Uses

An enormous amount of solar energy is intercepted by the Earth each day—equal to the output from 180 million thousand-megawatt power-stations. No wonder people consider the possibilities of harnessing such abundant and pollution-free power. Unfortunately, the energy is spread thinly and attenuated by air molecules, aerosols and clouds (Section 2.3).

An important use of solar radiation is in photosynthesis within green leaves (Note 1.B, Note 2.I). In fact, if the water supply, nutrients and temperatures are suitable, plant growth and hence crop yields are directly related to the amount of photosynthetically active radiation (PAR) (Notes 2.D and 2.I) intercepted by the foliage. The PAR determines the potential net photosynthesis (PNP), which is the maximum amount of dry plant carbohydrate that can be

Figure 2.11 The January and July mean global radiation onto a horizontal surface at the ground, in units of W/m2.

photosynthesised by absorbed radiant energy. In theory, this is 3.4 grams per megajoule of incident radiation, but less than this is achieved in practice, on account of several factors; these include shortages of water or plant nutrients, incomplete covering of the ground by vegetation, inappropriate temperatures at critical stages of crop growth, the inability of leaves to cope with the highest radiation intensities, pests, weeds, diseases and the inefficiency of senile leaves. Figure 2.12 implies values of less than 6 per cent for the ratio of actual to theoretically possible yields.

The important point is that, even when farming problems are overcome, the most food that can be grown is limited by the available solar radiation. That sets an upper bound to the sustainable population of the world (Note 2.I).

Table 2.2 Monthly mean global radiance at various places, mainly in Australia

Location Mean global radiation (W/m2)

Location Mean global radiation (W/m2)

Table 2.2 Monthly mean global radiance at various places, mainly in Australia

Name

Latitude (°S)

January

April

July

October

Rabaul

4

203

204

191

222

Darwin*

12

229

251

235

282

Townsville

19

251

216

189

275

Port Hedland

20

319

231

205

310

Longreach

23

333

231

189

302

Alice Springs

23

309

224

185

291

Brisbane*

27

275

161

135

250

Perth

32

316

173

114

261

Sydney

34

271

175

117

236

Adelaide

35

289

154

103

227

Canberra*

35

308

152

109

245

Melbourne

38

286

128

76

211

Hobart*

43

262

117

71

201

Macquarie Island

54

163

-

21

-

Mirny South Pole

67 90

190 242

1 0

* From satellite measurements by Nunez (1990:345)

* From satellite measurements by Nunez (1990:345)

Was this article helpful?

0 0
Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment