Principles And Construction 2:
Heat And Its Collection

Heat, Temperature, and Mass

'Heat' is a way of describing energy stored in a substance, such as a concrete or rammed-earth wall, or a bottle of water.

'Temperature' is a measure of how much energy is stored in the substance... but merely knowing the temperature of something is not sufficient to describe how much energy is stored in it. You also have to know how much of the substance there is. This quantity of substance is called the 'mass'.

The total amount of heat energy in a substance is proportional to both the temperature and the mass. This means that a given amount of heat energy can be stored in a small amount of mass at a high temperature, or in a large amount of mass at a lower temperature.

This is why we can put out a candle flame by pinching it between moist finger and thumb: the flame is hot enough to glow, but there just isn't a lot of mass there--it's just a wisp of gas, after all. It doesn't have all that much energy in total. When our water covered fingers hit the flame, the flame's energy spreads out through the fingers' much greater mass, and the resulting temperature rise is much lower. If the flame is small enough, no harm is done.

Heat energy flows from warm places to cooler places. Heat flows through different substances at different rates. It flows through water more quickly that through air, which is why you cool off faster in cool water than in air, through which heat flows very slowly. It comes in handy, as we shall see.

Different substances also store different amounts of heat energy per amount of mass. Water stores a lot of heat energy in each unit of mass; concrete and earth, less so; and air, not a lot. It therefore takes a lot of heat energy to warm an amount of water or concrete or earth through some difference in temperature... and when that water or concrete or earth cools back down, it will yield up that same large amount of heat.

The amount of heat energy stored (or released) per unit mass or unit volume of a given substance across a given change in temperature is called the 'heat capacity' of that substance.

All these things--heat, mass, temperature, insulation, heat capacity--help us build an efficient, comfortable, and low-cost house.

Technical details:

Units of Measurement: introducing the way we describe this whole affair.
Heat Capacity: looking at the amount of heat a substance can store.
Thermal Conductance and Conductivity: the rate at which heat flows into, flows through... or escapes from something.

Capture and Transport of Heat

The heat provided by the sun is diffuse. Sunlight tends to warm large areas by small amounts; only small rises in temperature, relatively speaking, are involved. A large amount of energy is involved in total; however, it is spread through a large mass, and each individual mass that the sunlight strikes--each part of the surface of the earth, perhaps--is only warmed a small amount.

The low-temperature diffuse nature of solar heat provided challenges to people who wished to heat their houses with it. They were used to using high-temperature heat sources built around a fire: furnaces, stoves, fireplaces, and so on.

Such high-temperature sources can easily provide high-temperature fluids--hot air, hot water, steam, for example--to efficiently carry heat energy to different parts of a building. The high temperature meant that a lot of heat energy was packed into each unit of the fluid's mass. At the destination, the heat from the high-temperature fluids could spread out into the destination rooms, warming the much greater masses of their contents by much lower temperatures.

Active Solar Heat Collection

When people think of solar heat, they often think of solar panels mounted on a rooftop, piping some hot fluid to a heating system. This was the most natural way for people used to hot-water, hot-air, or steam systems to think; the fire in their furnaces could be replaced by a solar collector, and the rest of the heating system would not need to change much.

The problem was the low-temperature diffuse nature of the heat provided by sunlight. It is difficult to provide a given amount of heat energy to a house from such low-temperature sources, when they are external. To do this directly, a large amount of low-temperature mass would have to be moved to deliver the heat energy. The designers knew that same amount of heat energy could be provided by a smaller amount of mass--if that mass is heated to a much higher temperature.

As a result, much effort was expended in concentrating the heat from the incoming sunlight. Reflectors focused the sunlight on tubes of some fluid, bringing it to a high temperature. This fluid with its concentration of heat could then be piped more easily to some destination, where the heat could be released.

This is known as 'active' solar heat collection, because it involves pumping a hot fluid from one place to another to deliver heat energy to a desired location.

While such solar panels can indeed be good for providing hot water, they were not the most efficient system for heating buildings, because they involved concentrating a diffuse energy, then diffusing it again. They also involved many mechanical components which required energy to operate. Their advantage was that they could work well with previously-designed heating systems.

Passive Solar Heat Collection

Like many solar-heated houses, the Potters' house takes an alternative approach. Since the heat from the sun is diffuse and low-temperature to start with, and the end use--a warm room--is also diffuse and low-temperature, why not simply collect the heat energy where it is going to be used, and not worry about transporting it at all?

A building can be designed to directly collect heat when necessary, store it as needed within its structure, and yield it without fuss. No separate collectors or heat-transfer fluids or pumps are needed. This is known as 'passive' solar collection. And this is what the Potters' house does.

Everyone who has seen a cat sleeping in a warm sunny window has seen passive solar collection in action. All that is needed is sunlight, a window to admit the sunlight and retain the heat, and some surface--mass--to store the heat.

Many passive solar houses have walls that are heated this way. In some houses, heat-retaining walls are placed directly behind sunny windows, yielding a flow of hot air that can be sent to rooms throughout the house.

The Potters' house is simpler.

Sunlight shines in through the windows as needed, warming the walls and floor. The walls are insulated so that the heat escapes slowly. All rooms are provided with such insulated walls; all but a few have windows--and the walls of those few are interior walls that are warmed directly by the other parts of the house. There are no remote rooms to which heat needs to be sent.

There is no need for a furnace. The windows are sized and arranged to admit sufficient heat during the winter. The walls and floor are thick enough and contain enough concrete and earth to store that heat. The house is insulated enough that the heat that does escape during the winter is easily replenished.

There is no need for a cooling system either. The windows and the walls are anrranged so that they work with the seasonal change in sun angle. A lot of sunshine and heat enters during the winter; only a little enters during the summer. Thus the walls of the house are not heated too much either.

But the way the walls and windows work together with sunlight across the seasons is the subject of the next section...

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