James G. Speight - Encyclopedia of Renewable Energy

See also: Agricultural Waste, Alternate Fuels, Anaerobic Digestion, Biogas.

The annual removal is the net volume of growing stock trees removed from the inventory during a specified year by harvesting, cultural operations, such as, timber stand improvement, land clearing, and removal of trees killed or damaged by natural causes (natural losses), e.g. fire, wind blow, insects and diseases.

An example is clipping (i.e., harvesting aboveground plant biomass) which is common in agriculture and for bioenergy production.

Forest ecosystems contain large amounts of nutrients in woody biomass that may exist either as standing material, on the soil surface, or within the soil profile. Logs removed during timber harvesting remove considerable amounts of nutrients, and the disturbance caused by the process may sometime later, if not immediately, affect the amount of nutrients left on the site due to increased soil erosion, mineralization, and leaching. Thus, intensive harvesting, which removes a greater proportion of the forest biomass than conventional harvesting and the associated nutrients, may cause a decline in forest productivity.

Separately, warming and clipping alter soil and plant properties in either similar or contrasting fashions. For example, in grassland areas, both warming and clipping were observed to increase soil temperature and decrease soil moisture. In contrast, warming increased net primary productivity and plant carbon input to soil, but clipping reduced both. Moreover, warming led to an extended growing season length, while clipping caused compensatory root growth and stimulated exudation of carbon. The interactive effects of warming and clipping on these soil and plant properties rely on the mechanisms governing each single factor and the degree to which these factors interact. The responses of soil microbial communities to two or more factors are even less predictable than those of soil and plant properties, owing to their extremely high diversity.

See also: Biomass.

The apparent density ( bulk density ) is the weight per unit volume of a material, such as coal, which includes the voids that exist in the tested material ( Table A-22).

Table A-22Measurement of the apparent density of materials.

The bulk factor is the ratio of the density of a material after molding to the density of the raw material and provides a measure of the volume change that can be expected during processing.

The apparent density is not an intrinsic property of a material; it can change depending on how the material is handled. For example, a powder poured in to a cylinder will have a particular bulk density; if the cylinder is disturbed, the powder particles will move and usually settle closer together, resulting in a higher bulk density. For this reason, the bulk density of powders is usually reported both as “freely settled” and “tapped” density (where the tapped density refers to the bulk density of the powder after a specified compaction process, usually involving vibration of the container.

Aquaculture is the raising of fish and other aquatic animals in a controlled environment – it is the farming of fish, shellfish, and other freshwater or marine (saltwater) creatures. Using geothermal water in aquaculture helps keep water temperatures consistent, which increases survival rates and makes the creatures grow faster. Low-temperature geothermal resources that are not hot enough to produce electricity are useful to fish farmers. Animals grown in water of the proper temperature grow faster and larger than those in cold water or water with fluctuating temperatures. They are also more resistant to disease and die less frequently.

Fish farmers with access to geothermal water can use it to regulate the temperatures of their fish ponds. Though the mechanism to accomplish this can be complicated, basically what happens is that the fish farmer opens valves to allow geothermal water to flow into the fish ponds until they reach the desired temperature. The valves are then closed to prevent the water from getting too hot. The mechanism is similar to adding hot water to a bathtub to bring the temperature to the desired level. Water flow can be adjusted throughout the year to account for air temperatures. Most ponds contain some mechanism to circulate the water and keep it all at an even temperature. Aquaculture operations usually have several ponds, which are kept small enough to be heated or cooled easily.

The main species of fish that are raised in geothermal waters are catfish, bass, trout, tilapia, sturgeon, giant freshwater prawns, alligators, snails, coral, and tropical fish. The warmth of geothermal water makes it possible to raise tropical marine (saltwater) species in cold, land-locked places such as Idaho.

Some creatures have a range of temperatures in which they thrive. For example, catfish and shrimp grow at approximately 50 of the optimum rate at temperatures between 20 and 26°C (68 and 79°F) and grow fastest at approximately 32°C (90°F), but they decline at temperatures higher than that. Trout thrive at around 15°C (60°F) but dislike lower or higher temperatures.

Like other direct uses of geothermal water, aquaculture allows an area to make use of groundwater that may not be hot enough to generate electricity, but is still hot enough to be useful as hot water. Arizona, for example, has a great deal of geothermal water that is under 150°C (300°F), which cannot generate electricity, but is very useful in aquaculture.

The fish grown in geothermal fisheries are healthier and stronger than fish grown in unheated fish ponds. Fish farmers can regulate temperature throughout the year to make sure the fish grow to a consistent size year-round. However, fish farmers must be careful to regulate water temperature. The water in and near the pipes bringing in the hot groundwater can get very hot, creating pockets that are too hot for fish. For aquaculture to work well, there must be a source of cool water in addition to the hot water. Some geothermal fisheries collect geothermal water in holding ponds and let it cool in order to regulate pond temperatures. If the water does not circulate evenly, there can also be cold spots. This can make the fish crowd into areas where the temperature is at the right level. The hot pipes also can be dangerous to human workers who must wade into the pools for repairs, feeding, and harvesting.

For the most part, using hot groundwater to heat fish ponds is good for the environment. A farm that uses geothermal water is not burning fossil fuels or other sources of heat to regulate water temperature and is therefore not emitting pollutants. Many geothermal aquaculture operations use water that has already been used by geothermal power plants or heating systems. The water has lost most of its heat but is still hot enough to raise the temperature of the fish ponds, so it can be put to a second use before disposal.