michaelminn.net / Energy / Renewables

Availability of Future Renewable Energy Supplies

This report provides a introduction to, and summary of, current and potential future energy resources. While there is considerable uncertainty in the quantification of available resources due to significant technical and political limitations, this information is provided to summarize some parameters that can inform further research and debate.

In contrast to concentrated fossil energy sources, renewable sources tend to be more sparsely dispersed, making them much more difficult and costly to exploit.

Annual electrical energy is converted to BTU using the thermal power plant heat rate of 10,339 BTU/kWh to make the numbers comparable to the non-renewable sources that will need to be replaced.

Hydroelectric

Hydroelectric power is the most mature and reliable of the renewable energy sources, providing 16% of the world's electricity in 2007 (EIA 2010c) and much higher percentages in many countries (IHA 2000). In the U.S. hydropower in 2009 only supplied around 7% of electricity (EIA 2010h).

Additional potential hydroelectric power pales in comparison to availability from non-renewable sources and overall energy demand. Additionally, political and environmental considerations present severe barriers to construction of new hydroelectric plants (TCPA, 2008, 268). While there almost certainly will be a place for hydroelectric in many locations and at different scales, there is not enough untapped hydroelectric power to replace significant amounts of energy currently supplied by non-renewables.

Wind

Wind is a rapidly-maturing renewable energy source that has become an increasingly important in recent years. In the United States, wind generation increased dramatically since the late 1980s.

There are significant limitations on the number of areas where the wind blows strongly and consistently enough to make utility-scale power generation practical. In the United States, most of the class IV or better areas are the Midwest and offshore, with few good areas in the Northwest or deep south (Archer and Jacobson, 2003). The intermittency of wind presents technical challenges, although Georgilakis (2008) indicates that impacts on the power grid are modest up to a penetration level of 20%, and Smith et al (2007) asserts that at penetration levels of up to 30%, the problems will be more economic than physical.

While domestic wind power could theoretically supply the total current energy needs of the United States, the geographic sparsity of the resource, the issue of intermittency, and the high expense present a significant challenge to exploiting that potential. For example:

• A typical wind turbine has a 2,000,000 watt nameplate capacity
• At a capacity factor of 30%, over a year (8,760 hours) that turbine would generate 5,256 mWh or around 54 billion BTU
• Completely replacing 98 quads of US energy usage would require 1.8 million wind turbines
• Wind turbines cost around $3.5 million apiece to install (Windustry, 2013)
• The total installation cost would be around $6.3 trillion dollars or 36% of total 2014 US GDP (BEA 2015)
• This does not consider maintenance, repair and renewal costs, or the cost of support infrastructure

Solar

Energy from the sun drives almost all biological and environmental processes on earth and is at the root of most energy resources. Fossil fuels are essentially ancient sunlight that has been preserved in the earth. Wind and waves are driven by solar heating of the atmosphere. The amount of solar energy that strikes the earth annually is around 5.2 million quads, or almost 10,000 times the amount of energy used by industrialized society as primary energy. Yet, in 2007, solar power only accounted for around 6 tWh of worldwide electrical generation (0.062 quads) and the question remains unanswered as to how much of the massive potential can be economically captured for useful work.

Because of the broad, sparse distribution of solar energy across the earth's surface, the complexities of land use, and the evolving technology for concentrating, capturing and converting that energy for useful work, it is impossible to give a firm estimate of how much solar energy will be available in the future for use with machines. However, a number of authors have imagined possible scenarios for increased solar power generation in the future:

• Fthenakis et al (2009) estimated that there is at least 250,000 square miles of land in the sunny American desert Southwest that is suitable for construction of industrial-scale solar plants. This land receives around 4,500 quads of solar energy each year and converting just 2.5% of that energy to electricity could supply all US energy needs
• Jacobson and Delucchi (2009, 2010; Delucchi and Jacobson, 2010) took a cue from former vice president Al Gore and proposed a complete transition of the United States to wind, water and sunlight by 2030 in a plan that included 49,000 300-MW concentrating solar plants, 40,000 300-MW photovoltaic solar plants, and 1.7 billion 3-kW rooftop photovoltaic systems, which could supply all US energy needs. Smil (2010, 148) dismisses the proposal as "claptrap" not only for its exponentially unprecedented scope and schedule, but also for it's estimated $100 trillion price tag.
• Lopez et al (2012) used GIS land-use modeling to assess total technical potential from renewables across the 50 states. The potential numbers are stunning, but no attempt was made to model political or economic constraints

Aside from industrial-scale plants and rooftop photovoltaic arrays, solar energy can be used in smaller-scale applications. Homes can be and are designed to take advantage of passive solar heating. Rooftop flat-plate water heaters have been used for decades. Small photovoltaic arrays are a common site for powering remote equipment that cannot be conveniently connected to the power grid. At the opposite end of the complexity scale, solar satellite systems and solar chimneys have been proposed.

Biomass

The Department of Energy defines "biomass" as, "Any organic matter that is available on a renewable or recurring basis, including agricultural crops and trees, wood and wood residues, plants (including aquatic plants), grasses, animal manure, municipal residues, and other residue materials. Biomass is generally produced in a sustainable manner from water and carbon dioxide by photosynthesis." (Perlack et al 2005). Biomass and biofuels derived from biomass can therefore be thought of as indirect solar energy.

Biomass can be incinerated directly for heat, or converted to liquid fuels through a broad variety of chemical and thermochemical processes that are still evolving. Indeed, perhaps the biggest attraction of biomass is that it is currently the only renewable source of liquid transportation fuel that can be used with the existing carbon-fuel-based transportation fleet, fuel distribution network and industrial infrastructure. Accordingly, biofuels have been widely marketed as the green fuels of the future, and there is an extensive amount of ongoing biofuel / biomass research.

Estimates of total annual biomass growth on the planet vary widely. Groombridge and Jenkins (2002, 10-11) give an estimate of total global biomass at 560 billion tonnes, with an annual growth in the range of 100 to 320 billion tonnes, 45% to 55% of that growth in the ocean. Parikka (2004) estimates that there is total of 420 billion tonnes of woody biomass on the planet (equivalent to around 5,900 quads) and 52 billion tonnes (734 quads) in North and Central America. The resource is significant - the question is how much of that can be economically and sustainably harvested for use in generating energy.

Biomass still accounts for a significant amount of global energy use, primarily burned for cooking and heating in the global south (IPCC 2011, 5).

In 2005 the Department of Energy and Department of Agriculture adopted a strategic goal of 1 billion annual short tons of biomass production (Perlack et al, 2005), which may be equivalent to as much as 9 quads. That figure included a broad range of crops and crop residues, as well as municipal waste and animal manure. However, since that only represents around 9% of 2009 American energy usage, biomass alone clearly can not provide the solution to America's sustainable energy future.

In addition, no generalized, universally-accepted EROI figures are available that consider the energy required to harvest, transport and process bulky biomass on an industrial scale. Accordingly, the net energy available from biomass may be significantly below those direct conversion figures given above. Ethanol from corn is a notable case, with some authors even asserting that the energy required to produce ethanol actually exceeds the resulting energy in the fuel (Pimentel and Patzek 2005). Smil (2010, 98-115) is especially scornful of biofuels, stating, "Using complicated, energy-intensive, environmentally disruptive, and actually nonrenewable processes to produce liquid fuels for oversized, highly inefficient machines -- which are operated all too often for dubious reasons -- adds up to compounded irrationalities."

Geothermal Energy

Geothermal energy is heat that flows from the earth's interior resulting from the slow decay of radioactive particles in rocks (EIA 2011e). The estimated continuous geothermal heat produced by the planet is 42 TW or 1,255 quads per year. This is resource with significant long-term potential since the interior of the Earth is expected to remain extremely hot for billions of years to come (GEA 2011). Geothermal power can be used directly for heating or, in locations where heat levels are adequate, to generate electricity. However, estimates of technical potential are speculative and, accordingly, vary by orders of magnitude.

The major problem with geothermal energy is that there are a limited number of geographic locations where there is enough of this heat concentrated close enough to the surface where it can be accessed and exploited with a practical amount of effort. Most American geothermal sites are located in the West: California, Nevada, Arizona, Idaho, Utah, New Mexico, Alaska, Nevada, Hawaii (USGS 2008). There is also an issue in that some geothermal injection wells have been linked to earthquakes (Glanz 2009). But while geographic and thermodynamic constraints make geothermal a relatively limited resource in terms of total power needs, exploitation makes sense in areas where the resources exist.

Ocean Energy

The world's oceans represent a vast sink and storehouse of energy. The UNDP (2008, 166) World Energy Assessment groups extractable energy into four categories with estimates from various sources of world annual potential:

• Tidal (75 quads): Tides are variations in sea level caused primarily by the gravitational pull of the moon, with additional help from the sun and the rotation of the earth. Therefore, tidal power is essentially indirect lunar power. Tides can be captured using barriers (two-way dams), lagoons and in-current turbines. Tidal energy facilities have been used for centuries and industrial-sized facilities are in operation, but the environmental impacts of barriers have constrained expansion of tidal power.
• Wave (62 quads): Ocean waves are small disturbances in the ocean surface caused by the force of blowing wind. Since the source of wind power is heat from the sun, wave power is essentially indirect solar power. A wide range of different devices for capturing wave energy have been developed or are in development, with most involving some kind of floating device that translates the kinetic undulations of waves into electricity.
• Ocean Thermal: Temperature differentials between warm surface waters and cold deep waters can be used with a heat engine to create kinetic energy to turn electrical generators. Although the potential power available is quite massive (6,800 quads per year) the practicality of large-scale facilities has yet to be demonstrated. High temperature differentials and low energy output requirements are desirable, which makes ocean thermal attractive on islands and in developing countries.
• Osmotic Power: Osmotic pressure differentials between ocean salt water and fresh water from rivers can be exploited to turn turbines and generate electricity. As with ocean thermal, technologies to exploit this resource are at an early stage in development and estimates of potential (80 quads per year) are highly speculative.

Older studies on this issue are an order of magnitude more pessimistic than the UNDP. The U.S. Minerals Management Service (MMS 2006), cited research estimating world economically feasible wave potential at 0.5 - 2.6 quads per year, with the caveat that projected improvements in capture technology could double or triple that amount. Hammons (1993) speculated that world tidal potential is 0.3 - 1.7 quads per year. The MMS estimated the total amount of wave energy on U.S. ocean shores (including Alaska and Hawaii) to be around 7 quads per year, but EPRI (2010) only believes the economically practical resource to be in the range of 0.3 - 0.6 quads per year.

Given the wide range in estimates and significant technical hurdles yet to be overcome, it is not possible to assess how soon or to what extent ocean energy will make a meaningful contribution to energy needs outside of a few niche applications in isolated areas.