Solar Photovoltaic Water Heating System

A solar photovoltaic water heating system is disclosed having a photovoltaic solar panel array, a storage tank containing water to be heated, a resistance heating element in the water to be heated. The water heating system matches the load resistance of the resistance heating element to the power that is available from the photovoltaic solar panel array in order to maximum energy transferred to the water in the storage tank.

DETAILED DESCRIPTION OF THE INVENTION

Turning toFIG. 1, a solar photovoltaic water heating system1is shown. The solar photovoltaic water heating system1includes a photovoltaic panel array31consisting of individual solar photovoltaic panels18, a water storage tank10with resistance heating elements15, a programmable control system16, and an alternating current power source32. The control system16is connected between the storage tank10and the photovoltaic panel array31and the alternating current power source32in order to control the delivery of power from either the photovoltaic panel array31or the alternating current power source32to the resistance heating elements15. One of the heating elements15may be a standard alternating current resistance heating element that is only connected to the alternating current power source32. Alternatively, both heating elements15may be alternating current and direct current compatible, resistance heating elements such as the resistance heating element15shown inFIG. 5and described below.

The photovoltaic panels18are conventional and produce direct current power when exposed to irradiance19from the Sun20. The amount of direct current power produced by each photovoltaic panel18depends on the level of irradiance19impinging on the photovoltaic panels18. Consequently, because the level of irradiance19varies based on the time of day and atmospheric conditions, the level of direct current power produced by each photovoltaic panel18varies accordingly.FIGS. 13A-13Carc graphs that show the typical energy available from the photovoltaic array31for different voltages, power levels, and averages.

The water storage tank10is generally conventional and is shown inFIGS. 2-4. The water storage tank10includes a cold water inlet22extending to near the bottom of the storage tank10for the introduction of cold water12into the storage tank10. The storage tank10further includes a hot water outlet21extending into the upper part of the storage tank10. A mixing valve17is connected to the hot water outlet21and to a cold water shunt33in order to provide a mixture of hot and cold water to the hot water outlet11, which in turn is connected to a consumer water system (not shown). The mixing valve17is thermostatically controlled to ensure that the hot water11is not too hot for a consumer water system such as residential hot water system. The operation of the mixing valve17allows the water in the storage tank10to be raised substantially higher than a normal residential consumer water system thereby allowing the solar photovoltaic water heating system1to store more energy in the storage tank10. The water storage tank10further includes a cover25for housing and protecting the control systems16and associated wiring. A pressure relief safety valve23is also connected to and in communication with the interior of the water storage tank10. If the pressure inside the water storage tank10exceeds a preselected level for the relief safety valve23, the relief safety valve23opens so that the pressure inside the water storage tank10can be relieved.

The resistance heating elements15are located in the water storage tank10at one or more levels to ensure consistency of the water temperature in the water storage tank10. As shown inFIG. 5, the resistance heating element15comprises, for example, four individual resistance heating rods29connected through a stainless steel fitting26. The resistance heating rods29may be formed of stainless steel, inconel, carbon steel, or copper. The stainless steel fitting26is threaded into the side of the water storage tank10to form a water and pressure tight seal. Each individual resistance heating rod29has a pair of connecting wires27that allow each resistance heating rod29to be individually connected to either the photovoltaic array31or the direct current power source32through a switching circuit, such as one of switching circuits34(FIG. 6) and 35(FIG. 7).

The switching circuits34and35are controlled by the control system16in order to select the optimum load resistance, such as direct current load resistance RloadDC37or RloadDC39for the direct current power source31(photovoltaic array31) or alternating current load resistance RloadAC36or RloadAC38for the alternating current power source32(public power grid). The switching circuit34illustrates a first configuration and is configured to provide seven resistance values for the direct current load resistance RloadDC37and one resistance value for the alternating current load resistance RloadAC36. The switching circuit34comprises a DC switch41, an AC switch42, and a resistance array including fixed value resistors R4, R6, R7, and R9with their associated switches. The resistors R4, R6, R7, and R9represent each of the four individual heating rods29in the resistance heating element15. The control system16opens and closes the DC switch41, the AC switch42, and the switches associated with each of the resistors R4, R6, R7, and R9in order to select the optimum direct current load resistance RloadDC37for the direct current power source31or the optimum alternating current load resistance RloadAC36for the alternating current power source32. Selection of the optimum load resistance RloadDC37or RloadAC36maximizes the energy delivered to the water in the water storage tank10by either the direct current power source31or the alternating current power source32.

Likewise the switching circuit35illustrates a second configuration and is configured to provide14resistance values for the direct current load resistance RloadDC39and one resistance value for the alternating current load resistance RloadAC38. The switching circuit35comprises a DC switch43, an AC switch44, and a resistance array including fixed value resistors R11, R12, R13, and R14with their associated switches. The resistors R11, R12, R13, and R14represent each of the four individual heating rods29in the resistance heating element15. The control system16opens and closes the DC switch43, the AC switch44, and the switches associated with each of the resistors R11, R12, R13, and R14in order to select the optimum direct current load resistance RloadDC39for the direct current power source31or the optimum alternating current load resistance RloadAC38for the alternating current power source32. Selection of the optimum load resistance RloadDC39or RloadAC38maximizes the energy delivered to the water in the water storage tank10by either the direct current power source31or the alternating current power source32.

With respect to configuration 1, illustrated by switching circuit34and by the table shown inFIG. 12A, the formula set forth below establishes the values for the fixed resistors R4, R6, R7, and R9(the resistance heating rods29of the resistance heating element15). Once the values for the resistors R4, R6, R7, and R9have been established, the control system16runs an algorithm to open and close the switches in the switching circuit34to produce the optimum load resistance for the power source that is available. In order to determine the values for the resistors R4, R6, R7, and R9, the formula first solves for the single resistor R4(one of the four heating rods29of the heating element15). The value for the resistor R4is then used in a ratio determination method, described in greater detail below, to determine the three other fixed resistance values for resistors R6, R7, and R9(the other three of the four heating rods29of the heating element15). The fixed resistance values of resistors R4, R6, R7, and R9are used by the control system16in various single and parallel connection arrangements to create up to seven different direct current load resistance values R1, R2, R3, R4, R5, R6, and R7, and one alternating current load resistance value R8(see table,FIG. 12A). The resistance values R1-R7are used in connection with the direct current power source31, and the resistance value R8is used in connection with the alternating current power source32. The resistance value R8is the optimum value for the alternating current power source32, and the resistance value R8is the lowest available resistance and results from the parallel connection of all four fixed resistors R4, R6, R7, and R9(resistance heating rods29of the resistance heating element15). R1is the optimum resistance value for the direct current power source31where the direct current power source is delivering maximum energy at 1000 watts per square meter of solar irradiance19. SeeFIG. 8, first line (resistance=23.82 ohms) andFIG. 12A(R1=23.82 ohms).

The formula for determining the value of the fixed resistor R4for configuration 1 (FIG. 12A) is as follows:

PW=Array Power in watts at solar irradiance level (w)

MPP=Maximum Power Point (MPP—the optimum transfer of energy from the power source to the resistance load)

W=Watts delivered to the load

W/m2=Watts per square meter of solar irradiance

VW=Photovoltaic array MPP voltage at a stated level of solar irradiance (w)

IW=Photovoltaic array MPP current at a stated level of solar irradiance (w)

Mv=Slope of linear equation for calculating Voltage (≈0)

Bv=y-intercept of linear equation for calculating Voltage (≈VW)

The majority of mono-crystalline photovoltaic panels follow the approximation for input energy between 200 and 1000 W/m2.

Formula for determination of optimal value of R4is as follows:

Once the value of R4for configuration 1 (FIG. 12A) is determined, the ratio formula is used to select the optimal resistance values of fixed resistors R6, R7, and R9:

R4is used to determine R6, and R7wherein

R6is a multiple of two (2) times the value of R4

R7is a multiple of four (4) times the value of the value of R4.

R9is determined by inserting the derived value for optimal AC operation as follows:WAC=desired power output for alternating current power sourceVAC=voltage for the alternating current power sourceIAC=WAC/VACRAC=VAC/IACExample: WAC=3500 watts

The calculation then proceeds as follows: IAC(current)=3500/240=14.58 mps

R4=Value of resistance to be determined using proprietary formula*

R0=∞ (all off)* (these are the fixed values of resistance for the four (4) pronged heating element15** (the R8value being the optimum resistance for operation on grid alternating current power)

With reference toFIGS. 6 and 12A, the direct current load resistance RloadDC37and the alternating current load resistance RloadAC36for the switching circuit34are selected by the switch configurations defined by the binary bits, in the column “Switch RS.” Each of the binary bits indicates the status of the associated switches for the fixed resistors R4, R6, R7and R9. For example, for the direct current power source31, the control system16, based on the output from direct current power source31, closes the DC switch41, opens the AC switch42, and sets the binary code to 1110. With the binary code set to 1110, the associated switches for fixed resistors R4, R6, and R7are closed, and the direct current load resistance RloadDC37equals the parallel combination of fixed resistors R4, R6, and R7, equals resistance R1, and equals 23.82 ohms (FIG. 12A, line R1). As a further example, for the alternating current power source32, the control system16closes the AC switch42, opens the DC switch41, and sets the binary code to 1111. With the binary code set to 1111, the associated switches for fixed resistors R4, R6, R7, and R9are closed, and the alternating current load resistance RloadAC36equals the parallel combination of fixed resistors R4, R6, R7and R9, equals resistance R8, and equals 16.46 ohms (FIG. 12A, line R8).

With respect to configuration 2, illustrated by switching circuit35(FIG. 7) and by the table shown inFIG. 12B, the formula set forth below establishes the values for fixed resistors R11, R12, R13, and R14(the four resistance heating rods29of the resistance heating element15). Once the values for the resistors R11, R12, R13, and R14have been established, the control system16runs an algorithm to open and close the switches in the switching circuit35to produce the optimum load resistance for power source that is available. In order to determine the values for the resistors R11, R12, R13, and R14, the formula first solves for the single resistor R11(one of the four heating rods29of the heating element15). The value for the resistor R11is then used in a ratio determination method, described in greater detail below, to determine the three other fixed resistance values for resistors R12, R13, and R14(the other three of the four heating rods29of the heating element15). The fixed resistance values of resistors R11, R12, R13, and R14are used by the control system16in various single and parallel connection arrangements to create up to 14 different direct current load resistance values R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13, and R14and one alternating current load resistance value R15(see table,FIG. 12B). The resistance values R1-R14are used in connection with the direct current power source31, and the resistance value R15is used in connection with the alternating current power source32. The resistance value R15is the optimum value for the alternating current power source32, and the resistance value R15is the lowest value and results from the parallel connection of all four fixed resistors R11, R12, R13, and R14(resistance heating rods29of the resistance heating element15). R1is the optimum resistance value for the direct current power source31where the direct current power source is delivering maximum energy at 1000 watts per square meter of solar irradiance19. SeeFIG. 8, first line (resistance=23.82 ohms) andFIG. 12B(R1=23.82 ohms).

The formula for determining the value of the fixed resistor R11for configuration 2 (FIG. 12B) is as follows:

PW=Array Power in watts at solar irradiance level (w)

MPP=Maximum Power Point (MPP-the optimum transfer of energy from the power source to the resistance load)

W/m2=Watts per square meter of solar irradiance

VW=Photovoltaic array MPP voltage at a stated level of solar irradiance (w)

IW=Photovoltaic array MPP current at a stated level of solar irradiance (w)

Mv=Slope of linear equation for calculating Voltage (≈0)

Bv=y-intercept of linear equation for calculating Voltage (≈VW)

The majority of mono-crystalline PV panels follow the approximation for input energy between 200 and 1000 W/m2.

RK≈Optimum resistance for Maximum Power delivery at 1000 W/m2

Formula for determination of optimal value of R11is as follows:

Once the value of R11for configuration 2 (FIG. 12B) is determined, the ratio formula is used to select the optimal resistance values of fixed resistors R12, R13, and R14:

R12is a 1.20 ratio of R11.

R13is a 1.50 ratio of R11.

R14is a 2.00 ratio of R11.

Thus creating the resistance values:

R8=1/((1/R12)+(1/R13)) Not needed as the step difference is negligible

R11=Fixed resistance value derived using proprietary Formula of Claim1.*

R12=Fixed resistance value equal to 1.20×R11*

R13=Fixed resistance value equal to 1.50×R11*

R14=Fixed resistance value equal to 2.00×R11*

R15=1/((1/R11)+(1/R12)+(1/R13)+(1/R14))*** these are the fixed values of resistance for the four (4) pronged heating element** this value is used for AC operation using grid power

With reference toFIGS. 6 and 12B, the direct current load resistance RloadDC39and the alternating current load resistance RloadAC38for the switching circuit35are selected by the switch configurations, defined by the binary bits, in the column “Switch RS.” Each of the binary bits indicates the status of the associated switches for the fixed resistors R11, R12, R13, and R14. For example, for the direct current power source31, the control system16, based on the output from direct current power source31, closes the DC switch43, opens the AC switch44, and sets the binary code to 1010. With the binary code set to 1010, the associated switches for fixed resistors R11and R13are closed, and the direct current load resistance RloadDC39equals the parallel combination of fixed resistors R11and R13, equals resistance R6, and equals 35.72 ohms (FIG. 12A, line R6). As a further example, for the alternating current power source32, the control system16closes the AC switch44, opens the DC switch43, and sets the binary code to 1111. With the binary code set to 1111, the associated switches for fixed resistors R11, R12, R13, and R14are closed, and the alternating current load resistance RloadAC38equals the parallel combination of fixed resistors R11, R12, R13, and R14, equals resistance R15, and equals 19.85 ohms (FIG. 12B, line R15).

Turning toFIGS. 9A and 9B, the control system16is programmed to implement the control method50that controls the selection of the optimized resistance for either the direct current power source18or the alternating power source32. The control method50has a Main Command Logic Routine52, a Sampler Logic Subroutine54and a Read Power Subroutine56. The control method50begins at loop step58of the Main Command Logic Routine52. The internal processor of control system16steps through the logic sequence continually while the system has power applied and is in operation. Using the example of the operational resistance values of the 4 resistor configuration 2 ofFIG. 12B, each resistance value corresponds to a “Mode” of operation of the solar photovoltaic water heating system10. There are therefore15distinct operating modes each corresponding to a lineup of resistors that have been connected in parallel to set a different resistance value depending on the direct current power available for 14 of the modes and one mode where all fixed resistance values R11, R12, R13, and R14are switched on in parallel thereby creating an optimized resistance for operation of the resistance heating element15in the storage tank using power supplied by the alternating current power source32.

Starting from loop step58of the main command logic routine52, the routine52moves to step60where the routine52sets the resistance to Mode14(R14of configuration 2,FIG. 12B), the highest value for the resistance of the resistance heating element15. From step60, the routine52moves to step62, where routine52imposes a two second delay. From step62, the routine52moves to step64, and branches to step104of the Read Power Subroutine56. From step104, the subroutine56moves to step106. At step106, the subroutine56uses the highest resistance value R14to read power available from the photovoltaic array31. The power (P) available from the photovoltaic array31is determined by measuring the voltage (V) from the photovoltaic array31and then calculating the power from the photovoltaic array31by using the formula Pcurrent=V2/R wherein resistance is a function of the Mode at the time Pcurrent is determined. Once the current power (Pcurrent) from the photovoltaic array31has been determined at step106, the subroutine56moves to step108and branches to step66of the routine52.

At step66, the routine52compares the previously determined power from the photovoltaic array31to the current power from the photovoltaic array31(Pprevious=Pcurrent). From step66, the routine52moves to decision step68, where the routine52determines if the current Mode is less than 15 and the routine54at step94is sampling the power from the photovoltaic array31using combinations the fixed resistor R11, R12, R13, and R14to create progressively lower resistances. If at step68the routine52determines that the current Mode is less than 15 and subroutine54at step94is sampling to a lower resistance, the routine52follows the yes branch to step72. If on the other hand, the routine52determines that Mode is greater than 15 or the subroutine54is sampling to higher resistances, the routine52follows the no branch to step70. At step72, the routine52sets Mdiff=+1. The term “Mdiff” means the difference by which the Mode changes when determining the next power reading by means of subroutine56. Particularly, at step72the routine52decreases the resistance by reconfiguring the fixed resistors R11, R12, R13, and R14to create a lower value of total resistance. Lowering the resistance is accomplished by turning on the next resistance value in parallel with the previous Mode resistance value, i.e changing from R14to R13inFIG. 12B(0001 to 0010).

If at step68the determination is no, then the routine52moves to step70where the routine52determines if the Mode is greater than 1 and sampling is to a higher resistance. If at step70the routine52determines that the Mode is greater than 1 and the subroutine for is sampling toward higher resistances, the routine52follows the yes branch to step74. At step74, the routine52sets Mdiff=−1, i.e. the routine52subtracts one resistance value to change the Mode to a higher resistance. If at step70the condition is not satisfied, the routine52follows the no branch to step78, which then returns the routine52to the beginning at step58.

After the routine52has processed Mdiff at either step72or step74, the routine52moves to step76that branches to step80of subroutine54. In the Sampler Logic Subroutine54, the subroutine54sets Mode=Mode+Mdiff. From step82of subroutine54, the subroutine54proceeds to step84where the subroutine54sets the resistors associated with the Mode (Mode+Mdiff). Particularly, the fixed resistors R11, R12, R13, and R14are selected in accordance with the switch configurations shown inFIG. 12B. From step84, the subroutine54proceeds to step86that imposes a 0.1 second delay. After the delay at step86, the subroutine54proceeds to step88, and then branches to step104of subroutine56. The Read Power Logic Subroutine56again determines the power value for the photovoltaic array31as previously described. Once the subroutine56, has completed its operation, control is transferred from step108back to step88any of the subroutine54.

From step88, the subroutine54proceeds to step90. At step90, the subroutine54determines if Pcurrent is greater than Pprevious. If at step90Pcurrent is greater than Pprevious, the subroutine54follows the yes branch to step92. At step92, the subroutine54sets Mode=Mode+Mdiff. Once the Mode has been set at step92, the subroutine54proceeds to step94where the sampling direction of the resistance is switched and a 0.1 second delay is imposed at step96. From step96, the subroutine54moves to step98. At step98, the subroutine54sets Mode=Mode−Mdiff. From step98, the subroutine54proceeds to step100, where the sampling direction for the resistance is again switched. From step100, the subroutine54proceeds to step102, which returns to step76of the routine52. From step76, the routine52proceeds to step78, and then returns to the beginning at step58.

If on the other hand, at step90Pcurrent is less than Pprevious, the subroutine54follows the no branch to step98. At step98, the subroutine54sets Mode=Mode−Mdiff. From step98, the subroutine54proceeds to step100, where the sampling direction for the resistance is again switched. From step100, the subroutine for proceeds to step102, which returns to step76of the routine52. From step76, the routine52proceeds to step78, and then returns to the beginning at step58. Consequently, method50continues sampling the power from the photovoltaic array31until a change is detected against the power trend. The control determination and switching process continues by constantly sampling values at varying time intervals as determined by the change rate of solar irradiance.

The programmable control system16of the present invention also monitors the water temperature and the water pressure in the storage tank10. Particularly, the storage tank10includes a temperature sensor28and a pressure sensor14. The temperature sensor28and the pressure sensor14are connected to the control system16. When the water temperature in the storage tank10falls below a preselected minimum temperature, the control system16connects either the direct current power source18or the alternating current power source32to the resistance heating elements15. Once the water temperature in the storage tank tenant reaches a maximum temperature based on the data received from the temperature sensor28, the control system16disconnects either the direct current power source18or the alternating from the resistance heating elements15. Once the direct current power source18has been disconnected from the resistance heating elements15, the direct current power source18can be diverted to a direct current power takeoff30that can be used to charge batteries, to power an inverter, or to drive a second resistance heating load such as a hot water space heating system. The control system16further monitors the data from the pressure sensor14so that the pressure in the storage tank10remains in a preselected safe pressure range. If the pressure in the water storage tank10rises above the preselected pressure range, the control system16disconnects either the direct current power source18or the alternating current power source32from the resistance heating elements15. If the pressure continues to rise in the water storage tank10, the pressure relief safety valve23will open relieving the pressure in the storage tank10. Further, if the pressure drops below the preselected pressure range, the control system16disconnects either the direct current power source18or the alternating current power source32from the resistance heating elements15so that the water in the storage tank10does not begin to boil at a low-pressure.

The control system16is programmable, either through a direct interface or remotely through a remote interface, and can be programmed to collect operating data including, but not limited to, the temperature and pressure data over time, the power delivered to the resistance heating elements15, the amount of energy delivered to the resistance heating elements15by the direct current power source18over time, and the amount of energy delivered to the resistance heating elements15by the alternating current power source32over time. Such data can be stored locally by the control system16or it can be transmitted to a remote data acquisition system (not shown) either over a wired network or a wireless network. Further, with advanced internal programming, the control system16is capable of learning to optimize energy delivery. For example, the control system16can monitor the time of day, the solar irradiance19, and the temperature of the storage tank10and thereby determine the optimal time to switch from the direct current power source18to the alternating current power source32by means of the switching circuit34, switches41and42or by means of the switching circuit35, switches43and44. Further, the time of the day, from around2-3 pm to 5-6 pm depending on geographic location, offers the greatest chance that the solar array31will provide sufficient energy to bring the storage tank10to its maximum temperature in which case the alternating current power source32will not be used thereby increasing the efficiency of the solar photovoltaic water heating system1. The time periods above are also the less likely time periods in which a high consumption of hot water will be used while the sun is still out.

Turning toFIG. 10, the graph shows an idealized power curve (power from the direct current photovoltaic power source31versus load resistance) that defines the maximum power point, i.e., the point of optimum transfer of energy from the power source to the resistance load where the load resistance is infinitely variable. The graph inFIG. 10compares the idealized power curve to the operating curve for the solar photovoltaic water heating system1of the present invention. Particularly, the graph inFIG. 10shows that by matching the resistance using the switching circuits34and35, the performance of the solar photovoltaic water heating system1of the present invention closely tracks the idealized power curve.

FIGS. 11A and 11Bshow plots of the step changes in the load resistance that occurs when using the formula and method of the present invention. To maximize energy delivery by maximum power point matching, the steps in the changes of the load resistance must be in as equal incremented values as possible. Resistance values that have changes that create anything other than a smooth curve will vary the load resistance above or below the maximum power point matching for a given level of solar irradiance. Resistance values that cause points to deviate from a smooth curve plotting will cause losses of energy delivery to the medium being heated.

FIGS. 13A-13Care examples of data tables derived from measured and published performance information regarding photovoltaic panels. The data then is used to determine the various photovoltaic water heating system operating parameters, which in turn is used to determine the resistance values of the heating rods29.

While the invention has been described in connection with heating water, the invention has applicability to heating other media for storing energy. Further, while this invention has been described with reference to preferred embodiments thereof, it is to be understood that variations and modifications can be affected within the spirit and scope of the invention as described herein and as described in the appended claims.