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PostPosted: Sun 23 May 2010 10:05 
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Solar panel: maximum power point tracking (MPPT)

In CREG journal 43 (March 2001, pp14-16) and again in journal 60 (June 2005, pp9-13) I discussed a method of ensuring that a solar panel was always operated at its maximum efficiency. The argument went as follows: If the panel is open-circuit it develops a voltage across its terminals but, because no current is flowing, it delivers no power. Conversely, if the panel is short-circuited then the current flows through the short-circuit but, because this has zero resistance, no voltage is developed and, once again, the solar panel delivers no power. This suggests that there must be an optimum load resistance, somewhere between short and open circuit, where the panel will deliver the maximum amount of power. My article, which you can read at http://bcra.org.uk/creg/jnl/public-pdf/j60_09.solar.pdf, described how to construct a special type of switched-mode battery charger for use with a solar panel, which regulated the power in such a way that the solar panel was always operating at maximum efficiency.

Such operation is now referred to Maximum Power Point Tracking (MPPT) and is routinely applied to solar power systems (That is not to imply that I was stating anything new – the principle was generally known before I wrote about it in 2001. What has happened now is that a snappy phrase has been coined to describe it).

Linear Technology http://www.linear.com now manufactures an IC, LT3652, that combines a switching regulator with an MPPT system in much the way I described. The IC’s index page is at http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1037,C1078,C1089,P89360 and includes some useful hyperlinks, including the datasheet http://cds.linear.com/docs/Datasheet/3652fc.pdf and several articles describing the IC and the principle of MPPT. Also see http://www.epdonthenet.net/article/32577/Streamlined-solar-cell-based-battery-charging.aspx.

Interestingly, MPPT can be applied to systems other than solar photovoltaic panels. Consider a solar thermal panel, for example. Such devices typically use a pump to circulate water, which is heated by the solar panel and which transfers its energy, via a heating coil, to a tank of water, in much the same way as a gas central heating boiler uses a closed circuit to heat up a domestic hot-water tank. The question is: at what rate should the pump operate? If does not circulate any water at all, the solar panel will get very hot, but can transfer no energy. Conversely, if the pump operates at a very high rate, it circulates a large amount of water, but this water has no chance to be heated by the panel. Clearly, there must be an optimum pumping rate, where the water is pumped slowly enough for it to be heated in the solar panel, but fast enough to transfer the heat at a useable rate. This is obviously a candidate for a maximum power point tracker and, in fact, a project to build an MPPT for a solar thermal system was described in the May 2010 edition of Elektor magazine, http://www.elektor.com/magazines/2010/may/solar-thermal-energy-regulator.1321945.lynkx.

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