GB2408096A - Oven featuring photocatalytic cleaning arrangement - Google Patents

Abstract

An oven comprises an oven interior defining a cooking space, a source of radiation 5 for photoexciting a photoactive surface 3 of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface 3 hydrophilic, or both, and a controller for controlling the source of radiation 5. The source of radiation 5 is controlled such that the photoactive surface 3 is irradiated within a pre-determined time period before use, at least once within a pre-determined period of time or within a pre-determined time after use. There may be provided a fan 9 for creating an airflow over the photoactive surface 3 during the irradiation. The photoactive surface 3 may comprise a layer of photocatalytic material deposited on the oven interior which may for example comprise titanium dioxide or tungsten trioxide. The radiation source 5 may be an ultra violet light. There may be provided a safety switch for shutting off the radiation source if the oven door 7 is open. The oven may have glass walls on at least three sides of the oven.

Description

OVEN FOR COOKING FOOD

The present invention relates to an oven for cooking food and to methods of cleaning an oven.

Self-cleaning ovens or ovens that make cleaning easier are widely known.

Normally, provision is made for very high temperature operation which allows organic matter deposited on the inside of the oven to be burnt off. Sometimes, catalytic surfaces are provided in the oven to enhance the thermal oxidation of the organic matter.

The oven may be designed to make cleaning easier. For example, the inside of the oven may be suitably treated. The company NEFF provide an electric oven which has a cleaning assisting facility. In this facility, water with a small amount of detergent may be placed in a part of the oven which is then heated to 60 C. The resulting water vapour helps to loosen organic matter deposited on the inside of the oven, facilitating cleaning.

High temperature self-cleaning ovens have the disadvantage that they require a large amount of energy during the self-cleaning cycle. They also give offunpleasant smells and gases which can be dangerous to pets due to the high temperature oxidation.

Ovens with cleaning-assisting facilities still require substantial effort to clean them once the dirt has been loosened.

WO98/41482 discloses a photocatalytically - activated self-cleaning appliance in which a photoactivated self-cleaning substance is coated on the inside of the appliance. This is capable of being activated by natural or artificial radiation to photocatalytically remove organic contaminants on the surface of the oven.

This appliance can be cleaned photocatalytically without raising the temperature inside the appliance to excessive temperatures. However, it is still possible that organic matter may build up on the inside of the appliance, which is difficult to remove.

The present inventor has set out to provide an oven which has a selfcleaning facility which is highly effective and which does not require high temperature operation, and which is capable of being operated in such a way as to give optimum self cleaning properties.

The present inventor has realised that the effectiveness of the selfcleaning is greatly improved if the photocatalytic surface is activated before cooking starts, before any organic material is deposited on it. The present inventor has further realised that the effectiveness of the self-cleaning can be greatly improved if a regular cleaning cycle is used, with the oven being cleaned at least once within a fixed period of time or within a fixed period after use. In this way, the build up of organic matter can be prevented.

Further, organic matter deposited during a cooking cycle can be degraded before it subsequently re-heated and hardened by further cooking. The inventor has also observed that irradiation of the photocatalytic surface can be used to make the photocatalytic surface hydrophilic. This provides the benefit of making such surfaces particularly easy to wipe clean after irradiation.

Accordingly, in a first aspect, the present invention further provides: an oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photoexciting a photoactive surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both and a controller for controlling the source of radiation, so that the source of radiation irradiates the photoactive surface: (a) within a pre-determined period before use, or (b) at least once within a predetermined period of time, or (c) within a predetermined period after use, or (d) in accordance with any combination of (a) (b) and (c) above.

The first aspect of the invention further provides: a method of cleaning an oven, comprising irradiating a photocatalytic surface of an oven interior of the oven with radiation to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both, wherein the irradiation is carried out.

(a) within a pre-determined period before use, or (b) at least once within a predetermined period of time, or (c) within a predetermined period after use, or (d) in accordance with any combination of (a) (b) and (c) above.

The present inventor has further realised that as the photocatalytic degradation process is largely driven by oxidation by atmospheric oxygen and atmospheric water vapour, the rate of degradation can be accelerated if a fan is provided for creating an airflow over the surface, whereby the transport of oxygen and water vapour to the surface is improved and the transport of oxidation products away from the surface is improved.

Accordingly, in a second aspect, the present invention provides an oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photoexciting a photocatalytic surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, and an airflow generator for creating an airflow over the photocatalytic surface during the irradiation thereof.

The second aspect further provides a method of cleaning an oven, comprising irradiating the photocatalytic surface of an oven interior of the oven with radiation to photocatalytically degrade organic matter deposited on the oven interior, wherein an airflow over the photocatalytic surface is created during at least part of the irradiation process.

The present inventor has further realised that, with the self-cleaning regimes described above, a very high level of cleanness can be obtained within a cooking appliance. Accordingly, it will now be possible to provide a clean appliance which is constructed on at least three sides out of glass. Although glass vision panels have been provided in cooking appliances for many years so that the state of the food being cooked can be observed, it has not been desirable to make a large part of the oven of glass, as it would rapidly become stained and unsightly. However, with the present methods, clean glass surfaces can be obtained so that an aesthetically appealing structure can be maintained over a long period of time. Accordingly, in a third aspect, the present invention further provides an oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photoexciting a photoactive surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both wherein the oven walls are made of glass on at least three sides of the oven.

Photoactive Surface and Photocatalvtic Degradation Photoactive coatings are already known for providing self-cleaning windows. A very thin layer of photoactive material, typically titanium dioxide, is applied to the window. The semi conductor material acts as a photocatalyst for photodegradation of organic pollutant. The thin film exhibits photo induced hydrophilicity, allowing rain water to form sheets that give effective dirt removal.

By 'photocatalytically degrade' it is meant that some change occurs in the organic material in contact with the photocatalytic surface when it is irradiated with radiation of a suitable wavelength. Suitable wavelengths are discussed below.

By using a photodegradation system, it has been found that an efficient cleaning of the oven interior can be obtained at low temperature. In particular, cleaning may be carried out at ambient temperature or at a temperature not higher than 1 00 C. In practice, a source of radiation will generate a certain amount of waste heat when running. Suitably, cleaning is carried out at a temperature produced by release of this waste heat into the cooking space in the oven. As discussed further below, this temperature may be in the range 30-70 , preferably 40-60 C.

The photo degradation process can be carried out until removal of the organic matter is substantially complete. Alternatively, it can be carried out sufficiently long to loosen the organic matter which is contact with the surface, facilitating manual cleaning.

It is particularly preferred that the photoactive surface is for photocatalytic oxidation of the organic matter.

The science underlying the invention will be described in more detail further below.

However, without being bound by theory, it is preferred that the photoactive surface comprises a semiconductor material in which an electron can be promoted to the conduction band of the semi conductor, leaving a hole in the valence band.

Photocatalytic oxidation can then occur if the band gap between the conduction band and the valence band is sufficiently great to generate radicals from atmospheric oxygen or water, as described further below Photocatalytic oxidation is particularly advantageous, because it employs atmospheric oxygen which is obviously readily available.

It is preferred that the photoactive surface comprises a layer of a photocatalytic material deposited on the oven interior. In this way, the oven interior can be constructed of normal materials, particularly glass or metal, while the photoactive surface can be applied to the interior before or after assembly into the oven.

Any suitable interior surface of the oven may be provided with a photocatalytic surface. The photoactive surface may be applied to any or all of the oven walls, the oven door and any interior fittings of the oven, for example, racks, grilles or grill pans provided in the oven. Conventionally, racks and grilles are treated with a chrome surface. The photoactive surface may be applied to such a chrome surface, or directly onto the material out of which the grilles or racks are made.

Preferably, the photoactive surface comprises titanium dioxide, tungsten trioxide, Nb2O5, SrTiO3 or mixtures thereof.

It has been found that the photoactive surface suitably comprises titanium dioxide, tungsten trioxide or a mixture thereof.

These may be supplied in any suitable form. It is particularly preferred to use crystalline titanium dioxide, for example anatase or futile. This material can be obtained from manufacturers of fillers with great ease. Titanium dioxide is also referred to as titania. The photoactive material is preferably applied as a thin layer of a finely divided solid material. Suitably, the particle size is greater than 50nm.

The thickness of the layer of photoactive material may be any suitable thickness.

However, it is preferably in the range 50nm-lOOOnm. In practice, it is found that increasing the thickness beyond 400nm does not increase the rate of photodegradation of organic matter in contact with the surface. The photoactive material is suitably applied over substantially the entire internal surface of the oven wall, so that there are substantially no places where organic material may accumulate after cleaning. Titania has many advantages. It is hard, durable, refractory and an excellent photocatalytic It has also been found that the photoactive coating may additionally or alternatively render the interior of the oven hydrophilic when irradiated. This helps to prevent adhesion of organic matter to the surface and which means that the oven interior can be subsequently cleaned manually with a damp cloth.

When ultraviolet light is absorbed by the photocatalytic material, it is believed that electrons and positive holes are generated which migrate to the surface of the photocatalytic material.

During photocatalytic oxidation, the organic debris left inside the oven can be converted to a greater or lesser extent into carbon dioxide and water.

As the photocatalytic degradation occurs at the surface of the oven interior, any adhering organic material is effectively loosened making it easier to remove manually.

An oven according to the present invention may be constructed as a new oven or it may be provided by taking an existing oven and applying a photoactive surface to the oven interior and providing a source of photoexciting radiation.

The photocatalytic surface is suitably applied to the oven interior of an existing or new oven by a process selected from chemical vapour deposition (CVD), physical vapour deposition (such as sputtering and evaporation methods), sol-gel spray coating, dip coating, spin coating or screen printing. Chemical vapour deposition (CVD) includes all forms of CVD such as atmospheric pressure CVD,low pressure CVD, plasma assisted CVD, molecular organic CVD, vapour phase epitaxy, laser enhanced CVD, aerosol CVD and liquid injection CVD.

Radiation Source and Irradiation Process The oven may be designed to be light tight, to prevent escape of harmful radiation. In practice, conventional oven designs provide an adequate level of light tightness without substantial redesign It is particularly preferred that the source of radiation is an ultraviolet light source. Ultraviolet light typically has an energy which is sufficient to promote electrons from the valence band of a semiconductor such as titanium dioxide to the conduction band, which are separated by an energy gap which is sufficient for oxidation of organic matter deposited on the surface.

Typically, the ultra violet radiation has a wavelength in the range 380230nm.

Preferably the ultra violet light source comprises a W black light bulb. The power of the light source need not be very high, being preferably in the range 5-20W, suitably around 8-1OW. A normal 8W germicidal lamp of the type available from BDH has been found to be suitable. The radiation density may be in the range 2-10 w/m2, preferably W/m2.

The use of ultra violet radiation also has the additional benefit of killing bacteria and viruses located within the oven, providing an additional cleaning effect.

In the method of the invention, it has been found that a cleaning effect is obtained after an exposure of at least 30 minutes, preferably at least an hour. More resistant stains may require longer exposure to remove. An additional manual or cleaning step is suitably provided to remove any remaining organic matter. However, for the reasons set out above, this additional manual cleaning step is made particularly easy by the present invention.

It is important that substantially the entire surface of the oven interior is irradiated. It has been found that, because of the relatively high reflectivity of photocatalytic material, particularly titania, substantially the entire surface area is irradiated due to multiple internal reflection.

According to the first aspect of the invention, the source of radiation irradiates a photocatalytic surface according to at least one following conditions: (a) Irradiation occurs within a tire-determined period before use. Preferably, irradiation of the photocatalytic surface occurs within a period of 24 hrs before use, more preferably within a period of 12 hrs before use and suitably within a period of 6 hrs before use.

There are number of practical way this may be attained. According to a first embodiment, it is assumed that majority of cooking operations will occur within a fixed period of the day, for example, from 7am to 7pm, or from 11 am to 1 1pm, depending upon domestic cooking pattems. Given this assumption, the controller may be programmed to irradiate the surface at a fixed time of day before the beginning of the normal time at which cooking occurs, for example at 6am.

The controller may be programmable by the user to irradiate the surface at a selected time of day, depending upon the likely use by the user.

(b) Irradiation occurs at least once within a ore-determined period of time The operation may be carried out to render the surface of hydrophilic, in preparation for subsequent use or in order to remove organic matter deposited on the surface. hradiation of the surface may occur at regular intervals, separated by a fixed period. The predetemlined period is suitably equal to the half-life of excited species on the photoactive surface, so that a high level of excited species is always present. For example, the controller may be configured to irradiate the surface every 12 hrs more preferably every 8 furs, and most preferably every 6 furs.

(c) Irradiation occurs within a pre-determined period after use Organic material can be easier to remove when it is still hot. Accordingly, irradiation may occur within a predetermined period after use, for example within one hour of use, more preferably within 30 minutes of cooking, more preferably substantially immediately after cooking.

(d) Irradiation may occur in accordance with a combination (a) to (c) above.

The controller may be programmed to operate a combination of the procedures described above.

For example, it may be programmed to initiate irradiation after a fixed period after cooking has ceased. It may then be programmed to initiate another irradiation operation a fixed period after the first irradiation period has ceased. Alternatively, it may be programmed to carry out irradiation operations at fixed periods of the day provided that irradiation has not taken place within a fixed period of time.

Further, optional additional irradiation procedures may be selected by the user.

Oven design In a preferred embodiment, a safety interlock is provided whereby the source of photoexiciting radiation is switched off if the oven door is opened to prevent risk to a user, particularly to the user's eyes The oven of the first aspect of the invention preferably comprises an airflow generator for example a fan, as in the second aspect of the invention. The air flow generator preferably comprises a fan.

Any suitable design of fan may be used. For example, a conventional propeller or impeller type of fan may be used.

Where the oven is of a design in which a fan is provided as part of the design of the oven, for example for cooling a microwave source or for enhancing conventional cooking, the same fan may be used to direct air to the photoactive surface during the irradiation thereof. Preferably, a controller is provided which is configured to switch on the fan when irradiation commences.

It is preferable that an airflow is created over at least 75% of the photoactive surface and preferably at least 90%. This may be obtained by using a plurality of fans for directing air to different parts of the surface. Alternatively, a single airflow generator may be used. For example, a single fan may be used if it is powerful enough.

Alternatively, the interior of the oven may be configured to direct airflow to all parts of the surface. Airflow may be directed by baffles, ducts or guides.

The present invention does not require a very high airflow per unit area of photocatalytic surface. For example, the flow rate may be of the order of 0.5L/m2s up to 20 L/m2s, more preferably about 1-5 L/m2s. The airflow generator may operate by blowing air on to the surface or by withdrawing air from the oven interior to thereby create airflow through oven interior.

Where the same fan is used for the cooking operation and for the irradiation operation, it may be operated at a different power for the irradiation operation, preferably at a lower power.

Glass Oven According to the third aspect of the invention, the oven may be formed of glass on at least three sides. Whilst the oven may be any shape, it is conventional to construct ovens from a polyhedral shape employing flat surfaces which are easy to manufacture.

It is most common to manufacture ovens as a rectangular prisms or square prisms.

It is particularly preferred that the oven is constructed as a rectangular prism with at least 3 and preferably 4 side walls and preferably the top wall formed of glass and, optionally, the bottom side as well. This will give a high level of visibility.

Preferably, the surfaces which are formed of glass are glass cover at least 50% preferably at least 75% of that area.

The inventor has found that the adherence of titanium dioxide to glass is extremely good, leading to a tough and durable surface which is particularly suitable for use in the present invention.

A fan unit and other oven accessories may be provided occupying one face of the cooking space or part of one face of the cooking space. Such oven accessories include controls operable by the user, a processor for controlling the operation of the oven, temperature sensors and other accessories.

The present invention will be described Further below with reference to the accompanying drawings in which.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a sketch cross section through an oven according to the first, second and third aspect of the present invention.

Figure 2 is a schematic diagram showing processes that occur when a semi conductor undergoes photoexcitation.

Figure 3 is a diagram showing comparative redox couples for anatase and tungsten trioxide.

Figure 4 is a schematic drawing of the control unit of the oven of Figurel.

Figure 5 shows the process steps in the operation of the irradiation cycle according to a first embodiment.

Figure 6 shows the process steps involved in an irradiation process according to second and third embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWING

Figure 1 is a sketch cross section through an electric oven according to the first, second and third aspects of the present invention. The oven, generally designated 1, comprises a wall 2 defining an oven interior providing the cooking space. The internal surface of the wall 2 is coated with a layer 3 of photocatalytic material comprising titanium dioxide (anatase). An electrical heating element 4 is provided within the cooking space. A source 5 of ultra violet radiation having a wavelength in the range 380-230 nm is provided. A control module 6 is provided for controlling the electrical element 4 and the ultra violet light source 5. The oven interior may comprise conventional fittings such as racks and grilles, which are not shown for clarity and which may be coated with a layer of photocatalytic material. An airflow generator 9 in the form of a fan is provided, for directing airflow over the interior surfaces of the oven.

The oven may also contain other conventional fittings, for example a light, which are not shown, for clarity.

According to the third aspect of the invention, the wall 2 of the oven is made of glass at least in its top and bottom faces and on three sides. The fourth side is closed by the structure 10 enclosing the control 6.

The cross section of figure 1 is taken facing towards the door 7 which allows access to the cooking space. An interlock is provided for automatically switching off the ultra violet source 5 if the door 7 is opened while the light source 5 is switched on.

In normal use, cooking takes place in the cooking space. As a result, organic matter is deposited onto the oven interior 2. In order to clean the inside of the oven interior 2, the oven door 7 is closed and a programme is selected or actioned automatically whereby the ultra violet light 5 is switched on. The ultra violet 5 may be switched on for a period of approximately one hour in order to provide degradation or loosening of organic material by photocatalytic reaction which will be explained below.

Without wishing to be bound by theory, it is believed that the principles underlying the present invention are based on semi conductor photocatalysis, explained below.

Semiconductor photocatalysis Semiconductor photocatalysis refers to the "acceleration of a photoreaction by the presence of a semiconductor catalyst".' A semiconductor can be activated by irradiation with photons possessing energy greater than the bandgap. Thus, an electron, e-, is promoted to the conduction band of the semiconductor and a hole, h+, is left in the valence band. These species can then recombine, resulting in deactivation of the semiconductor, or migrate to the surface of the semiconductor where the photogenerated hole can oxidise electron acceptors and the photogenerated electron can reduce electron donors, see Figure 2. For a photocatalyst to be efficient, the surface redox reactions must dominate over the electron - hole recombination reactions.

The photogenerated electron and hole can recombine either in the bulb (a) or at the surface (b). Alternatively, at the surface the photogenerated electron can reduce electron acceptors (c) and the photogenerated hole can oxidise electron donors (d).

It was realised in 1929 that the pigment titania (TiO2) caused fading in paints." This is due to the photodegradation of the organic polymer binder, otherwise known as chalking."' Furthermore, Fugishima and Honda'V discovered in 1972 that titania electrodes can photocatalyse the splitting of water to evolve oxygen and hydrogen Eqn. 1.

2H20 2H2 + O2 Eqn 1 Relating this reaction to Figure 3, protons are electron acceptors and are reduced by photogenerated electrons, and oxygen anions are electron donors and are oxidised by photogenerated holes. For the electron to reduce water the conduction band needs to be more negative than the hydrogen evolution potential (H+/H2); for the hole to oxidise water the valence band needs to be more positive than the oxygen evolution potential (O2/H20). Figure 3 illustrates that this is the case for anatase TiO2.

The splitting of water has received much attention because it can be applied to the production of electricity from photoelectrochemical cells. However, the most important application resulting from the field of semiconductor photocatalysis concerns the destruction of organic pollutants, both in air and in water, Eqn. 2 semiconductor organic pollutant + O2 > CO2 + H2O + mineral acids Eqn 2 hv 2 Ebg It was not until 1983 that the semiconductor catalysed photodegradation of organic pollutants was first realised, when Ollis and co-workersV v' demonstrated that TiO2 could act as a photocatalyst for the oxidative mineralization of halogenated hydrocarbons. Since then over 200 organic compounds' have been shown to be degraded by the reaction described in Eqn. 2including alkanes, alcohols, alkenes, carboxylic acids, aromatics, phenols, polymers, surfactants, herbicides, pesticides, dyes, bacteria and viruses. The majority of this work has involved semiconductor particles as photocatalysts, but there is also a great interest in the application of photocatalytic thin films, especially on glass, ie. self-cleaning windows.

The mechanism by which Eqn.2 occurs is not simple. As demonstrated by Fig. 3 because the valence band of anatase is sufficiently positive in energy, the photo- generated holes are capable of oxidising surface hydroxy groups to afford OH. radicals.

These radicals can then oxidise organic pollutants. The role of the photogenerated electrons is uncertain, but because the conduction band of anatase is sufficiently negative in energy, they are capable of reducing oxygen to yield Of-. It has been suggested that the consequent reduction of water by these Of- radicals generates species such as HO2., HO2-, H202 and OH. that can oxidise the pollutant.' Most of the research concerning the photocatalytic destruction of organic pollutants has concentrated on titania, because it is photostable, non-toxic, inexpensive and very photoactive. Titania has a large bandgap, ca. 3.2 eV, and hence requires irradiation with W light of wavelengths less than 400 nm to be activated.

There are two processes by which self-cleaning coatings are known to act, one being the photocatalysis of organic pollutants, as described above. The second of these processes is photo-induced hydrophilicity. The hydrophilicity of a film is quantified by the water contact angle, whereby large contact angles represent beads of water on the surface and a contact angle of 0 represents complete flattening of the water droplet on the coating. For self-cleaning coatings, a low contact angle ensures that water forms sheets across the surface of the material, resulting in effective dirt removal.

It has been shown that the contact angle of titania is reduced from ca.70 before UV irradiation to 0 after. Tungsten oxide films have also been shown to posses the same property, with contact angles falling to ca. 5 after exposure to UV radiation.

Watanabe, Hashimoto and co-workers have suggested that the mechanism by which photo-induced hydrophilicity occurs for titania arises from structural changes of the oxide material. Subjecting titania to W irradiation causes oxygen defects in the material, and hence some Ti4+ sites are reduced to Ti3+. Watanabe and Hashimoto suggest that these reduced sites are favourable for the absorption of dissociated water,'X and influence surrounding sites to produce hydrophilic domains. Tungsten oxide also readily forms oxygen defects (ie. WO3 X), and is thought to be hydrophilic by the same mechanism.

References i A Mills, S Le Hunte,J. Phcenz Phi! A: (fly, 1997, 108, 1 ii E Keidel, Fez Zen 1929, 34, 1242 iii SP Pappas, RM Fischer,J. Pa?= TV, 1974, 46, 65 iv A Fujishirna, K Honda, Name, 1972, 37, 238 v AL Pruden, DF Ollis,J. Cam, 1983, 82, 404 vi CY Hsiao, CL Lee, DF Ollis,J. (my, 1983, 82, 418 vii R Wang, K Hashimoto, A Fujishima, M Chikuni, E Kojima, A Kitamura, M Shimohigoshi, T Watanabe, Name, 1997, 388, 431 viii A Nakajirna, S Koizumi, T Watanabe, K Hashimoto, Larpnar, 2000, 16, 7048 ix R Wang, K Hashimoto, A Fujishima, M Chikuni, E Kojima, A Kitamura, M Shirnohigoshi, T Watanabe, A fizz Mater., 1998, 10, 135 Figure 4 shows a schematic view of the control unit 6.

The control unit 6 comprises a processor 11. The processor 11 is connected to a timer 15, a manual control unit 14, a radiation cycle data store 12 for storing records of when irradiation cycles have occurred and an operation data store 13. The operation data store can store operating cycle instructions for operating the heating element 4, the fan 9 and the ultra violet light 5.

The processor is connected to these elements and also to the interlock 8 which determines if the door is open or shut.

Figure 5 shows the processes which occur after a cooking operation ends. The processor 11 firstly determines whether the door is open by checking the interlock 8. If the door is open, a timer is set to zero and, after a short time period t, for example 5 minutes, the door is checked again.

When the door is closed, the irradiation operation begins. This will involve irradiation using the ultra violet light for a predetermined period of time, for example up to half an hour, together with simultaneous operation of the fan at a predetermined airflow rate.

Once the irradiation cycle is complete, the time at which the irradiation cycle ended is recorded in the irradiation cycle data store. The operation then ends.

Figure 6 shows the cycle of operation involved in irradiation according to a first embodiment of the first aspect of the invention, whereby regular irradiation of the photoactive surface can be obtained throughout the course of a normal day.

The cycle begins at START.

The processor 1 1 checks the time recorded by timer 15. The processor then checks the time from the timer 15 with a cycle recorded in the operation data store. If the time is equal to the time set for an irradiation operation, the processor then checks the irradiation cycle data store 12. In particular, it is determined whether the time since the last irradiation cycle finished is greater than a fixed amount or not. If it is not greater by the fixed amount, there is no necessity to carry out a further irradiation cycle and the system returns to START to go round the cycle again after a predetermined period of time.

If, however, the time since the last irradiation cycle exceeds the predetermined amount, irradiation is commenced.

The processor 11 first of all checks using the interlock 8 whether the door is opened. If it is opened, an alert may be given and the system may then return to the start, to go round the cycle again after a predetermined period of time. If the door is closed, an irradiation operation is carried out as described in relation to figure 5 above.

Finally, the time at which the irradiation cycle ends is recorded in the irradiation cycle data store 12 and the system returns to START.

In a second embodiment, instead of having fixed times for irradiation cycles recorded in the operation data store 13, it is only necessary to check whether a given time has elapsed since the last irradiation cycle. In this case, the processor 11 can proceed directly to checking the irradiation cycle data store 12 having obtained the time from the timer 15.

The present invention has been described above by way of example only and modifications can be made within the invention. The invention also extends to any individual features implicit herein or shown or implicit in the drawings or any combination of any such features or generalization of any such features or combination.

Claims (12)

1. CLAIMS: 1. An oven for cooking food, having: an oven interior defining a

   cooking space, a source of radiation for photoexciting a photoactive surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both, and a controller for controlling the source of radiation, so that the source of radiation irradiates the photoactive surface: (a) within a pre-determined period before use, or (b) at least once within a predetermined period of time, or ( c) within a predetermined period after use, or (d) in accordance with any combination of (a), (b) and (c) above.
2. 2. An oven for cooking food, having: an oven interior defining a cooking space, a source of radiation for photoexciting a photocatalytic surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, and an airflow generator fan for creating an airflow over the photocatalytic surface during the irradiation thereof.
3. 3. An oven according to claim 1 or 2, wherein the photocatalytic surface is for photocatalytic oxidation of the organic material.
4. 4. An oven according to any preceding claim wherein the photocatalytic surface comprises a layer of photocatalytic material deposited on the oven interior.
5. 5. An oven according to any preceding claim, wherein the photocatalytic surface comprises titanium dioxide, tungsten trioxide, Nb2Os, SrTiO3 or mixtures thereof.
6. 6. An oven according to any preceding claim, wherein the source of radiation is an ultra violet light source.
7. 7. An oven according to claim 6, further comprising a safety switch for switching off the source of radiation if the door of the oven is opened.
8. 8. A method of cleaning an oven, comprising irradiating a photoactive surface of an oven interior of the oven with radiation to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both wherein the irradiation is carried out.

   (a) within a pre-determined period before use, or (b) at least once within a predetermined period of time, or (c) within a predetermined period after use, or (d) in accordance with any combination of (a) (b) and (c) above.
9. 9. A method of cleaning an oven, comprising irradiating the photocatalytic surface of an oven interior of the oven with radiation to photocatalytically degrade organic matter deposited on the oven interior, wherein an airflow over the photocatalytic surface is created during at least part of the irradiation process.
10. 10. An oven for cooking food, having!: an oven interior defining a cooking space, a source of radiation for photoexciting a photoactive surface of the oven interior to photocatalytically degrade organic matter deposited on the oven interior, to make the photoactive surface hydrophilic, or both, wherein the oven walls are made of glass on at least three sides of the oven.
11. 11. An oven substantially as herein described with reference to the accompanying drawings
12. 12. A method of cleaning an oven, substantially as herein described with reference to the accompanying drawings.