I don’t think so. On sunny winter days, you can heat them in the sun, using a solar water heater (https://en.wikipedia.org/wiki/Solar_water_heating https://en.w

I don’t think so. On sunny winter days, you can heat them in the sun, using a solar water heater (https://en.wikipedia.org/wiki/Solar_water_heating https://en.wikipedia.org/wiki/Solar_water_heating) and let them cool down at night, heating your body. The low tech version is to paint them black and put them in the focus of a roughly parabolic reflector.