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Tuesday, 29th December 2009

How good is solar power

In a previous blog, I talked about the potential of renewable energy, in particular solar energy, in tackling the global issue of climate change. I mentioned solar thermal collectors which are gaining popularity in Hong Kong and elsewhere. As I shall explain, there is more than one good reason for this.

Solar thermal collectors (courtesy of Ho Fung School, Hong Kong)
Solar thermal collectors (courtesy of Ho Fung School, Hong Kong)

So how good are the solar thermal collectors? The most common ones you see in Hong Kong do not involve high technology. They consist simply of black or dark-coloured absorbers of sunlight. Sunlight heats up the water inside the pipes and causes it to rise, by natural convection, to a storage tank at the top. With good insulation, solar thermal collectors are capable of producing hot water even in wintertime, as the graph below shows.

Figure 1   Daily direct solar radiation measured at King's Park
Figure 1     Daily direct solar radiation measured at King's Park
(November 2008 to October 2009)

Figure 1 gives the average daily amount of solar radiation directly falling on a flat surface oriented towards the southeast (120 degrees from north) and tilted at 40 degrees from the horizon. The orientation is an optimal one based on available data.

As Figure 1 shows, the peak of incoming direct solar radiation occurs in wintertime. We believe that this has something to do with the amount and type of clouds at different times of the year in Hong Kong. One can think of the thick clouds in summertime associated with thunderstorms and rain, and the relatively thin clouds if any in wintertime associated with the dry northeast monsoon.

Based on the above, our calculations show that two solar thermal collectors each of size 1 square metre will generate over 100 litres of warm water per day in wintertime and 60 litres per day on average round the year. That is, more warm water comes at a time when you most need it, and you save on your energy bills. By 'warm water' we mean water raised 20 degrees Celsius from its original temperature. In our calculations, a conservative estimate of 50% for the efficiency is assumed, i.e. half of the incoming solar energy is absorbed and used in heating water.

Thus, solar thermal collectors are handy when it comes to providing your household needs. If installed on the roof, they help lower the indoor temperatures a little, resulting in further energy savings on air conditioning in summer. When used for generating electricity, they are also generally more efficient than solar panels or solar cells. A drawback is that solar thermal collectors require direct sunlight, as cloudy days tend to diminish the power output a lot. However, the intensity of sunshine in Hong Kong is enough to more than compensate for this.

B.Y. Lee

Last revision date: <17 Jan 2013>