HOT WATER SYSTEM

FIELD OF THE INVENTION

The technical filed of the present invention is over temperature protection and thermostatic control systems. An application of the over temperature protection and thermostatic control systems for hot water systems, in particular for hot water systems powered using solar energy.

BACKGROUND OF THE INVENTION

Globally there is a clear trend towards greater use of environmentally friendly renewable energy technologies. In some countries solar energy is a particularly attractive form of renewable energy. However, as an energy source, solar has the disadvantages of periodic supply - only available during daylight hours. Also the energy supply is variable, having relatively predictable temporally slow intensity variation due to time of day and seasonal variation, and temporally faster and less predictable variation due to intermittent weather or shadowing effects. A common household utilisation of solar energy is for supplying hot water. Typical solar hot water supplies utilise solar thermal energy to directly heat water during the day, effectively storing the solar energy in the water for use. The efficiency of such systems is typically low, but has the advantage of utilising a free and clean energy source. Such systems can also have the disadvantage of slow reheating after consumption of stored hot water.

The majority of solar water heaters heat water directly using solar radiation. Such hot water systems heat the water using solar thermal collectors. Water from a storage cylinder is pumped up onto a rooftop mounted solar collector, or collectors, for heating using heat energy from the sun that is collected in the solar collector panel. Such systems involve relatively complicated plumbing in inconvenient locations, increasing initial cost & maintenance expense & leading to loss of energy through water/heat transfer losses in pipework.

Another problem this type of system suffers from is overheating. The system will continually heat water in the collectors while the sun shines. Once the hot water in the storage cylinder has achieve the desired temperature, the continual heating can lead to a problem of overheating, particularly during summer when hot water use is low & temperatures are high. There is no way to turn off the heat so when the circulation system is stopped in response to the water in the storage tank reaching the desired temperature, the water that remains in the solar collector continues to be heated. This water remaining in the collector can be heated to stagnation

temperature, often over 200 degrees Celsius. Water at this temperature releases dissolved solids from solution, known as calcification, causing incredibly hard build up to collect in the piping of the collector causing blockages and degrading the performance of the collector. Such overheating temperatures and calcification can also degrade the pump, the valving, and the storage cylinder. Further, overheating presents a safety hazard, for example, should the collector fail and leak, 200 degree Celsius boiling water/steam can spray onto and off the roof, this can scald people and damage the roof and guttering (especially PVC guttering and down pipes).

Solar energy can be converted directly to electricity using photovoltaic technologies. Due to the variability and intermittent supply, utilisation of solar energy for electricity generation is typically utilised only in conjunction with other power supplies, such as mains power or backup generators. Households may utilise arrays of solar photovoltaic (PV) collectors for generation of electricity which can be utilised by the household (for example, for powering traditional electric hot water systems, lighting, electric appliances etc.) and excess energy is fed into an electricity grid (and the household's energy bills adjusted based on a feed in tariff). PV arrays are a direct current supply, so household systems require inverters for DC/AC conversion and power control for safe operation and to enable the generated power to be fed into the household power supply and electricity grid. Such systems are complex and expensive to install.

Globally there is a clear trend toward the use of solar PV collectors for the production of electricity and an emergent trend with developing battery technologies for people to be able to store the electricity they produce for later use rather than sell the electricity back to the grid at low feed in tariff rates. This adds complexity to the PV system, managing a combination of consumption and storage of direct current (DC) power supply from the PV array via the battery or battery bank and the inverter. There is a need for alternative systems for utilisation of solar energy.

It should be appreciated that any discussion of the prior art throughout the specification is included solely for the purpose of providing a context for the present invention and should in no way be considered as an admission that such prior art was widely known or formed part of the common general knowledge in the field as it existed before the priority date of the application. SUMMARY OF THE INVENTION

In one aspect of the invention, but not necessarily the broadest or only aspect, there is proposed a control system for a direct current (DC) powered hot water generation system, the control system comprising:

a water temperature sensor component configured to monitor the temperature of water stored in a water storage vessel of the hot water generation system;

a power supply control system component configured to receive water temperature data from the water temperature sensor component, and control switching on and off direct current power supply to the hot water generation system based on the water temperature data; and

an over temperature protection device configured to mechanically disconnect the DC power supply from the hot water generation system in response to an over temperature condition, the over temperature protection device comprising a safety circuit configured for mounting in thermal communication with a water storage vessel of the hot water generation system and in electrical connection with the DC power supply to a heating element of the hot water generation system, the safety circuit comprising:

thermal switch configured to mechanically disconnect the DC power supply responsive to exceeding a switching temperature threshold; and a low current fuse connected in parallel with the thermal switch the low current fuse being configured to temporarily conduct current during the mechanical disconnection by the thermal switch and be sacrificial below a minimum operating current threshold for the hot water generation system, to prevent arcing during mechanical disconnection of the DC Power supply.

In an embodiment the thermal switch is configured to switch from a closed current conducting state to an open non-conducting state responsive to exceeding the switching temperature threshold, the thermal switch connected in series with the DC power supply positive terminal and input to the heating element. In an example the thermal switch is a bimetallic switch.

In an embodiment the safety circuit can be further configured to cause disconnection between the DC power supply negative terminal and heating element. In this embodiment the safety circuit can further comprise a second thermal switch connected in series with the DC power supply negative terminal and output from the heating element, and in parallel with a second low current fuse configured to be sacrificial below the minimum operating current threshold for the hot water generation system.

In an alternative embodiment the control system comprises one thermal switch having four terminals and configured to provide two electrical connections in a closed state, each connection connecting a different pair of terminals, and no electrical connection between terminals in an open state,

the thermal switch being arranged in the safety circuit to provide a series connection between the DC power supply positive terminal and the heating element via a first pair of terminals, and a series connection between the DC power supply negative terminal and the heating element via a second pair of terminals,

and the safety circuit having a first low current fuse connected in parallel across the first pair of terminals, and a second low current fuse connected in parallel with the second pair of terminals, the first low current fuse and second low current fuse configured to be sacrificial below the minimum operating current threshold for the hot water generation system.

According to another aspect of the present invention there is provided an over temperature protection device configured to mechanically disconnect a direct current (DC) power supply from a protected system in response to an over temperature condition in the protected system, the over temperature protection device comprising a safety circuit configured for mounting in thermal communication a component adapted to output heat indicative of the over temperature condition of the protected system and in electrical connection with the DC power supply to the protected system, the safety circuit comprising:

a thermal switch configured to mechanically disconnect the DC power supply responsive to exceeding a switching temperature threshold; and

a low current fuse connected in parallel with the thermal switch the low current fuse being configured to temporarily conduct current during the mechanical disconnection by the thermal switch and be sacrificial below a minimum operating current threshold for the protected system, to prevent arcing during mechanical disconnection of the DC Power supply.

In an embodiment of the over temperature protection device the thermal switch configured to switch from a closed current conducting state to an open nonconducting state responsive to exceeding the switching temperature threshold, the thermal switch connected in series with a positive terminal of the DC power supply and input to the protected system. The thermal switch can be a bimetallic switch.

In an embodiment of the over temperature protection device the safety circuit is further configured to cause disconnection between a negative terminal of the DC power supply and the protected system.

In one embodiment of the over temperature protection device the safety circuit further comprises a second thermal switch connected in series with the DC power supply negative terminal and the protected system, and in parallel with a second low current fuse configured to be sacrificial below the minimum operating current threshold for the protected system.

Another embodiment of the over temperature protection device comprises one thermal switch having four terminals and configured to provide two electrical connections in a closed state, each connection connecting a different pair of terminals, and no electrical connection between terminals in an open state, the thermal switch being arranged in the safety circuit to provide a series connection between the DC power supply positive terminal and the protected system via a first pair of terminals, and a series connection between the DC power supply negative terminal and the protected system via a second pair of terminals,

and the safety circuit having a first low current fuse connected in parallel across the first pair of terminals, and a second low current fuse connected in parallel with the second pair of terminals, the first low current fuse and second low current fuse configured to be sacrificial below the minimum operating current threshold for the protected system.

The thermal switch can be a bimetallic disc type thermal switch.

In some embodiments the low current fuse is configured to be sacrificial in a range of 0.5 to 5 amps.

In some embodiments the switching temperature threshold is in the range of 70 to 99 degrees Celsius.

According to another aspect of the present invention there is provided a direct current (DC) powered hot water generation system, comprising:

a DC power supply connection component for connection to a DC power supply providing positive and negative DC power terminals;

a water storage vessel;

a heating element disposed to heat water stored in the water storage vessel; and a control system comprising:

a water temperature sensor component configured to monitor the temperature of water stored in the water storage vessel;

a power supply control system component configured to receive water temperature data from the water temperature sensor component, and control switching on and off direct current power supply to the hot water generation system based on the water temperature data; and

an over temperature protection device configured to mechanically disconnect the DC power supply from the hot water generation system in response to an over temperature condition, the over temperature protection device comprising a safety circuit configured for mounting in thermal communication with the water storage vessel of the hot water generation system and in electrical connection between the DC power supply connection component and the heating element, the safety circuit comprising:

thermal switch configured to mechanically disconnect the DC power supply responsive to exceeding a switching temperature threshold, and a low current fuse connected in parallel with the thermal switch the low current fuse being configured to temporarily conduct current during the mechanical disconnection by the thermal switch and be sacrificial below a minimum operating current threshold for the hot water generation system, to prevent arcing during mechanical disconnection of the DC Power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of the invention and, together with the description and claims, serve to explain the advantages and principles of the invention. In the drawings,

Figure 1 is a schematic example of a hot water system incorporating an

embodiment of the invention;

Figure 2 is a block diagram of an example of a controller;

Figure 3 shows the main components of the power supply and control for a solar PV hot water system ;

Figure 4 is a schematic illustration of an embodiment of an over temperature protection circuit; and Figure 5 is a set of photographs of components of a disc type bimetallic thermal switch.

DETAILED DESCRIPTION OF THE ILLUSTRATED AND EXEMPLIFIED EMBODIMENTS

Similar reference characters indicate corresponding parts throughout the drawings. Dimensions of certain parts shown in the drawings may have been modified and/or exaggerated for the purposes of clarity or illustration.

Use of PV array direct current supply for powering hot water generation has been proposed. In such a system PV array generated direct current electricity supply is utilised to power a resistive element to heat water. This can enable better energy conversion efficiency and water heating time than for thermal solar energy water heating. The PV system also avoids the overheating problem associated with solar thermal hot water systems as there is no water on the roof, no 200 degree water temperatures and reduced likelihood of leaks as no pumping system and associated piping is required. The PV system can be simply turned off when the water in the storage vessel has reached the defined temperature. This also reduces deterioration of the components due to calcification caused by excessive temperatures.

However, this hot water system concept presents significant safety concerns.

Firstly, hot water service heating elements typically have a fixed resistance, designed for use with a fixed voltage power supply to operate at a particular power rating. In a typical Australian hot water system, designed to operate using AC power supply, a

2400 watt element has a resistance of 24 ohms, to draw 10 amps at 240 volts.

However, DC power supply from solar panels is variable, meaning the constant power model is not applicable. The instantaneous power supply to the heating element will vary in a PV DC hot water system. Secondly, switching of DC at high power is more complex than switching AC due to potential risk of arcing across contacts.

Referring to the drawings for a more detailed description, there is illustrated hot water system 10, demonstrating by way of examples, arrangements in which the principles of the present invention may be employed. Embodiments of the present invention provide a safety circuit and controller for a direct current powered hot water system.

An example of a hot water system incorporating an embodiment of the invention is shown in Figure 1 . The hot water system comprises a water tank 120 for heating water and storing the hot water, with an inlet 160 for cold water and outlet 170 for hot water. In this example a heating element 130 in the tank 120 heats the water. The power supply to the heating element 130 is DC power from a solar PV array 1 10. A controller 140 monitors the temperature of the water in the tank 120 and switches power on and off to the heating element 130 based on the water temperature and power supply availability from the PV array 1 10. It should be appreciated that the representation of the hot water system in Figure 1 is a simplified representation of the key system components, rather than a true illustration of such a system, and in practice the invention can be utilised with many different hot water system configurations. Further, although the proposed power supply is a PV array, embodiments of the invention could be utilised with any DC power supply. The system should not be considered limited to PV power.

The controller 140 includes a water temperature sensor component configured to monitor the temperature of water stored in a water storage vessel 120 of the hot water generation system. A power supply control system component is configured to receive water temperature data from the water temperature sensor component, and control switching on and off direct current power supply to the hot water generation system based on the water temperature data. The controller can be configured to monitor the power output from the DC power supply and taken in to consideration varying power supply when managing water heating control . In some embodiments the controller can also be configured to be powered via the DC power supply, in this instance the controller will also turn off in low or no light conditions.

A problem for direct current PV powered electric hot water systems is thermostatic control as there are fundamental difficulties with using existing switching mechanical thermostats with DC. AC (Alternating Current) is a flow of current which periodically reverses its direction, the sine wave. In Australia it moves from +230 Volts to 0 volts to -230 Volts to 0 volts & back to +230 volts. It does this at 50 times a second. DC (Direct Current) only flows in one direction and does not have the cyclical nature. Disconnecting and connecting AC is relatively easy to do safely due to the constant flux in current flow. Direct current is more problematic as the current flow is constant and wants to continue flowing, even using the air as a medium to keep the electrons flowing. Particularly at high power, but even at relatively low power, attempts to shut off DC can result in arcing between switch contacts. The arc effect can damage contacts, in particular burning or even fusing contacts. This damage can make breaking contacts difficult. Other problems that may be cause include degrading system performance, disconnection delays, safely concerns and even system failure.

In the context of a hot water system, this arc effect can strip thermostat switch contacts causing premature failure of the thermostat. This may occur in an unsafe manner as the arc may weld the contact on causing overheating of the unit. It should be appreciated that DC switch arcing can be a problem for any mechanical switching, both in regular operation and safety cut off switching.

In an embodiment of the thermostatic controller, the power supply control system is configured to monitor and control the power supply production by the PV array and control operating parameters to maintain power supply output below a threshold voltage. In an example configured for Australian markets the voltage threshold is 120 volts. This target is chosen to minimise regulatory burden for installation of the system, as system designed to operate above 120V require special installation licenses. Complexity exists here in that to generate the heat in an element you want as much voltage and amperage as possible to generate as much heat as possible with a resistive element while keeping it under the 120V threshold.

The skilled addressee will however appreciate that the present invention is not limited to voltage at, or below, 120 volts and the voltage may be above this level, for instance 150 volts, 250 volts or otherwise, without departing from the spirit or scope of the invention. Accordingly, the example of a 120V threshold is provided for illustration purposes only and is not intended to limit the scope of the invention.

Embodiments of the inventors PV hot water controller uses a microprocessor to monitor and control load and current switching from the PV collectors to power the heating element. In an embodiment the controller utilises a small regulated current and voltage generated from the PV collector array to power the microprocessor. This is a relatively small voltage and current, compared to the voltage and current used for the heating circuit, enabling a microprocessor to be used to monitor the system operation even in low light conditions. Thus, embodiments of the controller can be configured to operate without the need for an additional fixed power source, such as an on-board battery. Even low light conditions the PV array can generate enough power to start the microprocessor, to allowing monitoring and controlling of the system.

A block diagram of an example of an embodiment of the controller 140 is shown in Figure 2. In this embodiment the controller 200 comprises a power supply control system 210 including a processor 240, and memory 245. The processor 240 may be implemented using any suitable programmable or non-programmable logic device, for example, a microprocessor, programmable logic controller (PLC), filed programmable gate array (FPGA) or application specific integrated circuit (ASIC). In the example embodiment discussed in detail the processor 240 is a microprocessor. Memory 245 can include ROM and RAM memory accessed by the processor 240. The memory 245 may be configured to buffer data from the temperature sensor and PV array power monitoring.

A user interface 248 may be provided to enable input of some user controllable data, for example setting a target operating temperature range. The user interface may also be used to output data such as fault indications. In an example the user interface comprises a display screen (such as a liquid crystal display) and buttons for data input. Alternative user interfaces such as touch screens or a wireless communication interface accessible via a remote device, computer or mobile phone may also be used in some embodiments. It should be appreciated that hot water systems are often installed outside, exposed to the elements or in hostile

environments. The type of user interface implemented may be chosen based on the anticipated operating environment. In an embodiment the power supply control system 210 may also include a power supply such as a battery or mains power connection, however this is optional. An advantageous embodiment of the controller is configured to operate powered by the PV array alone, without requiring an independent power supply. In an embodiment the controller may be configured to receive operating power from a separate PV source, for example a smaller panel beside a main array or from a single panel of the array.

In this embodiment the power for turning on the controller and enabling temperature monitoring is separate from a main PV array. This embodiment can have an advantage of enabling one controller to be configured for use with a variety of sizes of main PV array - for example reducing the number of different capacity controllers required to be carried by installers or allowing scale up of main array without needing to change the hot water system controller. The power supply control system 210 is configured to receive temperature data from a temperature sensor measuring the stored water temperature, and controls power switching hardware 220 for turning on and off DC power supply to the heating element 130. Illustrated as part of the controller in Figure 2 is also an over temperature cut out circuit 250 that will be described in further detail below. The controller 200 is configured to monitor the PV array 1 10 power output and temperature of the stored water. The controller 200 controls switching of power to the heating element 130, once the power output from the PV array is adequate for heating, and if heating of the stored water is required to maintain the stored water temperature with a target temperature range. In an embodiment the power switching hardware 220 includes two MOSFETS (metal-oxide-semiconductor field-effect transistor) used for switching electronic signals, and configured to be controlled by the microprocessor 240. The microprocessor controls two MOSFETS. The

MOSFETS have a gate that requires very little current to cause the device to switch on and switch much larger currents. The microprocessor is programmed to monitor the PV array output power supply and only turn the MOSFETs on once there is a stable voltage and current available. This avoids the MOSFET turning partially on with low start up voltages and currents, particularly in low light conditions. In particular, the controller is configured to avoid MOSFET partial turn on in low light or fluctuating power conditions as this can cause huge amounts of heat to be generated in the MOSFET leading to component failure. Although in this two MOSFETS are used, embodiments of the controller can be implemented using only one MOSFET. Alternatively, more than two MOSFETs can be used. One advantage of using more than one MOSFET is for reduction of heat in the controller circuitry. Particularly for a hot water system controller that must be enclosed for outdoor installation heat dissipation and reduction in heat generation in the controller are important.

Another advantage of utilising multiple microprocessor controlled MOSFETs is the ability to configure the controller to switch DC power generated by the PV array to systems other than the PV hot water system, for example, the controller may be configured to divert PV power supply to a battery storage system or even an inverter for grid feed in or household use. In an embodiment the controller is configured to monitor the PV DC power supply level and while a relatively stable DC power supply is available the controller can selectively switch the power supply to any one of the hot water system heating element or other connected systems such as a battery storage unit, inverter or other system, by selectively switching MOSFETS. Thus, the controller can select which system to power based on current operating conditions, for example when the hot water is within a target temperature range the PV DC power can be diverted to a battery storage unit or an inverter for grid feed in. In some embodiments the controller may also be configured to also draw DC power from the battery storage unit for water heating in inadequate PV power supply conditions (low light or dark). Another advantage enabled by using microprocessor controlled MOSFETS is providing earth leakage protection, enabling shutdown of both positive and negative sides of the circuit via MOSFET switching in the case of earth leakage. The use of microprocessor controlled MOSFETs allows control of current flow to the heating element even in fluctuating or failing light conditions (i.e. passing clouds, end of day) by the microprocessor holding the MOSFET hard on. The MOSFETs are turned hard off by the microprocessor once the voltages and currents produced by the PV array fall below a predetermined limit and are not turned back on again until the determined requirements are met. This stops the gates opening and closing rapidly in changing conditions which would cause heat build-up and component failure. Use of microprocessor controlled MOSFET switching enables controlled turning on and off of the power supply to the heating element.

An embodiment of the controller is configured to monitor the water temperature using a sensor to detect potential water overheating and cease power supply to the heating element.

For example, if the hot water system received direct sunlight, heat from solar radiation may contribute to water heating and increase the rate of water heating, which may be detected by the controller and the duration of activation for the PV electric heating adjusted to avoid overheating. Some embodiments of the hot water system controller can also be configured to operate a booster system powered using an alternative energy source (for example gas, or mains electricity) where the required heating cannot be achieved using the PV system, for example if water heating is required at night. In this embodiment the controller may include a battery (preferably rechargeable via the PV array) or alternative power supply (mains) to ensure night time operation. The controller monitors the water temperature and if the temperature falls below a threshold temperature when power form the PV array is not available (for example at night) or a target heating temperature cannot be achieved via the PV power (for example during winter in colder more overcast conditions) the controller is configured to activate an alternative (boost) heating system, for example a gas boost, for example LPG (liquid petroleum gas) or natural gas heating system. It should be appreciated that such a system can be configured to operate

independently (off -grid) of any mains connected electricity or gas supply, and be well suited for remote environments or areas where power supply is unreliable.

Another important element of any electrical system is a safety cut out to prevent continued, potentially unsafe operation if a fault occurs. Figure 3 provides an overview of the main components of the power supply and control for a solar PV hot water system, a solar panel array 310, a controller 320, and an over temperature protection device 330 configured in the circuit to cut off power supply from the heating element 340 in over temperature conditions - if the water in the hot water tank exceeds a set safe threshold temperature. The Solar panel array 310 generates DC electricity. In normal operation the controller 320 monitors the water temperature and PV power supply and controls switching on and off direct current power supply to the heating element. PV DC Hot water controller uses information from the temperature sensor on the tank and the power available to switch power to the element to heat the water.

The Over temperature cut out device 330 sits in series, attached to the hot water storage tank to sense the stored water temperature and only functions in response to the sensed temperature exceeding a set threshold. If it functions it separates the positive and negative connections to prevent further heating of water in the tank. The over temperature cut out device operates mechanically to break the electrical connection between the power supply and heating element. This is a requirement of safety regulations for hot water systems in many counties. In Australia the safety regulations at the time of writing also require disconnection from both positive and negative power supply terminals. As discussed above mechanical switching in DC circuits can cause arcing between switch contacts, which can cause switch contact to fuse and a failure to disconnect. Thus, traditional thermal switches or thermal fuses typically used in AC hot water systems are not safe for application in DC system.

An aspect of the present invention provides an over temperature protection device for use to protect DC powered systems. Embodiments provide an over temperature protection device configured to mechanically disconnect a direct current (DC) power supply from a protected system in response to an over temperature condition. The over temperature protection device comprises a safety circuit configured for mounting in thermal communication a component adapted to output heat indicative of the over temperature condition of the protected system and in electrical connection with the DC power supply to the protected system. The safety circuit comprises a thermal switch configured to mechanically disconnect the DC power supply responsive to exceeding a switching temperature threshold, and a low current fuse connected in parallel with the thermal switch. The low current fuse is configured to temporarily conduct current during the mechanical disconnection by the thermal switch and be sacrificial below a minimum operating current threshold for the protected system, to prevent arcing during mechanical disconnection of the DC Power supply.

In an embodiment the thermal switch is configured to switch from a closed current conducting state to an open non-conducting state responsive to exceeding the switching temperature threshold, the thermal switch connected in series with a positive terminal of the DC power supply and input to the protected system.

The safety circuit can also be configured to cause disconnection between a negative terminal of the DC power supply and the protected system. In one example the safety circuit further comprises a second thermal switch connected in series with the DC power supply negative terminal and the protected system, and in parallel with a second low current fuse configured to be sacrificial below the minimum operating current threshold for the protected system.

The thermal switches used can be bimetallic switches, however other types of thermal switches can also be used. The requirement for thermal switch choice include, operation temperature threshold, Bimetallic type switches are well suited to this application because these are available for a range of operation threshold temperatures and configured to cleanly separate electrical contacts. These components can also be economically sourced. This is advantageous for applications where the over temperature device is to be treated as sacrificial and replaced once activated.

In an embodiment of an over temperature protection circuit is illustrated schematically in Figure 4 and the thermal switch shown pictorially in Figure 5. This embodiment is configured for a solar PV CD hot water system as described above. In this example the over temperature protection requires dual poll (both positive and negative sides) disconnection. It should be appreciated that dual pole disconnection cannot typically be achieved using conventional thermal fuses in series with the power supply, as once one fuse breaks the other loses power so may not break. It is highly unlikely to have two thermal fuses blow simultaneously and therefore near impossible to meet the dual pole disconnection requirement using standard thermal fuses. In this embodiment the over temperature protection device 400 uses one thermal switch 410 having four terminals, 1 ,2,3,4 and configured to provide two electrical connections 412, 414 in a closed state, each connection connecting a different pair of terminals (as shown in figure 4), and no electrical connection between terminals in an open state. The thermal switch 410 is arranged in the safety circuit to provide a series 412 connection between the DC power supply positive terminal 430 and the heating element 420 via a first pair of terminals 1 , 2, and a series connection 414 between the DC power supply negative terminal 440 and the heating element 420 via a second pair of terminals 3, 4. A first low current fuse 450 is connected in parallel across the first pair of terminals 1 , 2, and a second low current fuse 455 is connected in parallel with the second pair of terminals3, 4. The first low current fuse and second low current fuse are configured to be sacrificial below the minimum operating current threshold for the protected system. The thermal switch will operate to disconnect all switch terminalsl , 2, 3, 4 at the threshold temperature. The sacrificial fuses are typically destroyed during operation but allow the contacts to separate cleanly by momentarily taking the full DC load allowing the contacts to open cleanly without arc, before blowing. Thus the switch can operate safely without generating an arc, and/or causing the contact to melt and fail or alternately, welding the contact closed and causing a device operation failure. The sacrificial fuses are configured to blow below the minimum operating current threshold for the protected system so that in a rare case where the fuses may not have blown initially as the switch was tripped and power cut, the fuse will blow as soon as power supply resumes at normal levels, if the thermal switch remains tripped. It should be appreciated that the threshold current for the sacrificial fuses and also the

temperature threshold indicating over temperature conditions will vary from system to system. The threshold current for the sacrificial fuses is chosen based on the minimum normal operating current range of the system to be protected. This minimum current will vary based on the nature of the system, for example a DC motor may operate at around 2-5 amps, whereas for an industrial process a minim operating current may be around 10 amps, thus different current rated sacrificial fuses may be used in over temperature protection devices used in these two systems. One consideration in choice of sacrificial fuse current threshold may also be potential current variation (current spiking) during regular operation, in particular to avoid choosing a fuse that may blow during normal operation and thus effect the safe operation of the over temperature protection device during an over temperature fault. Thus, using a fuse configured to be sacrificial at a current marginally below (for example 1 to 10% below) the minimum operating current for the protected system can reduces the risk of premature blowing of the fuse either during normal operation or too early during a fault to ensure that arcing is prevented. For example, for a DC hot water system the low current fuses may be sacrificial below current thresholds from 1 to 4amps depending on the system configuration. When determining the temperature threshold for thermal cut-out, consideration should be given to the nature of the over temperature condition and thermal conductivity to the over temperature protection device, to determine a threshold value for detection at the switch that will be indicative of the over temperature condition in the system. For example, in a hot water system an over temperature condition may be water in excess of 90 or 95 degrees Celsius within the water tank, in an embodiment where the over temperature device is attached directly to the tank, within the tank insulation this may trip at the over temperature condition temperature.

However, in an alternative system the over temperature protection device may be in thermal communication with a heat source via another element of the system, and therefore due to heat loss or thermal conductivity coefficient of the element the temperature indicative of the over temperature condition may be different from the actual temperature of the heat source.

In such systems the thermal switch is chosen to trip at a temperature sensed at the system element which is indicative of an over temperature condition in the protected system.

In this embodiment the thermal switch is a bimetallic disc type thermal switch as illustrated in Figure 5. The temperature cut out is triggered by the bi-metallic disc reaching the trigger temperature. A bi-metallic disc is a disc is made from a laminate of two different metal alloys with different coefficients of thermal expansion. The disc changes instantly from a concave to a convex shape when it is heated above its activation temperature. The mechanical displacement of the disc creates mechanical movement. The mechanical movement moves a pin that physically separates the contacts via a pin and bridge.

In the context of a hot water system, the thermal switch mechanism tends to only operate while under load as if there is no current flowing though the contacts there is no power being supplied to the heating element. The system tends not to go over temperature unless there is an alternate heating source. If the bimetallic thermal switch is used alone the contacts separate, while conducting DC current, the contacts tend to arc and fail. However, in the configuration described herein this arcing is prevented by placing a low current fuse across the contact to very temporarily take the load to allow the contacts to separate without arcing before the fuses fail.

The over temperature cut out is a safety device. It should only ever operate if there is a fault in the thermostat and controller. If the thermostat fails on and continues to supply energy to the heating element when it should have disconnected the supply, the cut out will operate and disconnect the heating element. In Australia the requirement is for dual pole operation, so the positive and negative are disconnected. The unit must be manually reset - often replaced - to ensure the system is checked before being able to be reconnected.

Although the over temperature protection device has been discussed above in the context of a hot water system the over temperature protection device can be used in any DC powered system where thermal cut-out protection may be required. For example, industrial process control systems, DC motors etc.

The skilled addressee will now appreciate the advantages of the illustrated invention over the prior art. In one form the invention provides a control system for a direct current (DC) powered hot water generation system including a bimetallic switch which mechanically disconnects a DC power supply in response to a switching temperature threshold being exceeded.

Various features of the invention have been particularly shown and described in connection with the exemplified embodiments of the invention, however it must be understood that these particular arrangements merely illustrate the invention and it is not limited thereto. Accordingly, the invention can include various modifications, which fall within the spirit and scope of the invention.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.