A minimalist solar powered chest freezer + passive icebox setup

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Published on The Doomstead Diner on April 11, 2017

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by Geoffrey Chia, March 2018

Unless you run heavy duty power tools or electric motors or pumps, the home appliances which consume the most electricity are the fridge/freezer and washing machine*. Whereas in the past, 1.5kW (1500W) of solar panels on the roof of a house was considered standard, nowadays 3000 to 7000W are becoming the norm, due to falling prices. A neighbour in a rural area told me he had 25 photovoltaic (PV) panels on his roof (=6000W if each panel is 240W) and after meeting all his domestic electrical needs, he was earning a healthy income exporting electricity to the grid. During the fortnight this January that my friend and I were staying in a caravan on a rural plot, that visiting neighbour was surprised to see how we were meeting all our fridge and freezer needs with a single portable 240W PV system, 400Ah of 12V AGM batteries (=2.4kWh energy storage, assuming 50% DOD) and a 60 litre 12V DC chest freezer.

Here is a very brief videoclip of that setup on youtube and the components are as follows:

  1. Monocrystalline 240W, 12V portable photovoltaic set (two 120W panels which fold together)

  2. Voltage regulator embedded at rear of panel (a relatively inefficient pulse wave modulation box)

  3. Wires going to

  4. Two 200Ah deep cycle lead-acid AGM 12V batteries in parallel (protected against direct sunshine in a very makeshift manner)

  5. Wires going to

  6. Sixty litre chest fridge/freezer – in this case used exclusively as a freezer

  7. Standalone passive cooler box containing ice blocks to chill the perishables

 

Notes regarding use:

  • The location was around 40 degrees latitude South. Being high summer there were long hours of good sunshine each day and there were only a few half days when it was relatively overcast.

  • During that period, daytime temperatures went above 30 degrees C on several occasions, but was down into the low teens at night

  • With the freezer set at minus 12 degrees C (not the minimum setting), in the day the voltage display was usually around 13+V without load and 12+V with load. At night however the voltage went as low as 11V with load (low voltage cutoff was set at 10.7V)

 

Notes on efficiency (or lack thereof) with numbers corresponding to the components mentioned above:

  1. Monocrystalline panels are the most efficient (around 18% energy conversion). Polycrystalline and amorphous PVs are less efficient but are also cheaper and may therefore be more cost effective per Watt in situations where the panels are permanently fixed and do not need to be small and portable. Amorphous panels are more shade tolerant.

  2. An MPPT regulator is around 25% more energy efficient than a PWM regulator but is also more expensive and bulkier. Worthwhile investment for the long term.

  3. Shorter length of larger calibre wire will reduce energy loss

  4. Lithium batteries are much lighter, with greater depth of discharge and longer cycle life than lead acid, but remain significantly more expensive. A battery management system to ensure proper balancing of cells is essential. Perhaps even more expensive and much harder to obtain are Nickel Iron (NiFe or Edison) batteries, which despite a number of drawbacks (eg high rate of self discharge) beat all other batteries in terms of longevity (possibly indefinite if care is taken with the electrolyte) and are highly resistant to abuse (excessive charge or discharge). What type you choose depends on your list of priorities. A proper battery box or cupboard is of course required in a more permanent situation.

  5. see (3)

  6. see Notes on the chest freezer below

  7. When starting off, before buying perishables, bottles of water were placed in the freezer. When frozen solid they were then transferred to the cooler box to keep the veggies, fruit and milk cold. Even when mostly melted, so long as some ice remained in a bottle, the temperature in that bottle would still remain around zero degrees C. In practice we replaced each bottle with a fully frozen one when about half of the ice had melted. This would occur sooner rather than later if we drank the cold water and refilled it with ambient temperature water

 

Notes on the DC chest freezer:

  1. When the icecream was initially stored on the high shelf of the freezer it was mushy, but when placed at the very bottom, it froze well. This indicated to me that the temperature display related to the coldest, lowest part of the freezer

  2. The best way to retain cold in the freezer is to pack it with as many ice blocks / ice packs as possible (in this case we filled plastic bottles with water, which we froze). This will ensure that if inadvertent interruption of electricity occurs (eg low voltage cutout at night), food will remain frozen till electricity resumes (the sun rises or the wind runs your turbine). This strategy of “cold capacitance” will also reduce frequent cycling by the compressor (and hence should increase compressor longevity). Some marine systems incorporate tanks containing brine or glycol within the freezer compartment, the fluid being described as a "eutectic mixture" (a universally adopted misnomer) which performs this function even better.

  3. In climates where it tends to be cooler outdoors than indoors (temperate zones and colder), the most efficient location for a fridge/freezer is outdoors in the shade. The efficiency gain is particularly notable at night when the ambient temperature can fall markedly, hence the compressor will have an easier time working against a smaller temperature gradient.

  4. The chest configuration minimises loss of cold air when opening the lid (little cold air escapes, being denser than hot air).

  5. Probably the most important factor is heavy insulation and in this case I also double bagged the freezer with two external covers.

  6. Camping, caravan and boat fridge/freezers mostly use super efficient Danfoss type compressors, a design which has been refined over decades to cope with being transported and knocked about.

  7. Even though the compressors of domestic AC fridge/freezers have improved in efficiency over the years, running a standard AC appliance off PV will require an inverter which will incur about a 10% energy loss

  8. An acquaintance who bought a domestic AC dedicated chest freezer (hence heavily insulated) and used a thermostat to run it as a fridge only (at perhaps +3 degrees C), promoted it as a low energy method of refrigeration, which it was, even though it required an inverter when running off DC solar panels. However he subsequently argued that his setup was superior to a front opening, less well insulated, double compartment DC fridge/freezer (operating in simultaneous fridge + freezer mode) running off solar PV. His setup may have used less energy (mainly because it was not being used as a freezer) however his argument was spurious because he was comparing apples with oranges. Physics dictates that all other things being equal [if the motors of the AC and DC appliances are of equal efficiency and the appliances have the same configuration (chest only or double compartment only) and have exactly the same insulation and are operated down to the same temperatures in the same mode], then the setup which requires an inverter will inevitably be less efficient due to energy loss by the inverter. You cannot cheat physics.

 

What will the solar panel and battery requirements be in winter? Probably less than double for each, despite the shorter duration, weaker sunshine in winter, because the ambient temperatures will be much lower (assuming the system is kept outdoors in the shade) and hence the energy requirements by the freezer will be correspondingly less. Optimum angling of panels is particularly important in winter http://www.solarpaneltilt.com/

In subzero locations where sunlight is reflected off a snowy landscape, energy consumption will be a pittance.

 

Notes on the caravan electrics:

This was a separate circuit with another 240W PV system (probably overkill) charging a 100Ah house battery which was more than enough for the LED lights, water pump, stereo system and to charge laptop computers, mobile phones and low power handtools (rechargeable drill, circular saw etc). I only checked the voltage of that setup at the beginning and end of the stay (13+V each time).

 

Final comments:

This article describes short term experience of a minimalist freezer and PV setup located outdoors. However for long term use, despite the inefficiencies, most of us prefer an indoor, front opening, double compartment fridge/freezer setup (ie the standard setup, which we all take for granted) due to convenience. Nevertheless this was a useful proof of concept demonstration of how to minimise energy consumption using minimal equipment, while still meeting modern lifestyle desires. It can be a good backup option if your primary system (the one dependent on 25 PV panels on your roof) fails.

Longer term use of this minimalist system will require better anchoring of the PV panels, better protection of (shorter, larger calibre) wires, a rainproof well ventilated battery box, electrical grounding (by convention, usually to the positive terminal of the battery) and proper fuse protection. If the two 12V batteries are not absolutely identical in all respects, “string” (diode) protection between them is recommended. This setup is not meant for prolonged (several months) storage of frozen food, where the freezer temperature should be minus 18 degrees C or below.

G. Chia, March 2018

 

*Footnote:

During this limited period, I used the “grape stomping” method of washing my clothes in the shower at the end of each day, which also saved water and detergent. A washing machine is not essential for a comfortable lifestyle, although it is great time saving device if you have the electricity available. The activity which requires most physical effort (and prunes the fingers) is wringing the clothes, which can be easily done with an old fashioned manual clothes mangler. This item may be difficult to source these days but is worth the investment. As for an electric tumble dryer, that energy sucking appliance is a pox on our civilisation. Unless you are a traveller in a hurry, you should air dry your clothes with a simple clothesline or rack!

2 Responses to A minimalist solar powered chest freezer + passive icebox setup

  • mo flora says:

    A good topic, and one that finds much discussion here.  Unfortunately, we have several months of cloud cover here in the winter and early spring (now) that makes any solar installation problematic.  We are planning therefore to store ice next winter so that it will be available to use for a few months refrigeration.  This used to be common practice in places where it freezes in winter.  Pack the slabs in wood chips or sawdust and they last a very long time.

    I tried to comment on this article in the doomstead area of the diner – but only found references to an older article.  Technology hates me.

  • Rebecca Zegstroo says:

    You can find everything you need for manual clothes mangling on Amazon – bucket or tub, plunger, and a choice of wringers. Drying racks and lines also. I just need to figure out how to get of the gray water at an RV site.

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