Simple Solar Boiler

The Simple Solar Boiler is a low-tech boiler that produces Steam from sunlight. While heated, the Solar Boiler has the same steam output as the of 150L Steam per 25 Ticks, or 6L/t overall.

Math
Every 256 ticks, the boiler checks to see if it can see the sun. If it can, it gives itself 8 points of fuel. During the (about) 5 seconds (8x12=96 ticks) it is active from those 8 points of fuel, its heat increases by 6 (+8-2). During the following 8 seconds, it loses 3.25 heat (-1 per 50 ticks), for a net change of ~2.75 heat per 13 seconds of daylight, or roughly 0.21 degrees per second. It therefore takes 6.3 minutes for a cold Solar Boiler to start producing Steam (temperature=20 to temperature>=100). As a Minecraft day lasts only 10 minutes, if you do not sleep, a Solar Boiler will only produce Steam for about 4 minutes out of every 20-minute full day. So in order to make Solar Boilers feasible, you need to constantly exploit the 'sleep resets time to 6am' feature of beds, and not have them chunk loaded at night.

It takes 38 minutes of continuous sunlight for a Solar Boiler to reach maximum temperature (500).