Answer to Question #110317 in Physics for Jj

Question

A farmer plans to generate all the electricity needed on her farm, using either a biogas generator or a small wind turbine.
The biogas generator would burn methane gas. The methane gas would come from rotting the animal waste produced on the farm. When burnt, methane produces carbon dioxide.
The biogas generator would cost £18 000 to buy and install. The wind turbine would cost £25 000 to buy and install.
The average power output from the wind turbine would be the same as the continuous output from the biogas generator.
Evaluate the advantages and disadvantages of the two methods of generating electricity.
Explain, with a reason, which system would be better for the farmer to buy and install.

Answer 1

In terms of the price of electricity produced, it is more profitable for a farmer to buy a biogas-based generator. The difference in price is £7,000.

Biogas plants have another significant plus. They not only produce the necessary gas for the energy sector, but also process the waste in such a way that enough nutrients remain in the residue and can be used as fertilizer. In other words, a biogas plant allows you to organize such a production in which even waste is used as raw material for obtaining the resources needed in the household: electric and thermal energy, fertilizers [1]

A biogas generator is no different from a conventional gas generator, because biogas and natural gas have a similar composition. The only difference is that a biogas plant is able to provide full autonomy to suburban households, since the feedstock for producing electric or thermal energy (gas) is produced directly on the farm, and not bought and delivered from outside. The figure shows a diagram of a gas generator using secondary heat. Under this scheme, waste energy recovery can reach 90%.

From an environmental point of view, bioenergy does not produce any extra pollutants. On the contrary, instead of an uncontrolled release of the dangerous greenhouse gas methane - $CH_4$ generated during spontaneous decay of waste, carbon dioxide $CO_2$ and water $H_2O$ vapor are released into the atmosphere. Although these emissions are the same greenhouse gases that are easily processed by vegetation, they contribute to its growth and enhancement of photosynthetic mechanisms releasing pure oxygen.

The wind turbine with the same average power output as from the biogas generator does not participate in any gas exchange and depends only on the strength of the wind [2]. However, usually installations of the same average power as fuel are quite large. Their blades should be raised to a great height and constantly rotate. In strong winds, energy must be stored in the battery. In light winds, battery power is consumed to maintain the average energy consumption of the farm. The environmentally friendly disposal of batteries is a complex and not completely resolved problem.

A farmer might think of other sources of energy [3].

[1] https://en.wikipedia.org/wiki/Biogas

[2] https://en.wikipedia.org/wiki/Sustainable_energy

[3] Letcher T. (ed.) Future Energy: Improved, Sustainable and Clean Options for Our Planet.

2nd Edition. — Elsevier Ltd., 2014. XXII, 716 p. — ISBN: 978-0-08-099424-6.