OVERVIEW

Energy is a concept familiar to each of us, so much so that we will not attempt a rigorous definition. It powers our cars and appliances, heats our homes, and lights our workplaces. The science of thermodynamics teaches that energy can take on many different forms, each equivalent. Energy can be stored in a compressed spring, a rotating shaft, a pressurized vessel, a magnetic field, etc. Heat is a form of energy. Equivalency implies that each type of energy can be measured in the same units. For example, we could sell electricity in calories or measure food energy in kilowatt-hours. This would baffle consumers, of course, but would be perfectly correct thermodynamically.

Table 1 shows the basic energy types and other characteristics associated with each renewable energy resource. The type of energy may restrict how the resource can be used or at least imply that some uses may be more economical than others. The second characteristic, intermittence, is an issue for some resources but not others. In biomass, for example, the energy is locked in chemical bonds and can be released when needed, whereas the kinetic nature of wind means that it must be used when available. Spatial variability refers to the range of the resource across a given region. Sunshine, for example, changes only modestly; annual global solar radiation varies by a factor of two from the sunniest spots in the nation to the cloudiest. Biomass yields, on the other hand, can vary 30-fold from fertile regions to infertile ones, due to variations in soil and rainfall.

Although Table 1 lists seven resources, in reality almost all are derived from the sun. Solar energy is the source of our weather, heating up the atmosphere, driving winds, and dictating the hydrologic cycle. Ocean waves are in turn produced from winds, and ocean temperature gradients result directly from absorbed sunlight. The energy fixed in biomass is likewise converted from sunlight via photosynthesis. Only geothermal energy, derived from the vast thermal reserve of the earth's interior, and tidal energy, influenced mainly by the moon's mass, are not truly solar resources. Even fossil fuels, while not renewable, can be thought of as a form of solar energy, as they are simply fossilized biomass.

Resource Quantification

One of the main efforts of this project was to estimate the size of each of Texas' renewable energy resources. This quantification, summarized in Table 2, warrants discussion. The total energy for each resource comprises the amount incident upon or available within the entire state per year. The accessible resource base is defined as that amount of the total resource that is technically feasible to extract with existing or near-term technology. Units are quads per year (see the inset at right for the definition of a quad). Note that no economic discriminator was used in the definition of accessible base, only a judgement as to technical viability. Energy density compares the relative concentration of the resources at a prime Texas location for each. Finally, typical applications of the resources are listed.

For reference, Texas consumed about 10 quads and the U.S. about 82 quads during 1992. Clearly then, the 4,300 quads of solar energy incident on the state each year is an immense resource. The other resources are substantially smaller since, as mentioned previously, most are derived from the solar resource. For example, only about a fourth of one percent of incident solar radiation is manifest in the kinetic energy of the wind, resulting in a statewide resource of 12 quads. Similarly, the annualized photosynthetic conversion efficiency of sunlight to biomass stands at just 0.3%. A low conversion efficiency, however, does not imply a poor resource. Wind energy may represent only a tiny fraction of the original sunlight, but at prime sites it is the most "energy dense" of the renewables. The 4 quads of accessible wind resource assumes that windy areas of the state are blanketed in turbines spaced 10 blade diameters apart.

The geothermal resource can be evaluated in two different ways. The continuous heat transfer from the earth's interior to its surface is minute, about 0.06 W/m2 or about 10,000 times less than the incident solar radiation on a clear day. Integrated over an entire year it yields just 1 quad of resource. However, the total thermal energy stored within the first 4 miles of the earth's crust is staggering, some 2.3 million quads beneath Texas alone. The sustainability of the resource would depend on how it is exploited, but the number is so large that this would not likely be a pressing concern.

Finally, the building climatology numbers merit a brief comment. This resource refers to employing the climate as a resource to minimize building energy demands through techniques such as ventilation and evaporative cooling. Climatic energies are huge, but the upper bound in potential energy reductions is clearly limited by how much is presently consumed in Texas buildings. The potential to reduce these demands is not certain due to an incomplete knowledge of the present Texas building stock, but the values in the table represent reasonable estimates.

Renewable energies have the reputation for being diffuse in nature and therefore very land intensive. Land acquisition is a central aspect of major development projects. It is interesting, therefore, to contrast the relative land use of several key renewable resources with fossil fuels as in Figure 2. Each square in the figure is sized to represent the area required by the respective resource to yield either a quad of electricity or a quad of primary fuel. Typical conversion efficiencies and Texas' standard spacing for oil and gas wells were used to develop the map. The very large biomass squares point out this resource's land-intensive nature due to its poor solar conversion efficiency. Furthermore, biomass uses virtually all the land it is developed on whereas other resources may not. For example, cattle can graze around wind turbines and oil wells, and solar technologies can be installed on rooftops.

Texas has among the best renewable energy resources in the nation. In addition, most other parts of the U.S. that possess good resources&emdash;sunny states of the desert Southwest or windy states of the Great Plains&emdash;do not presently possess the energy demand nor anticipate the growth that is predicted for Texas. Texas makes up 8% of the U.S. population but consumes 12% of its energy due mostly to the energy-intensive petroleum industry along the Gulf Coast. This fact is significant as new energy facilities, renewable or otherwise, will be constructed most rapidly in the context of a large, growing energy economy. Understanding the state's complex renewable resources is only the first step toward their development.