Physics is the science that defines how the physical world is structured and how each part of it interacts with all the others. Unfortunately, it's an incredibly complicated science because of this. The world is a very complex machine, after all!
As far as solar designers are concerned, there are some very basic laws of physics that must be understood. If you are a newbie in the solar energy field, here's a brief primer for you.
The sun is the only moving part in passive design
No matter where you build a passive solar system, the only moving part is the sun. Whether it's windows placed on a south-facing wall for solar gain or a massive solar chimney built up the side of a mountain, the sun is the only moving part.
The sun's path across the sky changes throughout the year. In summer, it travels much higher than in winter, as shown in the diagram. In practical terms, this means that the sun gives more heat directly overhead in summer and less heat at a lower angle in winter.
This is why south-facing windows are so effective. The low winter sun reaches further into the home, providing heat and light in cold months. The high summer sun generates a lot more heat but doesn't reach as far inside the home. It's easier to block the summer sun as well, using a simple awning or another kind of window covering.
Heat always moves in one direction
It is a common misconception that heat always rises. Heat does always move in one direction, but not always upwards: it always goes from warm to cold. That means it can travel sideways or even downwards.
In practical terms, this means that as your home heats up in summer, the heat in the walls tries to travel inwards towards the cool centre of the home. In winter, the heat indoors tries to travel outwards to the cold exterior. Insulation, plugging leaks and installing other barriers prevent the heat from escaping when you want it inside and from getting in when you want it to stay outside.
Well-designed passive solar homes use thermal mass inside the home to absorb and store heat for later release. This requires virtually no effort on your part, except perhaps to open or close shutters as required. In most cases, a passive solar setup can provide between 25% and 80% of your annual heating needs.
Water has strange properties
Water does strange things at extreme temperatures and the "extremes" for water are well within normal limits, unlike most chemicals. For example, propylene glycol, which is used as antifreeze in solar fluid, boils at 371°F (188°C) and freezes at -74°F (-59°C); those temperatures are not commonly experienced as a result of weather!
Water boils at 212°F (100°C) at sea level. When it goes above this temperature, it vaporizes into steam. Thankfully, steam can be compressed so it doesn't cause too much trouble (unless it escapes and burns someone). Vaporization itself – the point where a liquid turns into a gas – is extremely