Carl's Beloved Cooker
Energy Efficiency - How To Save Money In The Kitchen
Carl Legge |
Wednesday, 6th April 2011
How to save money by using a pressure cooker, an energy saving device from our permaculture smallholder and chef on 'The Dragon's Tail', the Llyn Peninsula in Wales
I'll come clean. I've been emotionally involved with my pressure cooker for many years now. It's always there when I need it, very fast and it produces tasty meals. What more could any person want?
There is more. It can also be a very energy efficient choice for your cooking.
In the last month I've used it to save energy cooking: Roast chicken, lamb curry, beef & vegetable pie, chicken & pearl barley soup but you can use pressure cookers very effectively with pulse based vegetarian meals as well.
In the first three meals, I used the pressure cooker to pre-cook the ingredients prior to adding them to the rest of the meal. The soup was cooked entirely using the pressure cooker.
Here's how my calculations work out on energy saving.
On my propane range (we live in rural Wales) I simmer on the lowest setting on the smallest ring which is rated at 0.3KW. I use a 3.5kW ring to bring the cooker to pressure. These ratings mean that each ring uses that amount of energy in a hour.
This means, that once the pans are up to heat* the pressure cooker consumes energy about five times as fast as simmering. Therefore, any cooking that takes more than five times as long as cooking 'normally' will use more energy. With the more flavourful meats that require long slow cooking, the pressure cooker is a real bonus.
For the roast chicken the calculation is a little more involved. A 1.5kg chicken will need about 20-25 minutes pre-cooking in the pressure cooker depending on whether you want it firm or falling apart! This uses about 1kW. In my small oven the elements combined are 1.5kW. After the pre-cook you can brown the chicken in the oven for 10-15 minutes using 0.4 kW. Including bringing the cooker up to pressure*, this method uses less than 2kW in total.
Using just the oven, the minimum roasting time (from warm oven) is from about 80 minutes to give just cooked, to 120 minutes for falling apart. So the energy usage would range from about 2kW to 3kW.
If I use my big oven I can cook other things such as roast potatoes as well. So this will affect the usage and saving. If you're not roasting other things there's no need to brown and you have a beautifully tender, moist chicken. And, either way, you get some 'free' stock for your gravy at the same time.
Being frugal, I reused the chicken carcass with more veg to make stock for the soup. This took 20 minutes pressure cooking. Jamie Oliver recommends 3-4 hours simmering for chicken stock. So pressure cooking uses 50% of the energy of the slow simmer method.
To make the soup, I strained out the stock, fried some veggies in the pressure cooker, added the pearl barley & popped the stock back in with some seasoning. 20 minutes pressure cooking later, I had a beautiful tasty soup ready to serve with a herb garnish.
I cooked the lamb for the curry (stewing cuts, neck etc) in 10 minutes from coming to pressure. It would have taken about 90 minutes or so to do that normally - almost a 50% saving. While that cooked I made the curry sauce and added the lamb & some cooking stock to that to combine flavours
With the beef I was under time pressure to get a meal ready super quick. The (welsh black) stewing beef was frozen. I didn't have the time to thaw it & then to gently simmer it for 2 to 3 hours. It cooked to super tender in the pressure cooker in 20 minutes from frozen. I then added the cooked beef and some of the thickened cooking stock to the vegetables for the pie mix.
Pressure cooking is not suitable for everything. I think vegetables by themselves and fish deserve quick cooking to preserve texture and flavour.
Some flavourings such as gentle herbs and many spices do not survive the pressure process and you need to think of other ways to incorporate these flavours afterwards. For example, the roast chicken had lemon slices under the breast skin for flavour and these survive the process. However, if I'd used say thyme and garlic to flavour these would not. The solution is to mix up some olive oil with the herbs & garlic and paint this on the chicken prior to the final browning in the oven.
Meats that are very lean don't really benefit texture-wise from the method.
So pressure cooking is not a solution for every meal. It is a useful contribution with the right ingredients for super succulent food cooked fast using less energy. And, if you're clever, you can do it all in the one pan and so save on your washing up too.
You could love your pressure cooker too...
*I've assumed that getting to cooking temperature takes about the same amount of energy for both methods. There is a difference but it's not material. It takes about 5 minutes using the 3.5kW ring to get the pressure cooker up to pressure using about 0.3kW.
Carl Legge writes a regular blog at http://llynlines.blogspot.com full of delicious recipes and more!
While you are right that a
While you are right that a pressure cooker may be more efficient than an oven or open hob pan, you caused me (and no doubt others) great confusion with your units of energy use.
I see now that when you said "kW", you really meant kWh (kilowatt hours), which are very different things.
Watts and kilowatts ("W" and "kW") are units of power, which is how much energy a device is using over a given length of time. In the case of a device with a power of 1.5kW, the device is using 1500J (Joules) per second.
The Joule ("J") is a unit of energy, which is static and not the same as power (W).
kWh (which you *should* have said) is a way of expressing total energy use (J), not use per second (kW). It's important to be accurate in your language and terminology when trying to spread an important message, so your message is understood, and not misunderstood.
Additionally, your calculations are way off; 20-25 mins of 1.5kW is only 0.5-0.6kWh, not 1kWh, and 10-15 mins at 1.5kW is 0.25-0.38kWh, making a total between 0.75kWh and 1.00kWh, very much less indeed than "less than 2kWh".
Finally, if you are using gas to cook with, if the heat from the cooker allows you to turn the house heating down or off for a period of time, you will not be saving money by using less gas for cooking, as you will use less gas/electricity/oil for heating. In fact, the gas cooker may be a more efficient way of heating your house than your electric heating, if you have it, so it may be better to cook less efficiently in this case!
This does not apply for electric cookers if you have gas heating.
At times of year when your house heating is off anyway, you *will* save some money by using more efficient cooking methods.
Calculations
Andrew
Thanks for taking the time to read the article and for commenting late at night.
In my cooker handbook, and others I have had the rings are 'rated' at X kW, not in kWh. So if a reader were to look at their own handbook this is what they are likely to see.
As you correctly observed, I assumed that an X kW ring burns that amount of kW in an hour.
I chose not to use joules, Btu's or any of the other more arcane units of measure for precisely the point you make that the language needs to be accessible. Similarly, I attempted to use everday language to make the points to what I believe to be a generally non-technical audience. This may lead to technical imprecisions but, as you say, it's the overall message that's important. You appear to have grasped that.
My calculations are correct. As I note at the end of the article, it's necessary to get the pressure cooker up to temperature/pressure. For this,, I use a ring rated at 3.5kW for about 5 minutes. This uses 0.29kW. This added to 20 minutes at 1.5kWH (0.5kW) gives 0.79 kW which I rounded up to 'about' 1kW. Similarly with the oven browning, I rounded 0.38kW up to 0.4kW. The total of which is, indeed, less than 2kW.
As you might have noticed, I've rounded up my figures for the pressure cooker method. I wanted to be conservative in my calculations so that the case for when the pressure cooker was energy efficient was very clear.
This was in part because I recognised that the gas ring does not convert gas potential energy into hot water with 100% efficiency. As you observe, some of the heat is used to heat the general atmosphere. I have figures for efficiency but thought that presenting the calculations for this would obscure the point. I think the actual amount of heating in the context of the overall system of the house is de minimis for the purposes of the article and so didn't include it.
As far as my house heating is concerned, I have a wood burning stove with a back boiler which heats my individually controlled radiators. I use local sourced sustainable wood in it, some of which is coppice from my own land.
I'll take another look at the copy and may ask Maddy to make some minor amenments to cover some of the detail in your comment.
Cheers
Carl
pressure cooking
This is still confusing, Carl (to this physicist anyway) - because you're still muddling units. The total amount of _energy_ required for a process should be measured in kWh - because the kWh is a unit of energy. The _rate_ at which energy is used is called _power_, and the appropriate unit for that here is the kW.
Hence your propane burners are rated in kW, but the energy required to completely cook a meal should be expressed in kWh.
So an X kW ring provides X _kWh_ of heating energy in an hour.
Using a 3.5 kW ring for 5 minutes uses (5/60) x 3.5 kWh (not kW) of energy - i.e. about 0.3 kWh
And so on.
Having said that, all your calcs look numerically right.
And I love your approach to adapting recipes so that you can use the pressure cooker.