Control and Conversion of Solar Power

Background

Solar energy conversion is one of the most addressed topics in the field of renewable energy. Solar radiation is usually converted into two forms of energy: thermal and electrical energy. Solar electricity has applications in many systems such as rural electricity, water pumping and satellite communications.

In the past, solar power was usually used for large-scale grid connected system and small remote photovoltaic plants or stand-alone systems (Bica, 2008). Recent technological development in thin-film photovoltaics (PV) is leading to new generations of consumer portable solar panels. These new solar panels are light weight, durable, flexible, and have been reported to achieve power efficiencies of up to 10%. The portable solar panels make solar power readily available for mobile power needs such as outdoor enthusiast, expeditions and campers. It also provides portable solar power for the military to extend the run time of military devices including satellite communications, two-way radios, laptop computers, thermal imaging cameras, GPS, and etc. Therefore, solar power is expanding beyond its traditional applications.

Solar power is harvested and stored by charging rechargeable batteries. Older solar battery chargers were mainly developed for stationary situations such as solar house and RVs. Lead acid batteries are usually used because light weight is not a major factor to consider. However, since the appearance of the foldable and light weight solar panels, the need to develop solar battery chargers for more portable batteries such as Nickel metal hydrid (NiMH) and Lithium-ion (Li-ion) batteries becomes essential.

Previous work has been done to compare battery charging algorithms for stand alone photovoltaic systems (Armstrong, 2008). Peak power from the solar panels was tracked for photovoltaic systems using various methods (Hua, 1998). To increase conversion efficiency, maximum power point tracking techniques as well as optimal control were studied and implemented (Boico, 2007; Masoum 2004) .

Charging a battery requires a regulated dc voltage. However, the voltage supplied by a solar panel can vary significantly depending upon the day, time, weather condition and irradiation from the sun. In order to charge the battery with a regulated voltage, a dc-dc converter is connected between the solar panel and the battery. The output voltage, current and power can be regulated using feedback controllers (Mohan, 1995). In this chapter, the following dc-dc converters are discussed.

• 1.

  Buck converter (step-down)

• 2.

  Boost converter (step-up)

• 3.

  Buck-boost converter (step-down/step-up)

• 4.

  Cuk converter (step-down/step-up)

• 5.

  Sepic converter (step-down/step-up)

Of these five converters, buck converter and boost converter are the basic converter topologies. Buck-boost, Cuk and Sepic converters are derived based on the two basic topologies. In a dc-dc converter, the switching device such as transistor or MOSFET is switched on or off completely, therefore efficiency of dc-dc converters is generally higher than linear regulators.