Monday, 30 July 2012

Carbon emissions from freezing your garden produce

Gooseberries, tomato sauce etc. store well in the freezer
If you have an allotment or a large vegetable patch in the garden you probably store some of the produce in your freezer to eat later in the year. Eating fresh food in season saves energy and carbon emissions but people have been preserving food in various ways for millenia. These days freezing is the method which is easiest and most widely used. Is the extra energy use an issue - or not?


I asked my friends what they store in the freezer and the answer was lots: tomatoes, green beans, broad beans, peas, courgettes, carrots, raspberries, blackberries, redcurrants, blackcurrants, gooseberries, stewed applies and pears and quince (not raw because they would take up far too much space) ...
NB. Root crops like potatoes and parsnips and also onions and some others do very well in a cool, dark, dry place which requires no energy at all.

I also asked my friends how much space this takes in their freezer and if they have a bigger freezer because of it. If you would have had your freezer anyway then there is virtually no additional energy use or carbon emissions due to freezing this stuff. The energy required to actually cool the produce is very little. Most of the energy used by a freezer is in keeping it cool. Heat leaks in through the walls of the freezer, but this depends only on the insulation in the walls and the temperature difference between inside and outside. Over time, the amount of energy a freezer uses is much the same however much food you put in it.*

My friend Dave suggested I should also consider the energy for blanching the vegetables. However I have ignored this because blanching means the food is partly cooked and will take less time to cook properly later. You can be quite energy efficient with blanching by doing it in batches in a large saucepan: use a basket to lower the vegetables into the saucepan and fetch them out again when they are done. You don't need fresh water for each batch, at least not until it gets mucky.

Some of my friends have very productive allotments and rely on home grown produce for a large part of their food through the year. This produce takes up a large proportion of their freezer space and one or two said they had a dedicated freezer. In these cases, there is extra energy usage and carbon emissions due to the freezer.

Fridges and freezers have improved dramatically over the last few years in terms of energy consumption. Although most examples on sale now are rated A+ if you look hard you can find A+++, a least for smallish freezers (200 litres). Lets look at the easy case first - a smallish freezer dedicated to the garden produce. I am assuming the food density is 0.5 kg/litre, based on my measurements of a bag of frozen peas, which means the full freezer contains 100 kg food and you run it through the whole year. If you don't pack your freezer quite full then there are more emissions per kg food.

RatingkWh/yearkWh/kg foodkg CO2/kg food
A+2302.31.3
A+++1151.150.6
Freezer data based on Liebherr chest freezers: GT2121 (A*)  GTP2356 (A***)

If this is fruit, then it might not need any cooking at all. If it is beans, say, then it will need some. Based on my experiments with potatos as reported in my book, cooking 0.5kg (2 servings) on a gas hob used 0.3 kWh gas which generates 0.06 kg CO2. For 1kg, that comes to 0.12 kg CO2. The emissions due to storage are at least 5 times that.

Some people had a large chest freezer and filled it 1/3 full. Say you have a 300 litre freezer instead of a 200 litre freezer which gives you room for 100 litres of produce, so 50kg. The 50% larger freezer does not use 50% more electricity than the smaller one - probably only 30% more (it has a lower surface area/volume ratio). Considering just the A+++ case the calculation looks like this:

Ratingextra kWh/yearkWh/kg foodkg CO2/kg food
A+++350.70.4

This is better than the dedicated freezer case, but still quite a lot of energy and carbon emissions.

To compare this with commercial frozen food, I dug out the carrots example from my book. Cultivation, packaging processing etc. came to around 0.7 kg CO2/kg food.  Commercial freezers are many times more efficient than domestic ones but commercial food production with chemical treatments and tractors, then washing, packaging and distribution largely makes up for those savings. We can add on transport - say 1000 km - which comes to another 0.27 kg CO2. The chart below shows the final results. The home produced and frozen produce has carbon emissions less than the commercially frozen goods but not as much as one might have hoped. An older freezer is likely to use twice as much energy in which case the home frozen produce will have more carbon emissions than commercial produce, at least for carrots.
Greenhouse gas emissions associated with home grown and frozen produce compared with fresh and commercially frozen. The commercial case is based on frozen carrots. Home freezing is based on an A+++ freezer. Most freezers in homes today are A rated with at least double the energy use.
If you would have had that freezer anyway, then fine. Otherwise, there are good reasons to growing and freezing your own but your carbon emissions argument is probably not as strong as you expected. Where you will seriously win is with fruit and vegetables like tomatoes that have high chemical or energy inputs when commercially grown. In other cases, the home produce savings on transport, packaging and processing are partly or wholly outweighed by extra energy for home freezing.

* It is sometimes said that an empty freezer uses more energy. Two reasons are given. The first is that when you open the door you let more warm air in, because there is more space for it. However, the heat capacity of the air is actually very low, even allowing for water vapour in it. Secondly, when the freezer is nearly empty, it has a low heat capacity and warms up and cools down quicker which is supposed to make the heat exchanger cycle more quickly and less efficiently. However, firstly this can be allowed for by an intelligent heat exchanger and secondly it is the air in the freezer which will warm first as heat leaks in and this has a low heat capacity whether or not the freezer is full.

1 comment:

  1. Hi. Like your blog, Nicola, which I have just found. Your discussion of the economics of running freezers less than full seems to miss something. The key thing is that no part of the freezer that contains food should be warmer than -18C. Therefore as the food level falls, the thermostat can safely be adjusted so that the compressor runs less frequently and the freezer consumes less electricity. The upper levels of the freezer will be warmer than -18C, but this does not matter as there is no longer any food there. A good, reliable freezer thermometer is a must-have purchase for anyone freezing home produce and wanting to keep their energy bills as low as possible.

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