TECHNICAL FIELD

The present invention relates to an indoor system and a method of manufacturing an indoor system.

BACKGROUND ART

Energy generation has gone through a number of evolutionary stages, progressing from whale oil and charcoal through to petroleum and now entering a new generation of the so-called renewable energy sources. These include solar energy, biomass, geothermal, ocean current and tidal, wave and wind power. Petroleum-based generation is by far the main energy resource used in developed nations today, for reasons including abundance, ease of transportation, maturity of industrial and refining processes, high usefulness of by-products, and commercially the significant added -value that can be achieved at every point in the supply chain. World annual crude oil consumption reached 85 million barrels per day in 2009, and continues to increase. However, petroleum fossil fuels are depleting and availability around the globe will fall sharply in next few decades. Burning fossil fuels is also widely accepted to be a major cause of climate change and a significant source of environmental pollution. Endeavours to reduce reliance on petroleum fuels are presently focussed on renewable energy sources, but also on reducing energy use overall by efficiency measures. This invention has roots in both, relating to the utilization of an indoor energy harvesting device to provide renewable power to a device that function to increase energy saving inside buildings, such as sensors, controls and low power displays.

Of energy usage worldwide, approximately 40% is accounted for by buildings. And out of that 40%, half is wasted. In other words, 20%, of the world energy consumption is wastage and could be saved.

Minimizing this waste may effectively reduce the energy consumption.

In the prior art, sensor application using DSSC has been described (US 2007/0132426-A1). However, we find the description given in this patent is crude and the output parameters are too broad. In its claim 4, it is said that "it has a photoelectrical conversion efficiency of 8% or more, an open circuit voltage between 0.6V and 0.7V, and a short circuit current density between lOmA/cm ² and 12mA.cm ² unit cell." This is a very wide discrepancy from a DSSC for sensor application. Although the data can be produced by a DSSC, a cell suitable for indoor, or outdoor, sensor applications should work on a significantly lower current range. It is widely accepted in the indoor energy harvesting industry, that 200lux intensity of fluorescent or LED light is the standard test condition representing a moderately lit room. A state of art DSSC can provide 6 - power output under this condition. Considering the size of the module specified in the patent claim 14 (30mm x 30mm), which is equivalent to 9cm ² , the power delivered at short circuit by this cell will not exceed 72μ\Λ/, which is two to three orders of magnitude lower than the claim suggests. Therefore, the actual operating condition described in this patent is under strong doubt. The operating voltage stated is also questionable. In claim 5, as well as in claim 13, it is stated that "a voltage of the electricity supplied by the solar cell is within a range between 1.6V and 3.5V". Since the voltage output of a DSSC falls only between 0.5V and 0.7V, and under indoor light the output is towards the lower end of this range, in order to acquire a 1.6V to 3.5V output voltage, to achieve this four to seven cells must be connected in series within the module. This is not a trivial task given the small size of the claimed DSSC module, of 9cm ² . No description was given in this patent to explain how the DSSC module can supply 1.6-3.5V output voltage. For these reasons, we are sceptical about the validity of this prior art.

SUMMARY OF INVENTION

According to the present invention, there is provided an indoor system, comprising: an electronic control unit; a rechargeable energy storage medium configured to provide electric power to other components of the indoor system; and an energy harvesting module configured to generate electric power for charging the rechargeable storage medium.

According to the present invention, there is provided a method of manufacturing an indoor system, wherein the indoor system comprises: an electronic control unit; a rechargeable energy storage medium configured to provide electric power to other components of the indoor system; and an energy harvesting module configured to generate electric power for charging the rechargeable storage medium. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts the principle of DSSC operation;

Figure 2 shows the performance of DSSC in power output;

Figure 3 shows the performance of DSSC in power output;

Figure 4 demonstrates the application of defect passivation to gain high performance low light DSSC; Figure 5 shows results showing comparisons for DSSC performance at low light with varying Ti0 ₂ layer structure and thickness;

Figure 6 represents a schematic drawing of a DSSC module made of multiple cells connected in series; Figure 7 shows a detailed device top view drawing;

Figure 8 represents a schematic circuit diagram of a DSSC single cell module; and

Figure 9 represents the device top view drawing of the DSSC single cell module. DETAILED DESCRIPTION

There are many aspects of energy use in buildings that can be modified for efficiency, including temperature control (heating and cooling), lighting, humidity control, security and safety systems. Varying degrees of efficiency can be implemented using systems with different degrees of complexity, intelligence or 'smartness'. To create a 'smart' building, all possible energy waste must be minimized. In the residential consumer market, efficiencies can be introduced in lighting, heating, consumer- electronics, such as audio-visual, computing and related equipment and air conditioning. Offices and commercial buildings can achieve efficiencies in very much the same areas, and in these settings both the absolute and relative savings can be even greater. Most energy in offices and commercial buildings are used only in working hours, so if power supplies are not reduced or disconnected at the end of the working day, they will continue to supply wasted energy until the next working hour. Especially great energy waste may occur over a weekend or in holiday periods where the proportion of non -working to working time increases.

These energy saving systems are also applicable to, and often go hand in hand with, monitoring and controlling the level of living comfort, for example by air quality monitoring, or other environmental requirements, for example horticultural control, while minimizing energy expenditure as far as possible. Intelligence and complexity of this sensing and control system rely on the design and operation.

However, the more intelligent and more complex the system is, the greater power consumption it will have. Therefore, a sophisticated, intelligent and complex monitoring and control system which results in effective operational energy saving and high degree of comfort, will necessitate devices which demand increased power. Considering the amount of energy that may be saved as well as the environmental impact, development of intelligent control systems is still perceived as beneficial. Yet still, adoption of these systems is in its infancy, in no small part due to challenges and costs related to installation and maintenance.

The components in each device in an intelligent system may comprise a functional unit, an electronic control and signal transceiver unit and a power source. The functional unit may be one or more of a sensor, an actuator, a display or other component that detects or conveys information locally. The choice and design of the power source is especially important because it determines the allowable complexity of the device, the cost of components and maintenance, and the reliability and functionality of the entire system during its service life time. Therefore the selection and/or design of the power source are crucial.

In building management systems, 'smart' sensing and control relies on using data from the internal environment to determine whether energy use in building systems can be cut, reduced or otherwise modified. To achieve this effectively, a variety of different sensing heads are required for different purposes, for example light, temperature, humidity, motion or presence and air quality. Unfortunately, the function of one sensor head typically differs greatly from another. Therefore, sensors of multiple- task functions required for effective energy saving have higher energy consumption. Regardless of how many sensor heads are used in the sensor unit, a single power supply (or power source) is desirable. However, the demand requirement on this single power source therefore increases with the number of sensing heads. In other wireless systems inside buildings, displays and actuators increase in power requirement depending on their frequency of change of state.

Basically, there are three types of energy source possible. The first is direct wiring or other physical connection to local power facilities. The second is to use a disposable primary cell (a standard battery). The third is to use an energy harvesting device which can generate energy for the entirety of the device lifetime.

The first, a wired source, requires installation of electrical lines and connections. These are usually ducted cables through and along the walls and within building cavities. This is costly during building construction and installation, especially when retrofitting and for maintenance and repair if the power line or connections are damaged due to age or other deterioration.

The second, using the dry battery, appears a convenient solution. But a dry battery typically has a very limited lifetime. It also creates an environmental problem, of clean disposal within the regulations set by local law. The necessity of frequent replacement is yet another management issue with an associated cost. Where a battery power is exhausted without replacement, the building may lose its sensing and control system.

The third is an energy harvesting device such as a solar (photovoltaic, PV) cell, coupled with power management and energy storage components, which provides a reliable power source for the device.

For PV cells, most of the known energy harvesting devices work only under outdoor light intensity and spectrum. Very few solar cell devices work efficiently under indoor, low light and in highly diffuse light environments. This is because indoor light is typically no more than 2.5% of outdoor intensity. Most indoor light sources are shifted towards a white light spectrum and are highly diffuse - this is typical light source design to give uniformity and energy efficiency while maintaining a perceived comfortable light level.

The amorphous silicon (a-Si) photovoltaic cell is the only known commercialized energy harvesting device that has been successfully used in indoor light. Its capability arises from its spectral response, centred around 1.7 eV (730nm), and it can be slightly tuned to shift this peak. However, a-Si suffers from two issues. First there are long term stability issues under light exposure which are unlikely to have an easy solution. This is due to the so-called "Staebler-Wronski" effect, caused by unstable dangling bonds formed in cell when a silicon thin-film is deposited. The effect becomes even more severe if the cells are operated in an environment which is damp and humid. The second issue is related to the desire for higher power density generation (i.e. greater power per unit area of photovoltaic), to give smaller modules still capable of powering more sophisticated devices. Since the technology of a-Si has been in existence for nearly four decades, and has been considered mature for close to two decades, the possibility that a new breakthrough will bring a step-change in performance is very low. Organic Photovoltaics (OPV) may also be considered a competing technology to a-Si along with Dye Sensitized Solar Cells (DSSC); however as yet power conversion efficiency of OPV is reported as very low relative to both a-Si and DSSC. In this invention, Dye Sensitized Solar Cells (DSSC) are applied as the energy-harvesting power source for indoor sensor applications, as an improvement to a-Si that can enable the described systems. DSSC has several advantages to surpass a-Si as the preferred photovoltaic for indoor use. Firstly, DSSC has been proven to be extremely stable and able to survive for over 20-years equivalent lifetime in accelerated tests. Secondly, DSSC, even at its current early maturity development status, produces power at a higher area density than a-Si. The principle of DSSC operation has been widely published and is accepted to be as drawn in Figure 1. The key description of DSSC is given from published material below (Hagfeldt et al, Chem. Rev. 2010, 110, 6595-6663).

At the heart of the device is a mesoporous oxide layer composed of a network of metal oxide (for example Ti02) nano particles that have been sintered together to establish electronic conduction across the photoelectrode, also called the working electrode (WE). Typically, the film thickness is 10-15μιη and the nano particle size 10-30nm in diameter. The porosity is 50-60%. The mesoporous layer is deposited on a transparent conducting oxide (TCO) on a glass or other substrate. The most commonly used substrate is glass coated with fluorine-doped tin-oxide (FTO). Attached to the surface of the mesoporous oxide layer is a monolayer of the charge-transfer dye sensitizer. Photo-excitation of the dye sensitizer results in the injection of electrons into the conduction band of the oxide, and leaves the dye in its oxidized state. The dye is restored to its ground state by electron transfer from an electrolyte, which is typically an organic solvent containing the iodide/tri-iodide redox system. The regeneration of the sensitizer dye by iodide is therefore by intercepting the recapture of the conduction band electron. The 13- ions formed by oxidation of I- diffuse a short distance (<50 μιη) through the electrolyte to the cathode, also referred to as the counter electrode (CE). The CE is coated with a thin layer of platinum catalyst, where the regenerative cycle is completed by electron transfer to reduce 13- to I-. The circuit is completed via the load applied externally between the CE and WE.

The potential generated under illumination corresponds to the difference between the Fermi level of the electron in the mesoporous layer and the redox potential of the electrolyte. Therefore the open circuit voltage (Voc) of a typical DSSC is a function of the metal oxide semiconductor Fermi level in its WE and the chemical potential of the electrolyte which contains the redox system of iodine and tri- iodide, as well as the concentration of the electrolyte. The maximum possible Voc of DSSC containing Ti02 WE and an iodide/tri-iodide redox system in its electrolyte is approximately 0.9V.

The WE can be formed by printing the nano-porous metal oxide paste, for example Ti02, on FTO coated transparent conductive glass substrate. Doctor blade printing or screen printing are among the most common printing techniques. After Ti02 is printed, dried and sintered at approximately 500°C, the WE is then soaked in a dye bath which contains the dye sensitizer. Ruthenium based dye is commonly used. The CE is formed by depositing a thin layer of platinum as catalyst on FTO coated glass, by using vacuum sputtering, electro-plating , electro-less plating, or printing followed by sintering at approximately 450°C. The electrodes are then assembled and a fluid-tight barrier around the cells is formed using a sealant between the two electrode plates. UV curable sealant or thermoplastic films are commonly used as the sealing material. A controlled cell gap, which is the distance between the two electrodes, can be established by choosing a correct forming process of the sealing material. After the cell is assembled and sealed, electrolyte is then introduced into each cell void through a pre-drilled hole on the counter electrode glass substrate. After filling, the holes are then sealed.

Electrons can thus be regenerated via the sensitizer from the electrolyte iodide/tri-iodide redox system and supplied to the WE from the CE repeatedly. The device generates electrical power from light without suffering any permanent chemical transformation. To the external load, a DSSC behaves very similarly to other types of photovoltaics.

However DSSC photovoltaic has many unique characteristics. The output potential changes little across a very wide range of light intensity. Even at low light down to 200 lux, the Voc remains as high as 0.5V, and remains at 0.45V even at 50 lux. Therefore proper design of DSSC cells and modules can enable an electronic device to charge and function properly under very dim light.

The efficiency of a state-of-the-art multi-junction a-Si cell, which consists of a-Si/nc-Si/nc-Si structure reaches 12.5%. Reported state-of-the-art of DSSC produces 11.4% of power conversion efficiency, which is comparable to a-Si. However a key point is that these records were set under the standard solar test of "one sun", being 1000W/m ² with an AMI.5 spectrum (a simulated intense sunlight) and therefore not generally relevant to indoor use.

Indoor devices operate in much lower light intensities and under significantly different spectra to the standard solar test. Without exception, PV cells with reported record performances have been configured and optimised to the standard solar test conditions. Therefore, for indoor applications, novel cells must to be engineered to be optimized for indoor light intensity and spectra. For these indoor light environments, intensity is usually expressed in units of luminous flux per area ('lux') rather than the standard 'fraction of one sun', 'W/m ² ' or 'mW/cm ² ', due to difficulty of expressing total incident power in watts/cm ² accurately due to complex combinations of light intensities, spectrum and diffuseness.

As a rule of thumb for approximate comparisons, the 'one sun' standard is approximated as 120,000 lux. The typical indoor light environment may span from 50 lux to 3000 lux, which is therefore (as mentioned earlier) no more than around 2.5% of "one sun" - a very low intensity compared to the standard test. All types of solar cells behave very differently between outdoor and indoor light conditions, especially between high light (such as One sun') and low light conditions. This phenomenon was reported, e.g., by Randall and Jacot (Renewable Energy 28, 2003, 1851-1864), the part-findings of which we now describe. Crystalline silicon cell efficiency can drop from above 12% down to 0% when the incident light intensity falls from 1000 to lW/m ² (lW/cm ² ) light intensity; at this level a crystalline silicon cell effectively produces zero power. The drop in an a-Si cell over the same range is much less severe, from 7% to 5.5%.

In this invention, we focus on the energy harvesting device circuits, electronic and application requirements, specifically on managing the DSSC photovoltaic module voltage output and maximising its power density. The indoor system of the invention may have a functional unit such that the indoor system can perform a function other than generating electric power for charging the rechargeable storage medium and recharging the rechargeable energy storage medium. For example, the functional unit may be a sensing unit, a visual display, an actuator or an audio device.

In PVSEC, Hamburg 2011, Texas Instruments reported a comparison of indoor light energy harvesting capacity between Organic PV (OPV), a-Si and DSSC. In Figure 1, there are three columns at each light environment data point. The three left columns, from left to right, represent the power densities of DSSC, OPV and a-Si, respectively. DSSC in this literature outperforms OPV and a-Si by a factor of three. We report that DSSC made by the inventors further outperforms the DSSC in this original literature by 40%. Consequently, the DSSC made by the inventors outperforms OPV and a-Si by a factor of 4.2, from 50 up to 10 000 lux light intensities, under fluorescent light sources.

This validated test data gives the clear sign that DSSC produces much higher power density in indoor, low-light conditions than both the a-Si and OPV.

Under LED light (used as energy-saving lights in the most modern commercial lighting environments and reasonably similar in spectrum to fluorescent light which is more common) the state of art a-Si solar cell produces a nominal 3μ\Λ/ per cm ² under 100-lux illumination. This power output continues increasing from 100 lux at least up to 100 000 lux, and it is safe to assume linearity in output from 50 lux up to 1000 lux. As shown in figures 2 and 3, DSSC as described in this invention under the same light conditions can easily outperform in power output, with similar linearity as has been demonstrated by DSSC also up to 5000 lux and further. Therefore, for low light regimes it is practical to describe the PV power output at low light in μ\Λ/Λ:ιτι ² /100 lux, or μνν/cm ² per incremental 100 lux. In this invention, the DSSC output power density has surpassed the performance of the state of art a-Si, which is

across the range of 50-10000 lux. This high conversion efficiency in the context of energy harvesting devices indoors is an enabling factor for indoor device applications such as sensor networks, and will become more crucial as device power requirements for the network nodes increase.

Modifications may be made on the DSSC described by the inventors in order to optimise the

performance under low-light, indoor environment.

Optionally, any embodiment of the present invention may include passivation of defects on one or more internal surfaces. Defects on the internal material surfaces have a negative impact to DSSC performance. There are two major defect locations present. One is the defects on a transparent conducting surface such as a fluorine-doped tin oxide (FTO) surface. The other is the defect on the surface of a working electrode such as a nano-crystalline porous titania working electrode.

FTO is an exemplary transparent conductor that may be used for DSSCs in this invention. It's surface typically has a high number of defects, being non-uniformities in the FTO coverage. Electrons injected from the excited sensitizer to the Ti0 ₂ working electrode should be transported to the external load through the FTO conductor. However, defects on the FTO surface can cause electrons to recombine with the oxidized sensitizer or with iodide (the oxidized part of the redox couple in the electrolyte), thus the overall cell efficiency is reduced.

On the other hand, the working electrode of the DSSC may be composed of Ti0 ₂ nano-crystalline particles. These nano-crystalline particles are normally manufactured at very low cost with minimal refinement and purification process steps. Therefore, similar to the FTO surface, the Ti0 ₂ surface is also filled with high density of defects, being non-uniformities in the structure and/or composition of the titania. Under strong light intensities such as 1 sun (i.e. about 120,000 lux), the photocurrent is large and the number of recombination events and the resulting leakage current are not significant in comparison. However, under low light condition when the photocurrent is much lower compared to the 1 sun condition (i.e. about 120,000 lux), the leakage current caused by the same defect density becomes more significant relative to the photocurrent.

The defects in the FTO and/or Ti0 ₂ can be passivized using techniques such as immersing the electrode in reactive chemical solutions, e.g., about 40mM TiCI ₄ solution at about 70°C for about 30 minutes, during the fabrication process. After dipping in the chemical bath, the coated material, either the FTO coated glass or the Ti0 ₂ nano-crystalline particles, may be sintered in air in a furnace for a time period of about 15-90 minutes. This passivation process is especially critical for a low light environment. This phenomenon has been studied and reported (L. M. Peter, J. Phys. Chem. C 2007, 111, 6601-6612). Parameters of the immersion chemical can vary within a range around the optimized conditions. For example, the TiCI ₄ concentration, the operating temperature and immersion time can vary between about 20-60mM, about 45-85°C and about 15-45 minutes, respectively.

Although a technique has been described by e.g., Ito et al. (Chem. Commun., 2005, 4351-4353), or patented by Kay et al. (US005525440A), none of these prior arts reports the advantage of this technique being used for DSSC energy generation in a low-light, indoor environment. In Figure 4 the inventors demonstrate the application of defect passivation to gain high performance low light DSSC.

Optionally, any embodiment of the present invention may have a modified Ti0 ₂ working electrode layer structure. Standard DSSCs, designed for operation for under a 1 sun condition (i.e. about 120,000 lux) have been extensively studied and the Ti0 ₂ layer structure and thicknesses optimised for high absorption and energy conversion under strong light. The structure comprises an absorption layer of 12- 14μιη thickness and a scattering layer of 5-6μιη thickness (Ito et al., International Journal of

Photoenergy Volume 2009, Article ID 517609). The absorption layer consists mainly of ca. 20nm diameter Ti02 nano-particles, giving rise to a sintered porous structure with very large surface area . The scattering layer comprises mainly ca. 400nm diameter Ti0 ₂ , which has a significantly lower surface area. The DSSC photocurrent is almost entirely supported by the absorption layer due to its very large surface area.

The inventors of the present invention have found that in a very low light environment, this layer structure and thickness is not the best design. In order to optimize DSSC performance for low light environment, the layer structure may be modified. The scattering layer thickness may remain unchanged, but the absorption layer thickness may be reduced, for example to between about 2 and 4μιη for optimum performance. This is to minimize the effect of the defects on the Ti0 ₂ surface by reducing the overall surface area. Defects on Ti0 ₂ surface areas can cause increased numbers of recombinations and result in a leakage current and poor overall conversion performance. The maximum photocurrent obtainable is a function of the Ti0 ₂ surface area. Therefore, higher photocurrents will require higher Ti0 ₂ surface area to support them. Since the Ti0 ₂ surface is filled with defects, an increased surface area can contain more defects. In a low light environment, the photocurrent is much lower than that under strong light conditions. For 10,000lux incident light, we expect less than 10% the photocurrent at 1 sun (i.e. about 120,000 lux). Therefore to supply the photocurrent, the surface area may be reduced.

Theoretically, Ti0 ₂ thickness can be reduced to 10% of that used for the 1 sun condition, for example a thickness of approximately 1.5μιη. To stay within the limitations of screen printing of the titania layer, between about 1.5 and 5μιη thickness of titania on the working electrode has shown significant performance improvements in the final cell, and performance remains high when the absorption layer thickness approaches about 6-7 μιη. Therefore, the absorption layer thickness may be about 1.5-7μιη for low light conditions. In Figure 5, the inventors disclose results showing comparisons for DSSC performance at low light with varying Ti0 ₂ layer structure and thickness. According to these experiment data, a 3μιη absorption layer of Ti0 ₂ results in higher performance compared to thicker layers.

The most deployed solar energy harvesting power and communications module on the market to enable sensors is the EnOcean STM110. This product contains several innovations that reduce the power consumed by the constituent components. However, it is still only able to power a limited range of simple function sensors (contact, temperature, temperature and humidity) at a useful measurement cycle. A multiple-task sensor or an advanced device such as an infra-red-based gas or occupancy sensor will require higher operating power sourced from its energy harvesting module to obtain and transmit data at a viable rate.

ENERGY HARVESTING IMPLEMENTATIONS

One way for DSSC cell(s) to continuously drive a device is by charging a rechargeable storage medium such as a rechargeable battery, alternatively a supercapacitor or other capacitor, and let that store provide the energy to the sensor units to keep it functioning without interruption. Therefore, two parameters are critical for the DSSC module to drive the sensor unit: first, a sufficient output voltage (potential difference) to allow charging if the battery, and second, a sufficient amount of energy generated to operate the device over a given period. The device requires a certain driving potential, provided by the storage media, to be available when power is required for a measurement, control or communication activity. Therefore the energy harvesting (EH) module which charges the battery must also deliver a sufficient potential above a threshold potential so that during generation, the battery is effectively charged. The total energy fed into the battery storage, with all losses taken into consideration, must be equal or exceed the energy consumed by the device operation. Both these conditions should be satisfied under the operating environment, which becomes crucial when the device is required to operate under a dim light.

A single DSSC cell provides power at 0.4-0.75V depending on light intensity. Typical sensor device power inputs are of the 3-5V range, therefore requiring battery storage charging at a similar level. In this invention, we introduce three methods for a DSSC EH module to provide the necessary voltage and power output for an indoor sensor application. The first is a "Module Comprising Multiple Cells Connected In Series", the second is a "Module Comprising A Large-Size Single Cell With a the output Voltage boosted to a fixed potential", and the third is a "Module Comprising A Large-Size Single Cell With a the output voltage boosted to a Variable potential".

DSSC MODULE COMPRISING MULTIPLE CELLS CONNECTED IN SERIES

The first solution is to use a module made up of multiple cells connected in series so that DSSC output voltage is multiplied. This method is widely used in solar cell industry. However, it has a drawback, for series cells must be firstly physically separated and secondly be connected through interconnections. The separation and interconnection areas cannot generate optical power and therefore are referred to as "dead" or "inactive" zones. All series-cell-interconnected PV modules encounter the same dilemma, that dead zone cannot be avoided despite the undesirable loss of generating area. With the module area fixed by the device design, the module design is usually optimized for the highest possible efficiency by reducing the number of interconnected cells to the minimum so as to reduce the dead zone. Loss due to dead zones in an interconnected DSSC module can vary from 10% to 40% depending on the number of cells, as well as the module size, geometry, structure and production process.

DSSC MODULECOMPRISING A LARGE-SIZE SINGLE CELL WITH THE VOLTAGE OUTPUT BOOSTED TO A FIXED POTENTIAL

The second method is to use a large-size single cell DSSC module to construct an energy harvesting power source of a fixed output voltage. A module made of this type of large cell needs no inter-cell connections, which therefore results in no loss of active area. However, this design produces only a single-cell output voltage, which is usually not sufficient to charge a battery. Nevertheless, this issue can be resolved by boosting the output voltage using a voltage booster external to the DSSC module. A voltage booster can convert up to 92% of the total power, depending on the working conditions.

Therefore power loss due to voltage boosting is between 8% and 20%. The single-cell module size produced may be as small as 1 cm ² up to as large as necessary. Since the device is operating under low light and therefore with low current, sheet resistance by square does not limit individual cell area as is the case for traditional solar modules. However, the power generated from a 1cm ² area is too small for any useful applications at present. Therefore, considering both useful application and manufacturing feasibility, the practical size falls between a 2 x 2cm square (or 4cm ² equivalent area) and a maximum of 50cm x 50cm (or 2500 cm ² equivalent area) if the light is dim. Since power loss due to sheet resistances I ² *R, where / represents current and R represents sheet resistance. In a dim light, the current is low enough so that the power loss is negligible. A square shape is proposed only for simplicity in description. In fact, a cell of practically any shape, such as a rectangle with its aspect ratio below ten (10) will work as well. Aspect ratio is here defined as the ratio of its length divided by its width. Therefore, from 4cm ² to 2500cm ² area is the dimension range of this single-cell module.

Not constrained by a square or a rectangular shape, a DSSC with boosted voltage has high flexibility when designing the shape. Since only one cell exists, there is no need to be concerned with matching of cell sizes to maintain equal current flow though each cell. For example, in a device with a screen made of e-paper ^® , LCD or other display technology, the border often represents significant area even when narrow in width, and it could be desirable to use it for light energy harvesting. For this narrow but large border area, it is not spatially economic or technically feasible to make a series-interconnected multiple- cellmodule for energy harvesting. A single cell DSSC, with voltage boosted to the supply requirement of the device, which conforms to the display border form, can serve as a perfect energy harvester. As another example, a large size DSSC with a fixed boosted output voltage can be very useful to work as a centralized energy harvester to supply energy to multiple storage media, or directly to multiple energy and environmental control devices in a localised area such as in a greenhouse.

DSSC MODULECOMPRISING A LARGE-SIZE SINGLE CELL WITH OUTPUT VOLTAGE BOOSTED TO A VARIABLE POTENTIAL

A favourable feature of a voltage booster is that the output potential can be varied within a certain range (e.g., Texas Instruments bq25504 "Ultra Low Power Boost Converter" has the output between 2.5V and 5.25V).This gives a good flexibility for its applications. Devices made by different

manufacturers have different specifications for voltage input. A single-cell DSSC module with the voltage boosted to a variable range can supply storage media and devices with variable voltage input requirement within that range. This enables simplification of device power management design, with significant savings to development and manufacturing costs.

EM BODIM ENT 1 (DSSC MODU LE COM PRISING M ULTI PLE CELLS CON NECTED I N SERIES)

A DSSC module is made of multiple cells connected in series as represented in a schematic drawing in Figure 6. A detailed device top view drawing is shown in Figure 7. The space available for the cell area is 6cm x 6cm, or 36cm ² . Part of that space is used for interconnections and therefore cannot be used to produce power. In this embodiment, each DSSC cell produces at least 0.5V under 200lux light intensity from a fluorescent light source. There are eight cells connected in series, so the total voltage output (open circuit voltage) is therefore 4.0V or higher. This is sufficient to charge a battery or alternative storage medium of 3.0-3.5V rated voltage, and thereby to drive the connected sensor unit. Between the DSSC module and the storage medium there may be included rectifying circuitry of a linear or buck or boost nature, over/under voltage protection and shunt charging circuits.

Under the light range 50 - 10 000 lux, the DSSC module output power is approximately proportional to the total active (generating) area. A module in this design has a typical active area of 27cm ² , with power loss due to dead zone in the interconnection on this module is 25%. By example, a DSSC generating

This output power and voltage are sufficient to drive a sensor unit in constant operation.

EM BODIM ENT 2 (DSSC MODULECOMPRISI NG A LARGE-SIZE SINGLE CELL WITH A FIXED VOLTAGE OUTPUT BOOSTED)

A module of the same available cell area (6cm x 6cm) and under the same light condition (200lux fluorescent) as in embodiment 1 is described in this second embodiment. Figure 8 represents the schematic circuit diagram, and figure 9 represents the device top view drawing of the 6cm x 6cm DSSC single cell module. With 0.5V output voltage, it produces 23C^W output power. Through the use of a voltage booster (e.g. Texas Instruments bq25504 "Ultra Low Power Boost Converter"), the DSSC output voltage can be converted to anything from 2.5 to 5.25V, capable of charging a wide range of storage batteries. The conversion efficiency of the bq25504 is 80% (at 200 lux fluorescent light) and can reach 92% at higher light intensities. After voltage is boosted, the EH module therefore produces 184 μW net power output. The net output power is still slightly higher than that produced by series interconnected module in embodiment 1, which produces 172μW.

In this invention, although a battery has been described for the energy storage medium, it is obvious that in most cases other types of energy storage medium, for example a regular or super-capacitor, can be used as an alternative and to serve the same function, although they are not explicitly mentioned in the description.