Solar energy and thermal pollution

A previous post in this blog stated that if 100% of the energy used by man came from solar energy, the Earth would warm up, and although air pollution and the greenhouse effect would decrease, there would still be thermal pollution. Can we give figures? By how much would the Earth's temperature rise in that case?

According to various sources ([1] and [2]), world power consumption by humans is currently about 18.5 Terawatts (18.5 trillion watts). To find the energy consumed during a given period of time, we should just multiply this figure by the given time. For example, the total energy expenditure during a non-leap year, expressed in Terawatt-hours, will be found by multiplying 18.5 by 365 and by 24 (the number of hours in a year), which is equal to about 162,000 Terawatts-hour.

The second question is this: How much solar energy does the Earth receive? The total power that reaches us is estimated to be about 173,000 Terawatts, of which 30% is directly reflected and sent back into space, so the power that comes from the sun and heats the atmosphere, or reaches the surface of the Earth, where it heats the continents and the sea, and is used by plants, which transform it into chemical energy that keeps them alive, and which also feeds all animals (and ourselves), can be estimated as the remaining 70%, i.e., about 120,000 Terawatts. Compare this figure with 18.5 Terawatts of human consumption, equivalent to 0.015% of total solar energy.

What would happen if all the energy we consume came from solar energy? With current solar panels, we couldn’t convert more than 20% of that energy, i.e., for each usable watt generated, an amount of heat equal to five watts would be released. Thus, to generate the 18.5 Terawatts that we currently consume, a heat equivalent to just above 90 Terawatts would be released, which would be added to our energy expenditure, giving a total of about 110 Terawatts.

Currently, the origin of the energy we consume can be summarized as follows:

• Oil and gas: 54.4%
• Coal: 27%
• Nuclear: 5%
• Renewables: 13.6%

If we were to replace all the gas, oil and coal that we consume by solar energy, we would greatly reduce the emission of greenhouse gases, which are probably causing the warming of the Earth we have detected. On the other hand, there would be an increase in chemical contamination, because solar panels are made from substances difficult to recycle. As we have seen, there would also be global warming, by increasing the total amount of energy associated with the use of current solar panels, which could be reduced if the performance of said panels were increased. But let's talk about what we currently have, although we should keep investigating to improve them.

The final question is this: How much would the Earth's temperature rise, due to the extra heating generated by solar panels, if gas, oil, and coal were completely replaced by solar-powered generators?

The final result is this (if you're interested in where it came from, see the appendix): the rise would be roughly equal to half a tenth of a degree, which is a lot less than predicted (mainly because of the greenhouse effect) if current energy consumption continued to be based on the currently used sources. Not to mention that these sources, sooner or later, will run out, so whether we like it or not, we will end up having to replace them with others.

In conclusion: today and in the foreseeable future, the problem of global warming is not due to the heat that we humans generate, but to the increase in the greenhouse effect caused by the alteration of the chemical composition of the atmosphere. In any case, saving energy is always advisable, because every energy source has some drawback, and by doing this, we are saving money.

The same post in Spanish

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Fernando Sols and Manuel Alfonseca

Appendix: Calculation of temperature rise.

The extra heat generated by the massive use of solar energy (about 90 TW) would be added to what reaches us from the sun (about 120,000 TW), which is equivalent to one part in 1333. The average temperature of the earth's surface is equal to 287 K. As the thermal power varies as a function of T⁴, the relative variation of the temperature is 1/4 of the relative variation of the thermal radiation, that is, (1/4) × (1/1333) = 1/5333 = 0.00019, i.e., about 2 parts in 10,000. For a temperature of 287 K, the change would be equal to 287/5333 = 0.054 K