to be set or synchronized with the actual time of day. This synrchoni.altion is required alter the controller has lost the lA time of day information c. g. due to a power Ifilure. The synchronization can be done 4

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by a user or in other known ways to set a clock or by synchronising the internal clock of the controller with the solar activity detected by the temperature rise caused by solar activity.

Mieasurements on typical solar systems show, that the maximum tank temperature on sunny days will be reached at a certain time of day. This time may vary depending on the orientation of the solar collectors, season, location and hot water consumption.

Because the controller is controlling the booster it has the information, if a temperature rise inside the tank has been mainly caused by solar activity or by the booster. If the to booster in a typical solar hot water systems is switched on, the temperature rise is mainly caused by the booster and only a small contribution is made by solar energy, An example of this synchronisation is as Iollows: 'T'he controller has detected, that the maximum temperature in the tank will be eiched at 14:00h, if the booster has not been switched on. The controller stores this information in its non volatile memory. Now the time of day information is lost due to a power filure. In this case the controller wvill rcsynchronize the internal clock using i the information that the maximum tank temperature will be icached at 14:00h11. So when the controller detects the maximum temperature during 24h it will set the internal clock to 14:00h. D)uring the synchronisation period the booster should be kept switched off.l Also the gradient of' the temperature rise. the maximum temperature value and the number of hours a temperature increase has been detected Can be used to prevent setting the clock to an incorrect value dlue to weather conditions.

Although many dillfrent possibilities to realise the liFunctions of the controller exist, for example analogue circuits or discrete digital circuits or a combination ol both, one pref eired embodiment is to use a microcomputer to implement the control method. A power switch is required to switch the booster, because microcomputer are not yet able to control the booster directly. The power switch can be a relay or a semiconductor switch. The power switch can be located in the controller or external. The microcomputer could be, but is not limited to, a single chip microcomputer, having all the necessary functions like oscillator, central processing unit (CPU), program and data memories, timers and AD converter plus digital in- and outputs on a single silicon chip.

Fig. 1 shows one example of a passive mains pressure solar hot water system including the control system.

Fig. 2 shows one example of the internal configuration of the controller.

Fig. 3 shows die electrical heating clement used as heating element and temperature 1 sensor.

Referring to Fig. 1 a hot-water tank 1 is connected to the cold-water inlet 5. which supplies the tank with water at mains pressure, and a hot-water outlet 4. A solar collector 2 is also connected to the tank and is used to heat up the water in the tank. In the example drawn, the water is cycled through the solar collector by natural convection, thermosyphon. It could also be done by a pump. then it would be an active system. An electrical heating element 3 is mountcd in the tank together with a thenrostait 8. A controller 6 is supplied with electrical power 7, for example 240V AC, and controls the heating clement 3 in the tank. The controller receives input 2" information from a temperature sensor 10 in the tank and an optional flow sensor 9 at the hot-water outlet. It is also possible to mount the flow sensor at the inlet of the tank.

A flow sensor generates a signal proportional to the amount of water flowing through the sensor. The heart of the controller is a microcomputer, which can be programmed and also has a memory to hold information about consumption patterns etc..

In Fig. 2 the internal configuration of a typical controller 1 is shown. The controller is supplied with electrical energy at terminal 5 and 20. Terminal 20 should be connected to the neutral and terminal 5 to the active side of the supply, for example 240V AC.

Block 8 converts the external supply voltage to a supply voltage suitable for the controller, for example 5Volt DC. Block 8 also provides a reference voltage, which is connected to the input termnninal 3 via a resistor 7. Block 9 represents a single chip microcomputer. It contains a central processing unit 19, which has access to an A'D converter 13, a timer counter unit 14, a program memory 15, a data memory 16. an oscillator and reset circuit 17 and a digital output 11. The AD converter is used to measure the voltage at terminal 3 of the controller. Block 10 is a signal conditioning 15 circuit, which filters the input signal using a low pass filter. In the control system the temperature sensor is connected tc terminal 3 and 22. Although many different .9 temperature sensors are known, a temperature dependent resistor (NTC or PTC) is proposed here. A change in water temperature results in a change in the voltage at terminal 3. The result of the analogue-to-digital conversion is further processed by the 20 microcomputer to gain the temperature information, This normally includes liicarisation, digital filtering and sensor fault detection. The counter timer unit is used to measure the signal generated by the flow sensor, which is generally a lifrequency, Shis signal is connected to terminal 2 and 23 of the controller. Block 6 contains a comparator with hysteresis and an input protection circuit to form signal, which can be used to measure the frequency. The program memory 15 holds the program, which is processed by the microcomputer. At power up the reset and oscillator circuit ensures, that the microcomputer starts the processing of the program at the right AL memory location. The oscillator also generates the clock signal for the central processing unit. The signal also serves as a time reference for the controller. The Block 11 is the output port, which is used to transfer the output signal, the information to switch the booster on or off, to the relay driver 18. This relay driver controls the relay 12, which connects electrical power from terminal 5 of the controller to terminal 4, if it is activated. The heating element of the solar hot water system is connected to terminal 4 and 21 of the controller.

Fig. 3 shows the system using the heating element 1 as a temperature sensor and heating element. The heating element consists of a material which changes its resistance with temperature.

In a typical application the controller 4, which could be the same controller as described in Fig. 2. switches the relay 3 into position 6 to measure the temperature and in position 5 to heat the water. The relay is switched by the signal at output 10 of the controller. W\hen switching from position 5 to 6, the controller waits a certain time 15 before the resistance measurement is performed to ensure, that the element has cooled down to the water temperature. The temperature is measured by measuring the electrical resistance. The input 9 of the controller is internally connected to a reference voltage in the controller via a resistor. This resistor and the resistance of the heating clement are forming a voltage divider. The voltage at input 9 is converted by the controller to a value representing the temperature of the heating element. Switch 2 is the thermostat switch. The thermostat switch opens, if the temperature is above the switch-off point. In this case the measurement of the heating element's resistance is not possible. Because the switch-off and switch-on temperature of the thermostat is above the temperature range of interest. this is no problern. At 7 the active and at 8 the neutral tenninal of an electrical lower supply is connected, e.g. 240V AC.

Claims (9)

1. A control system to control a passive or active mains pressure solar hot water system, the control system consisting of a temperature sensor which measures the water temperature in a solar hot water tank, an electronic controller which receives temperature information from the temperature sensor and which is able to switch a booster of the solar hot water system on or off. either direct or by means of an external power switch, and the electronic controller performing a special control method to activate the booster, wherein the booster can be either electricity, gas or oil powered and is switched on or off by an electrical signal and the control method having all of the following characteristics, the temperature of the water in the tank is adjusted by the controller using closed loop control. wherein the value the temperature is adjusted to, depends on the time of day, this value of temperature versus time of day 15 being a temperature prolile, which can be set in a way to make efficient use of solar energy,

2. A control system according to claim 1. wherein the temperature profile is dclined by calculation or any other suitable method to enable the solar hot water system, within its physical limitations, to satisfy the demand given by a consumption pattern the consumption pattern being the consumption of water from the hot water system as a unction of time.

3. A control system according to claim 2, wherein the consumption pattern is measured by the controller using a flow sensor or any other method suit ble to obtain the consumption pattern and wherein the temperature profile is calculated by the controller based on this information.

4. A control system according to claim 2, wherein the consumption pattern is generated based on the temperature drop caused by cold water entering the tank. when hot water is drawn fiom the tank.

5. A control system according to claim 3, wherein a low pass filter algorithm is applied to the consumption pattern so that new consumption information measured is combined with previous consumption infonnation already in the consumption pattern.

6. A control system according to claim 1. wherein the booster is switched on by the controller, if the temperature at a specific time is below a defined value and the booster is switched off again after an upper temperature limit is reached. the upper temperature limit being variable depending on the time of day.

7. A control system according to claim 1, wherein the temperature sensor is calibrated 15 using the switch-olf-temiperature or switch-on-temperature or both of a thermostat as a reference, the switch-off-temperature being the temperature at which the thermostat switches off and the switch-on-temperature being the temperature at which the *A thermostat switches on.

8. A control system according to claim 1. wherein different temperature profiles are stored in a memory of the controller and selected by the controller or the user of the hot water system.

9. A control system according to claim 1. wherein the booster is also used as the temperature sensor. A control system according to claim 1, wherein the time ol day information is Ssychnchronized with the solar activity. (Wolfgang Reisch) (Date) C. CC C C CC *9 C C C C CC., Cs C C C C CCC. C. C C 'C A* CC s~ CC C C C*C C CC C CO IC C I bt-w 0 ~NT O~ ABSTRACT A system to control a solar water heater is disclosed. The system controls the booster of a passive or active mains pressure solar water systemn. The invention describes a s-Ystem to provide con-venient, automatic operation and use solar energy efficiently. This is achieved by a special control method to regulate the temperature in the solar hot water tank The control method is using closed loop control and a sct value. which varies depending on the time of day, called a temperature profle. The invention further proposes to d~crive the tenpcrature profile fromn a consumption pattern. 0* S S0 0. %:S so