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Monday, December 23, 2013

Limits on the Growth of Wind and Solar Power -- Part I, Area

The scalability of solar photovoltaic and wind power is considered in this post.  Calculations performed by David MacKay (author, Sustainable Energy Without the Hot Air) show that solar and wind power would occupy extremely large physical areas to achieve meaningful replacement of fossil fuels. 

Replacing Fossil Fuels with Wind Power and Solar Power
Atmospheric CO2 is rising due to fossil fuel emissions.  If the science is correct (and I believe it is), it will result in serious problems due to climate change. 

The natural reaction to this knowledge is to try to replace fossil fuels with renewable energy.  Renewable energy – power from the sun, the wind and from growing plants – is a growing part of our energy supply.   Renewables are regarded as clean, of low impact to the environment, and most importantly, emit no carbon dioxide.  The use of renewables, together with conservation, have reduced CO2 emissions in some Western European nations, and slowed the growth of CO2 emissions in the United States.  Nevertheless, global CO2 emissions continue to rise due to expanding use of fossil fuels, relating to population growth and economic development. 

Environmental advocates propose large-scale investments in renewable energy, and governments have implemented subsidies and policies to encourage these investments.  It is a worthy goal, but before we make a wholesale commitment to building renewable energy infrastructure, we must ask whether it is possible and reasonable to replace fossil fuels with renewables. 

Several criteria must be met in order for renewable energy technologies to prevent climate change by replacing fossil fuels.  Renewable energy technologies must be efficient, scalable, timely, and reasonably certain to perform as expected.

This post and the next post will consider the scalability of wind and solar power; we will consider limits to the growth of solar and wind electrical generation. Part one discusses the physical footprint required for significant replacement of fossil fuels.  Part two will discuss the limited availability of key elements used in solar panels and wind turbines. 

Growth Rates of Wind and Solar Power
From 2005 to 2010, world electrical generation from wind power grew at an average annual rate of 27%.   Solar power grew at a faster pace, at the rate of 53%.   But these growth rates are on the basis of small numbers.   At the beginning of this period, wind power provided only 0.6% of world electrical generation, and the contribution of solar power was negligible.  By 2010, wind power provided 1.7% and solar power provided 0.2% of world electricity.  The current growth rates are remarkable; if we assume a 20% growth rate for wind power, wind would meet world demand by the year 2036.  If these growth rates could continue, renewables would soon eliminate CO2 emissions from electrical generation.  But there are limits to the growth of renewable energy. 

Sustainable Energy – Without the Hot Air
David MacKay has written an extraordinary book, Sustainable Energy – Without the Hot Air.  The book is available for free, in an electronic edition at this site:  http://withouthotair.com/

MacKay puts hard numbers to the question of renewable energy in Britain, as compared to energy demand.  MacKay is clearly an advocate of renewable energy.  However, his analysis reveals the low efficiencies and difficulty of replacing fossil fuels with renewables.

MacKay addresses the huge physical scale of generation facilities required to replace fossil fuels, given the known efficiencies of various kinds of sustainable energy. 

MacKay considers paving 5% of Britain with solar panels, to produce about 50 kWhr/day per capita.  The cost of power would be about 4-fold higher than today’s electrical rates.   The number of photovoltaic panels would require more than 100 times the total number of photovoltaic panels existing in the world (2008).   This installation would produce only slightly more than the energy required for heating, cooling, lighting and gadgets in the UK, or about 44% of the average per capita energy consumption in the UK.   The estimate does not include transformation and transmission losses, or implicit energy consumption in the form of imported goods, for which the energy of production is expended in another place.

MacKay also suggests covering the windiest 10% of Britain with wind turbines.  This would provide 20 kWhrs/day per capita, and require about twice the number of wind turbines in the entire world (2008).  This number of turbines would provide about one-half of the energy needed daily for automotive transportation.   MacKay gives further consideration to offshore wind, while noting that some existing offshore windfarms have had serious difficulties with mechanical lifetime due to corrorsion.  MacKay proposes covering one-third of the shallow water (<30 m) offshore Britain with wind turbines.  It is an area equivalent to putting a belt of wind turbines 4 kilometers wide around the entire coast of Britain.   These turbines would produce 16 kwh/day per capita, or less than half the energy required for automotive transportation in Britain.   Added to the onshore wind assumption, it would involve nearly four times the number of wind turbines in the entire world (2008). 

Energy Density
Let’s use the term Energy Density to indicate the energy produced per unit of surface area on the earth. 
Let’s compare the energy density for wind, solar and petroleum.    MacKay provides numbers for wind and solar power.   Wind power, in the windiest parts of Britain, amounts to about 2 watts/m2.  Solar power, after accounting for panel efficiency, latitude, clouds, time of day and darkness, achieves about 5 watts/m2.    By comparison, the energy density of new “shale-play” onshore oil and gas developments in the United States ranges from about 200 to 1100 watts/m2, assuming a 2.5 acre well pad and a 15 year well life.1

The energy density of wind and solar power is quite low compared to petroleum.  Replacing even a part of the energy provided by fossil fuels would require a physical footprint many times larger than the land currently occupied by petroleum infrastructure.

Area Required to Replace Fossil Fuels with Renewable Energy
The average Briton consumes about 125 kWhr/day of energy (MacKay).   If supplied entirely by wind power, this amount of energy would require a land area of 0.65 acres per capita, or 2.6 acres for a family of four.   The average American consumes about twice as much energy, 250 kWhr/day.   To supply an American with energy from wind power would require 1.3 acres, and more than 5 acres for a family of four.   Solar energy is somewhat more efficient in terms of energy density, but a Briton would still require a quarter acre of solar PV panels, and his family would require an acre.  An American family would require two acres of solar panels. 

Next, consider the area required to supply the full energy requirements of large population centers with solar or wind power.   The New York City metropolitan area contains nearly 20 million people, and covers 13,300 square miles, or 24% of the state of New York.   If we provided the energy requirements of the entire population with wind energy, it would require covering 74% of the state with wind turbines.2   If we assume the same solar efficiencies as Britain, we would cover 30% of New York State with solar panels, in order to provide all of the energy needed by the people of New York City.

The dedication of such large areas of land to energy production is clearly absurd.   We need land for agriculture; we must preserve lands for nature.   Wind and solar power can make some contribution to reducing global CO2 emissions, particularly in places where they are most efficient.  But sooner or later, the growth of solar and wind energy will meet a limit in terms of the land area which can reasonably be dedicated to energy production. 

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1.  The energy density of an onshore oil or gas well assumes the performance of the shale-gas and shale-oil wells currently under development in the United States.   Wells in the Eagle Ford Shale are expected to produce an average ultimate recovery of 200,000 barrels of oil (SPE).  Wells in the Bakken shale are expected to produce 500,000 to 900,000 barrels (EERC).   Wells in the Marcellus shale are expected to produce an average recovery of 1 BCF gas, or 166,000 barrels equivalent (USGS).   The energy density calculation assumes a 2.5 acre well pad per well, which is the area required during drilling.   After the well is on production, much of the area of the drilling pad can be reclaimed for the producing life of the well.

2.  This is under the generous assumption of the best wind power productivity of Britain, which would not exist through most of New York State.  

References:
David MacKay, 2008, Sustainable Energy Without the Hot Air, http://withouthotair.com/

USGS technically recoverable reserves, Marcellus Shale
USGS Marcellus Per Well Recovery

Bakken Oil EUR per well

Eagle Ford Oil EUR per well
Swindell, G. S., 2012, Eagle Ford Shale, an Early Look at Ultimate Recovery, SPE.

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