The science of a new oven

In the kitchen of Vanderlyle restaurant in Cambridge, Chris Smith and Katie Haylor drilled down into the physics of exactly how this new oven, the Eco-Oven as they’re dubbing it, works with inventor Mark Williamson...

Mark - As you can see it look superficially just like any other built-in oven. It's the same size, it makes the same kind of noises, it in fact is plugged into exactly the same kind of electrical supply - 16 amp supply.

Chris - And our chicken is cooking away merrily inside and there are some delicious smells emanating. If that were a conventional oven how would it be cooking whatever we put into it? How does a normal oven work?

Mark - Okay. Well actually the way normal ovens work, a traditional fan oven, is using gently moving hot air - that's the fun oven bit of it. You may use the grill that's in the top of the oven and you may possibly use a hidden element underneath the base of the oven - that's basically it. So a conventional oven just cooks for a certain time at a certain temperature.

Chris - So basically you’re feeding electricity, if it's electric oven, and that electricity heats up a heating element and the overall body of the oven getting hot is then transferring heat to the food?

Mark - Exactly right. That's exactly how it works.

Chris - So what are you doing that's different?

Mark - We've got fundamentally different kinds of heating elements in our oven and our aim has been to get the heat into the food rather than heating up the fabric of the oven. The first heating mode we’ve got in the oven is infrared heating and we are using a very different infrared heating to that you would find coming from a conventional oven with a red hot grill.

Infrared heating is in the form of different waves light and they're space differently apart, and the spacing has very different effects on the food depending on the temperature of the thing that's radiating. So in our oven we’re actually replicating the kind of energy will come from burning wood.

Chris - That that cooks the food differently?

Mark - Oh absolutely. The food actually absorbs the infrared radiation very differently depending on that wavelength. So it goes all the way from really not absorbing anything in terms of the radiation from a traditional oven, to absorbing nearly everything in our oven.

Chris - And how do you make that different flavour - for want of a better phrase - of infrared?

Mark - The way you do it is by using a different temperature for the electrical element that's shining brightly. If it's at a much higher temperature you get a different spacing of the waves.

Chris - And that's what you're doing to get them to be a tighter spacing, so the energy gets in better?

Mark - Exactly. So when you burn a log of wood in a traditional pizza kiln actually that flame is that about 2000°C. The grill in your oven when it glows red hot is only 700°.

Chris - So what are you doing to make the 2000 in your oven then?

Mark - Well, that's a lot of secret stuff in there Chris!

Chris - You're not going to tell me are you?!

Mark - But basically these are very special infrared lamps which run at a very hot temperature.

Chris - Got it. So that's the infrared source. What else does this do differently?

Mark - Okay. So you can see in the oven there, in the base of the oven we actually have an induction heating system which you will not find in any conventional domestic oven. It's the same technology as induction hobs, which some of you may know about. Basically it works by having a coil hidden underneath the bottom of the oven which generates a magnetic field and that transfers the energy into the cooking dish.

Chris - Why do we want to basically create an induction frying pan in the bottom of the oven? Why is that better?

Mark - Well, in a traditional oven, actually the amount of heat that we can get into the bottom of the food is very limited. By doing it this way we are able to dramatically increase the rate at which we can heat the bottom of the food and we do it in a way that heats the food and not the oven.

Chris - Is that because when we plonk a chicken in the middle of oven we often put it on the middle shelf so the energy that's coming into it from below is largely coming from the air circulating, so there is a limit on how much energy you can get in, is that why?

Mark - Exactly right. And in fact, if you look at industrial cooking processes, they go out of their way to ramp up the amount of energy going into the bottom so that they can cook good food quickly because time is money when you're doing this thing in an industrial setting.

Chris - And what else have you got that’s a special modcon?

Mark - Convection air - that's why the traditional oven are called fun ovens - we've got air moving in our oven but it's moving 50 times faster than in a conventional oven.

Chris - Why does that matter?

Mark - Because it transfers heat the food much more quickly. The last thing we've got in there is a really quite sophisticated humidity control system. That actually controls the amount of moisture in the oven, allows us to prevent the surface of certain types of food from drying out.

Chris - So you’re injecting effectively steam into the oven?

Mark - Yeah. We have a small water reservoir in there. The water's converted to steam, a bit like a miniature kettle inside your oven. We think it's a rather clever way of measuring the humidity in the oven and that then controls the amount of steam that's generated. And we can set a recipe - we can have more at the start unless the end, whatever we want.

Chris - How do you know the humidity? Because that's pretty tricky to measure that at oven temperatures, how are you doing that?

Mark - Well, it is tricky and, in fact, in the context of a domestic appliance you can't really put sensors in it that need recalibrating regularly because the thing has got to last for 10 or 15 years. So we found a way of measuring the way that the convection fan in the oven is behaving, from that information, measuring what the humidity is.

Chris - Is that because once the steam's in the air, the air is a bit thicker, a bit denser so the fan's working a bit harder to move it so you can work out how much work the fans doing, and that tells you how in the air is?

Mark - You're on the right track Chris. It’s actually the other way round. The air is more dense than the steam and so in fact the performance of the fan changes to a lower power when you have more steam in the oven. That's what we patented in this oven.

Chris - I want to say it's cool, but it's not it's really hot and it smells amazing actually.

Katie - Once this is cooked, how much energy will you have put into that chicken to make it perfect and delicious? And how does that compare to me using my electric oven at home?

Mark - The first scary thing is that all the domestic ovens are really really inefficient. I mean a typical efficiency for a conventional oven is less than 20%, so the energy you pay for only about 17% of it actually goes into the chicken.

Katie - Is that counting the amount of time - and mine seems to take about three hours - the oven takes to actually to warm up to the right temperature before I even put my chicken in the first place?

Mark - Yes it is. So the pre-heat on oven - I'm not quite sure what's going on with your oven - but on a typical oven, the preheat normally takes about 10 to 12 minutes something like that to get to say 190° C. So yes, it does include that. And there is the time to cook a chicken. So this chicken would have normally haven taken an hour and a half in a regular oven with 10 minutes preheat, say an hour and 40 minutes.

Katie - But you don't have to preheat this oven, is that right?

Mark - No we don't. Basically it starts cooking immediately, within a few seconds it's at full heat and then, as I say, about 35 minutes we're done.

Chris - How much do you think a person would save then if they install this in the house? How much energy, over a working year, are they going to save?

Mark - Depends on the type of food. But if I had to pick an average number the tests so far would indicate somewhere between 15 and 20% saving on electricity usage.

Chris - And what fraction of the household energy bill in the average household - people cooking Sunday roast and the evening meal?

Mark - It's very significant Chris. Actually in many households the built-in oven is the biggest consumer of electricity in the home.

Katie - So how much am I going to save then if I cook my roast chicken in your oven compared to in my oven?

Mark - That depends on how much you're paying for your electricity! It's difficult to put an absolute number on it because of that, but we've worked out that in the lifetime of oven we can probably save you more than half the cost of buying the oven.

Chris - The timer has ticked down to zero, so if you're right Mark, that chicken which we put in 35 minutes ago should be ready to eat.

Katie - Well it certainly smells amazing. I'm guessing that needs to rest a little bit and I heard that we might be having some Yorkshire puddings. As someone from Yorkshire I'm very excited about that.

Mark - Most people will tell you you don't eat meat straight out of the oven. You must let it rest typically 15 minutes is a good amount of time, so we'll use that time to cook some Yorkshire puddings for you.

Katie - So how long do Yorkshire's normally take and how long they take in your oven?

Mark - If we were to pick an average size, a middle size if there is such a thing, probably about 30 minutes/35 minutes in a conventional oven, and we're going to do it in about 18 minutes.