Solar Arrays

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Recall that in Chapter 6 we (figuratively) went into the garden and presented a square meter of area to the Sun, and found that the Sun's power falling this was roughly 1.4 kW, neglecting the losses that occur due to passage through the atmosphere. In Earth orbit, this power flux is essentially a free resource that is just too good to miss, so spacecraft are usually equipped with solar arrays designed to convert some of this solar power into electricity. Most

Table 9.1: Primary electrical power sources used on spacecraft

Type Usage and principle of operation

Primary batteries Primary (unrechargeable) batteries are used for short duration missions, for example, to power a launch vehicle during the few minutes' climb to orbit.

Fuel cells Fuel cells are essentially chemical engines that produce electrical power, with water as a by-product. This makes them particularly suitable for manned missions. The duration of their operation is limited, however, by the requirement to fuel the chemical reaction continuously with oxygen and hydrogen. Fuel cells provide the primary power source for the space shuttle.

Solar arrays Solar arrays operate by converting sunlight into electrical power. With the intensity of sunlight at Earth (about 1400 W per square meter), solar arrays on Earth-orbiting spacecraft provide about 100 W of useful electrical power for every square meter of solar array surface area. So, although solar arrays are commonly used, they are inefficient (see text for more detail).

Solar dynamic devices A solar concentrator, such as a large parabolic mirror, is used to focus the power of the Sun to heat a working fluid, such as water. The high-pressure steam produced can then be used to drive a turbine generator—the dynamic part—to produce electrical power. Solar dynamic devices are more efficient than solar arrays, but are generally much heavier. As such, they are only considered for use on large spacecraft, such as space stations. Overall, they are rarely used.

Radioisotope thermal The heat energy obtained from a radioactive material generators (RTGs) (such as an isotope of plutonium) is converted into electrical energy. RTGs are widely used on spacecraft that operate a long way away from the Sun, where sunlight is so feeble as to make the use of solar arrays impractical. Each RTG is cylindrical in shape, typically 1 m in length and 30 cm in diameter (see Figure 9.8). Such a device is around 40 kg in mass, and produces about 200 W of useful electrical power. A quick calculation (200 W divided by 40 kg) gives a rough estimate of the amount of power produced per kilogram—approximately 5 W. So if large amounts of power are required for payload and subsystem operation, then the mass of the power supply can be significant. There is also political opposition to the use of RTGs by the "Green" lobby, which fears the effects of the dispersal of radioactive material in the event of a launch failure.

Nuclear reactors These are scaled-down versions of the nuclear reactor systems found in terrestrial power stations. They are used only for applications requiring large amounts of power— on the order of hundreds to thousands of kilowatts. To date, they have been used rarely, and then only on military space systems such as large active radars for surveillance.

1 min 1 hour 1 day 1 month 1 year 10 years Duration of mission

Figure 9.6: The most suitable primary power source is shown, depending on the level of electrical power required by the spacecraft and the duration of its mission. Note: SA, solar array; RTG, radioisotope thermal generator.

1 min 1 hour 1 day 1 month 1 year 10 years Duration of mission

Figure 9.6: The most suitable primary power source is shown, depending on the level of electrical power required by the spacecraft and the duration of its mission. Note: SA, solar array; RTG, radioisotope thermal generator.

solar arrays used for space applications are made out of the semiconductor material silicon, and they have an efficiency of around 10%. This means that of the 1400 Wof solar power falling on each square meter of array, only one tenth of this (140 W) is produced as useful electrical power to run the spacecraft's systems.

Unfortunately, there are a number of other factors that have to be taken into account, that further reduce the solar array's efficiency. For example, for best performance the array surface needs to be presented to the Sun so that the Sun's rays fall at right angles to its surface. This requires the array to be accurately pointed by the spacecraft's attitude control subsystem (ACS) (see Chapter 8), and this maybe done only to a certain tolerance—say, to within 5 degrees of the ideal. Such a pointing error will reduce the array's efficiency.

Another factor is temperature. Arrays get hot while in the Sun, typically up to around 50°C (in Earth orbit), and the hotter they get the less efficient they are; for example, silicon arrays can lose 10% of their electrical performance with a 25°C increase in temperature. A third issue is that the spacecraft's solar array panels cannot be covered completely in useful silicon material. Each panel is usually composed of smaller silicon cells, and the required electrical interconnection between these means that about 10% of the panel area is dead with respect to power generation. The final factor is damage to the array caused by particle radiation (see Chapter 6). After 10 years of operation in GEO, the electrical output from a solar array can be reduced by around 30% of its beginning of life performance.

All these factors vary according to the spacecraft's mission and orbit, but in Earth orbit a useful rule of thumb is to expect about 100 W of useful electricity—enough to run an old-fashioned domestic light bulb—for every square meter of silicon solar array on the spacecraft. Many modern spacecraft, particularly communication satellites, can have power requirements up to 10 kW, which means that an lot of solar array area is needed. Accommodating such large arrays can have a significant impact on the overall configuration of the spacecraft.

The above discussion applies to Earth-orbiting spacecraft, which are effectively at a distance from the Sun of 1 astronomical unit (AU). As we have seen, the Sun's intensity at 1 AU is about 1.4 kW per square meter, but this intensity varies with distance from the Sun. In fact it decreases in proportion to the inverse square of the distance; Chapter 1 discussed the inverse square law in the context of Newton's law of gravity. Basically, the idea is that at twice the distance from the Sun at 2 AU, the Sun's intensity will fall to a quarter (1/22) of what it was at Earth. At 3 AU it will decrease to a ninth (1/ 32), and so on. So, for example, if our mission is to take us to Saturn, which is about 10 AU from the Sun, the solar power flux there is about 1400 W divided by 100 (102)—about 14 W per square meter. On top of this, there are all the various inefficiencies of the solar array in converting this power flux into electricity, so we can see that at this kind of distance the use of solar arrays to generate electrical power becomes impracticable.

What is the maximum distance from the Sun where solar arrays can still be used effectively to generate spacecraft power? This is difficult to answer, but I would suggest 5 AU (the distance of Jupiter from the Sun) as the outer boundary. A quick calculation tells us that at 5 AU the solar intensity is 1400 Wdivided by 25 (52), which is about 56 W per square meter. Taking account of the efficiency of the array, we might expect about 6 W of electrical power per square meter of array, which still sounds marginal. But one saving grace here is the temperature of the array, which out in the cold reaches of Jupiter's orbit will typically be less than -100°C. As mentioned above, solar arrays become less efficient as their temperature rises, but then by the same token they become more efficient as their temperature drops. Consequently, the array can achieve a useful level of power generation even at these distances. Interestingly, at the time of this writing (2007), the European Space Agency (ESA) spacecraft Rosetta is on its way to rendezvous with a comet in 2014, and the interception will take place at a distance of about 5^ AU from the Sun where the solar intensity is about 50 W per square meter. The Europeans are generally averse to using RTGs, on environmental grounds, and so the spacecraft is equipped with solar arrays for power generation. However, to generate the required 395 W of electricity to run the spacecraft—equivalent to four light bulbs—64 square meters of solar array are needed! A quick calculation, 395 W divided by 64 m2, gives about 6.2 W of electrical power per square meter of array. The overall array efficiency (6.2 W/m2 of electricity divided by 50 W/m2 of solar intensity) is about 12%, which is just about feasible. It is at these great distances from the Sun where it is advantageous to think about the use of RTGs.

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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