How can I keep things cold without mains electricity

What are you trying to chill? Cool drinks are good for morale, but hardly essential for life. Chilling food can be crucial for hygiene. Keeping vaccines cool is a life saver. Knowing the value of your payload helps prioritise and weigh up how much effort and expense to allocate.

Non-critical items can be partially cooled using an evaporative cooler. This uses a porous shell from which water evaporates, drawing heat from the inside. They work best in conditions of low humidity. Air movement also helps. A simple version is a large earthenware pot with a lid, sitting in a bowl of water or saturated sand. Another approach is to build a box with mesh walls filled with charcoal and with a leaky water container or pipe dripping into the top of the walls. At its most basic, you can use a damp cloth over a metal box. There are plans and an explanation of a charcoal cooler here: https://www.appropedia.org/Charcoal_Co...

Powered options

Thermo-electric or Peltier effect coolers pass electric current through two different materials causing heat to flow from one junction to the other. Cheap examples are often available as small cool boxes for use in vehicles as a picnic cooler. They are inefficient compared to other powered systems.

Absorption fridges use heat from burning LPG or kerosene to drive an ammonia-based cooling circuit. They are fairly cheap but require a fuel supply. They are generally simple and reliable as there are no moving parts, but have poor temperature control and can freeze contents. Historically, they were used for vaccine and medicines but are no longer considered best practice. The multi-fuel fridges (230Vac, 12Vdc, LPG) found in camper vans and caravans use this technology.

Compression refrigeration cycle is used in conventional mains powered fridges. Beyond basic domestic versions there are specialised medical fridges, direct solar and 12 Vdc fridges. If you are using an off-grid power supply there are several options:

Use batteries, an inverter and an AC fridge. The fridge is relatively cheap but you will need the other components. If you are using a battery-charger-inverter system to supplement a generator or unreliable mains, this is an option, but bear in mind that a fridge is relatively energy hungry compared to a laptop or lights.

Use a specialised DC fridge which cost more but are more efficient. They use a specialised compressor and controller and will run off a 12Vdc battery system. Typically they are better insulated to reduce energy demand.

Solar direct drive fridges have been developed for the medical market that run direct from PV modules with no need for batteries, which are often the first component to fail in a DC fridge system. To keep the fridge cold overnight, they store cool in ice or phase change materials. There are also versions that work with intermittent mains supplies.

Vaccine fridges

WHO have a performance standard (PQS) to which all fridges used in UNICEF supported cold chains should conform. This stipulate the internal temperature ... (more)