Renewable Energy Review

The intent of this publication is an ongoing investigation of the progress and potential of renewable energy in our world. Our goal is to collect the best writing and news on the subject of renewable energy projects and policies. We have observed that humanity is innovating rapidly as the energy security of the future becomes a global priority. Current trends indicate that the age of coal will end before we run out of coal.

This is a Blog Carnival, where people can submit their blog posts and articles on this topic for inclusion in the next issue. For more information on article submission, see the launch post for the Renewable Energy Review.

What is renewable?

In order to be precise about what it means for an energy resource to be renewable, it is necessary to look at the original source of the energy in question. We have written a longer piece on the subject of what does ‘renewable’ mean for those interested in a deeper look. In short, renewable energy is a term to describe energy resources that are not depleted through use. All energy resources that we currently know of have limited lifetimes. Even renewable energy resources can be depleted on time scales of millions to billions of years. For all intents and purposes though, energy resources that can last that long are considered ‘renewable’.

Most forms of renewable energy that we are familiar with such as wind, hydro and solar depend on energy emitted by our sun. The sun uses nuclear fusion as its power source. The sun’s estimated lifetime is several billion more years. Similarly, if we had fusion power plants, we would have billions of years worth of energy available just using materials such as deuterium, which we can find in the earth’s oceans. For more information, see how we create power from nuclear fusion.

‘Green’ but not renewable

There are a number of energy sources that are regarded as ‘green’ to some extent, but are not strictly renewable. In some cases, such as biomass, an energy source could be renewable but the way in which we currently use it is not.

Landfill gas

Landfill gas plants require a source of waste. Currently there is a lot of such waste to be had, but the supply is not guaranteed for the long term, especially since we are likely to pollute our environment badly if we continue on this path. Landfill gas plants might be making more efficient use of our waste, but they are not an actual solution to the waste problem, nor are they technically renewable unless all of the waste they use is of renewable origin.

Geothermal

Geothermal is expected to be renewable on very long time scales. The source of the earth’s central heat is not well-understood, but there is a very large amount of it. It is very unlikely that our efforts will significantly change the temperature of the mantle and core. However, the practical difficulty of extracting heat from ever-deeper in the earth will eventually cause the price of deep geothermal to be too high for us to bother constructing it as long as alternatives exist. This is because in theory we would have to drill deeper and deeper to attain high temperatures. For the foreseeable future however, we can regard geothermal as renewable for the time scales that we humans normally plan for.

Biomass

Biomass energy relies on the sun’s light to power plant growth. As long as the plant growth can be sustained, biomass can be considered renewable. Non-renewable biomass consumption can happen in two ways:

If we consume our biomass resources more quickly than they grow, such as chopping down old growth forest. This is an ongoing problem in our world today. Using external chemical inputs such as fertilizer, pesticides, or herbicides to help the growth of biomass. If these inputs depend on non-renewable resources such as potash and natural gas, as they often do, the biomass production is somewhat non-renewable. Similar arguments can be made if petroleum products are used since they are also non-renewable.

Material Cycles

On a related note, renewable industry, when considered in a general sense, has some additional overarching goals. Materials should be used in cycles driven by renewable energy. Materials should be used until they need replacement. Renewable energy should be used to make them useful again. There should be theoretically little to no ‘waste’.

As elaborated in the book Cradle To Cradle by McDonough and Braungart, we can distinguish between two general categories of cycles: biological and technical. Biological cycles are driven by life and the sun. Technical cycles are everything else we create. For instance, we might get aluminium from rocks, use it to make a drink can, and then use some renewable energy to reform it into a new can after it has been used. Renewable industry of this sort will require rethinking our designs from the ground up.

Types of Renewable Energy

Hydroelectricity

Hydroelectricity, or ‘hydro’, is energy drawn from the water cycle of the earth. The sun’s light powers the evaporation of water on the surface of the earth, causing it to rise up to form clouds. Clouds eventually form droplets, which then rain down to the surface. The cycle then starts again. Driven by the sun, the water cycle is a truly renewable resource. Hydro is the most well-established form of renewable electricity production. It accounts for about 20% of global electricity production . Hydro in 2006 produced about 88% of all of the ‘renewable’ electricity in the world.

New and planned large-scale hydro projects are producing progressively more electricity. That is, the scale of projects has been increasing throughout the 20th century. This has led to the recent development of hydroelectric mega-projects such as the Three Gorges Dam in China, which can produce up to 22.5 GW of power. Most of these large scale installations utilize reservoirs to stabilize power production and water levels. The construction and use of a water reservoir can have both positive and negative effects. Reservoirs have many uses, such as irrigation and flood control, but they also tend to force the migration of people, flood valuable cropland, and damage local ecosystems. Flooded land also contributes to greenhouse gas emissions, since decaying biomass underwater tends to produce a lot of methane.

In the developed nations, the majority of the ideal reservoir hydroelectric locations have been developed. Newer development is often forced to use locations that are not as ideal as those used for earlier installations. However, advancing technology and societal wealth have also unlocked opportunities that were infeasible in the past. Additional development of hydro will eventually be restricted as a result of us having developed all of the really good locations.

On the other hand, there has also been a resurgence of interest in the smaller forms of hydropower and non-reservoir forms such as run-of-river hydro. Run-of-river systems generally have a reduced environmental impact as compared to reservoir-based techniques. The problem is however that they are generally not dispatchable, limiting their usefulness. If a river flows year-round, then some fraction of a run-of-river hydro system’s power capacity can be considered baseload.

Wind

Wind power has been seeing rapid development all over the world. The installed capacity of wind power has been increasing in recent years at nearly 30% per year. Growth is expected to continue into the next few years at the very least. The cost-effectiveness of wind turbines has improved as larger and taller turbines are built.

A recent trend in wind turbines is towards bigger turbines that reach their maximum power at a slower wind speed. For more information on the subject, see the CNet News: Green Tech article on The next big thing in wind: Slow wind, huge turbines.

Overall the wind power sector has seen tremendous growth and interest in the last few years. It has attracted interest as a cost-effective renewable energy resource. It is important to keep in mind that wind power is intermittent and cannot be dispatched. This means that while it can provide useful energy for our society, it cannot be relied upon by itself. We need dispatchable sources to use in harmony with wind.

Solar

Direct solar energy can be captured in many different ways, and for many different uses. Major implementations that we have looked at in our work are solar photovoltaics and solar thermal power. We give here a brief summary of each, along with a link to our more in-depth articles on each subject.

Solar photovoltaics (PV) are commonly called ‘solar panels’, and they are what most people think of when they think ‘solar electricity’. Solar PV capacity in the world has been growing at a tremendous rate in the last few years, around 60% per year! This means that the installed capacity of solar PV is growing faster (by percentage) than any other power source.



Solar thermal power is created by focusing the sun’s light to create heat. The heat is then used to run a heat engine such as a steam turbine. Some variants of solar thermal power are cost-competitive today and show promise of being extremely cost-competitive in the near future. Solar thermal power has the potential to be both baseload and dispatchable due to its ability to utilize thermal energy storage effectively. These properties make it extremely desirable for use in conjunction with intermittent renewable sources such as wind, solar PV, and run-of-river hydro.

Geothermal

Electricity

The earth’s crust is a relatively thin shell covering a molten layer of material called the mantle. As we drill deeper into the earth’s crust, we find hotter and hotter ambient temperatures. Geothermal electric power uses these hot temperatures to turn water into steam to drive electric turbines. Additionally, the heat can be used for industrial purposes or for space heating.

The feasibility of geothermal power development is very strongly connected with the cost of drilling to a sufficient depth. Deep drilling is often required to obtain hot enough temperatures to make cost-effective electricity. Some areas of the world that are developing geothermal power are Iceland , the United States , and the Philippines . These countries have access to geothermal heat that is relatively close to the surface. This makes tapping the resource much more cost-effective, since the drilling does not have to go as deep. This has contributed to the development of geothermal energy in these nations.

Cost estimates regarding the future of geothermal energy vary greatly. It has been noted that the advancement of drilling techniques for oil field discovery and development has been contributing to a gradual reduction in cost for geothermal development.

Heat pumps

Another way to use geothermal energy is to use the earth as a thermal heat sink. For instance, in many areas of the world, the temperature around 4 meters underground is very stable, even season-to-season. Many homes in the world are now using geothermal heat pumps, or geoexchange systems, to help regulate the temperature of buildings . The ground can provide both cooling in summertime and heating in wintertime. Leveraging this energy resource can lead to dramatically reduced energy use for air conditioning and heating.

Biomass

Biomass energy is essentially using living things like plants, fungi, and bacteria to change some of the sun’s energy into other forms that are useful for us. There are many different ways to accomplish this. However, we will not go into too much detail here. In general, biomass energy is used for electricity, heating, and as high-density fuel for transportation.

Humans have been generating heat by burning plants for thousands of years. In the last two centuries, we have used the heat from burning plants to produce electricity. In the last few decades we have begun to turn plants into high-density fuels such as ethanol and biodiesel on a large scale. This is new implementation but not new knowledge since it has been known since the invention of the Diesel engine that plant-based oils could be used as vehicle fuel .

A lot of research is being conducted into possibilities for biofuels to replace petroleum fuels on a large scale. Some intriguing research is being conducted, but the challenges are very great. Not only is the goal to replace the largest human industry ever created, this task must be accomplished by leveraging one of the oldest and most well-established industries: farming.

Our arable land is already under great pressure to perform in order to feed our world’s population along with our gobally increasing meat consumption. In order to fuel this land productivity, we are drawing on enormous amounts of mineral wealth such as potash, and energy wealth from fossil fuels. The Haber-Bosch process for producing plant fertilizer is responsible for most of the gains in cereal crop production in the 20th century . This process requires a tremendous amount of energy, most of which we still draw from non-renewable sources. For this reason and others, the path we are currently taking with our agricultural production is unsustainable. Better methods of land management and crop choice need to be implemented such as biodynamic agriculture.

Tidal Power

The Earth-Moon system creates predictable movements of water, called tides. Tidal power production has seen relatively little development, but presents some intriguing potential for the future. There are a number of different techniques for capturing the energy of tides.

Some of these have already been implemented in power plants such as tidal stream generators, which can be likened to wind turbines that are underwater. Another type that has been implemented already is barrage tidal power, which involves creating a dam across the mouth of a river or bay area which has a large volume of water flow during tide changes. Electricity is produced from a barrage system in a similar fashion to reservoir-based hydroelectric stations. Dynamic tidal power is an ambitious and very interesting idea for large-scale tidal power. It requires very specific conditions and a very large dam, but it has the potential to harness a tremendous amount of energy.

Tidal power has not been implemented extensively, but it is being investigated more seriously in recent years. Tides are much more easily predicted than wind or cloud cover, making tidal power easier to forecast than wind or solar PV. Predictability is important with regards to intermittent resources. Knowing when you need more power from other sources means you can ramp up other generators exactly when you need them. Uncertainty about power production means that extra power needs to be available to cover any sudden blips. This extra power availability entails additional cost. Tidal power may be more intensely developed in the coming decades thanks to its predictability and possible cost-economy.

Dispatchable renewable energy

Update: A later publication of the renewable energy review includes a more in-depth discussion of how we can produce dispatchable power from renewables.

Call for submissions

This concludes the first installment of the Renewable Energy Review Blog Carnival. For a complete list of all publications in this series, see our post regarding the launch of the carnival. If you are interested in submitting an blog post or article to this carnival, see our submission page on the Blog Carnival website. This carnival is published regularly, and we are always interested in seeing new material.