**4.1.1 Choice of composite material**

474 Solar Radiation

temperatures above 500oC. Type E has the highest thermopower above 0oC of any of the

Type N, Nicrosil (nickel-14% chromium, silicon) (+) vs. Nisil (nickel-4% silicon, magnesium) (-). This type differs from type K by having silicon in both legs and containing magnesium in the negative leg. It was developed to be more stable (exhibit less calibration drift) than type K when used at temperatures above about 1000°C (1800°F). Both type N wires are similar in color and both are non-magnetic, so identification is usually made by gently

Thermocouples employing platinum and platinum-rhodium alloys for their thermoelement (Noble-metal thermocouple types B, R and S) have been used for many years and exhibit a number of advantages over the base metal types. They are most resistant to oxidation, their thermoelements have higher melting points, and they have generally been found to be more reproducible at elevated temperatures in air. They are therefore used when higher accuracy

Of all the standardized thermocouples, Type **S,** Platinum-10% rhodium (+) vs. Platinum (-) is the oldest and perhaps the most important. Type B, Platinum-30% rhodium (+) vs. Platinum-6% rhodium (-), was adopted as a standard type in the US in the late 1960s primarily to serve requirements in the 1200 to 1750oC range. At elevated temperatures, it offers superior mechanical strength and improved stability over types R and S, and it exhibits comparable thermopower. Its thermopower diminishes at lower temperatures and

Identification of noble metal thermocouple wires is made difficult by the fact that all alloys are nearly identical in colour and all are non-magnetic. Sometimes it is possible to distinguish the positive wire from the negative one for types R or S by observing the 'limpness' of the wires. Pure platinum wires tend to be slightly more soft, or limp, while the rhodium-alloyed conductors are a little stiffer, enough so to permit identification. The differences, however, are subtle, and it is not possible to tell one rhodium alloy from another by these means. Proper connections for these thermocouples can be reliably determined by gently heating the junction and observing the resulting polarity on a

The core of the harvesting module (solar panel) is the harvesting circuit, which draws power from the solar panels, manages energy storage, and routes power to the target system. The most important consideration in the design of this circuit is to maximize efficiency and there are several aspects to this. Solar panels have an optimal operating point that yields maximal power output. The harvesting circuit should ensure operation at (or near) this maximal power point, which is done by clamping the output terminals of the solar panel to a fixed

A DC-DC converter is used to provide a constant supply voltage to the embedded system. The choice of DC-DC converter depends on the operating voltage range of the particular

heating the junction and observing the polarity of the resultant emf.

and longer life is sought, though more experience with lower thermopowers.

standardized types.

**3.4 Noble metal types** 

sensitive indicator.

voltage.

**4. Solar harvest circuit design** 

is vary small in the room-temperature range.

A material for fabricating solar cells should be cheap to acquire and must be pure. Attempt on polymer and composite material based cell is a good development. Composites in general showed good physical properties and improved mechanical strength, 0-3 type super conducting composites with epoxy and phenolic thermosetting plastics have advantages of high toughness, superior abrasion, dimensional stability and heat, water and chemical resistance.

The composition of naturally occurring or pyrolytically obtained composite material (Bitumen), is complex but separation, by both physical and chemical methods, into different chemical groups has been made (Oyekunle, 1985). The fractions so obtained consist of asphaltic hydrocarbons (asphaltenes) viscous naphtheno-aromatic hydrocarbons (heavy oils), heterocyclic and polar compounds (resins). Asphaltenes are hard, non plastic high, molecular weight compounds ranging between 1200 and 200,000 and are thus responsible for temperature susceptibility (Gun, 1973).
