June 21, 2010 | General

Digester Biogas And Energy Security (Cuba)

BioCycle June 2010, Vol. 51, No. 6, p. 33
With its oil supply strained, Cuba implemented an “Energy Revolution” in 2006. Among the self-sufficiency strategies is increased generation of biogas, especially in rural areas.
Hilary Flynn

DURING the early 1960s, the United States imposed strict economic sanctions on Cuba in response to the Communist revolution that brought Fidel Castro to power. As a result of these sanctions, Cuba developed a trading alliance with the Soviet Union under which Cuban sugar was traded for Soviet oil. For more than three decades, this arrangement provided an energy lifeline to the economically isolated island nation.
After the fall of the Soviet Union in the early 1990s, and increased restrictions on trade by the U.S. government, Cuba’s economy collapsed. Since this time, Cuba has experienced persistent shortages of basic commodities including severe energy shortages. The end of subsidized Soviet oil imports and the recent rise in crude oil market prices have caused a “peak oil” crisis in the Cuban economy.
This new energy reality has forced Cuba to conserve resources and find innovative ways to meet its needs. After years of relying on poor quality fuel and operating inefficient appliances and antiquated power generators, the country recently devised a plan to improve its energy situation. In 2006, the Cuban government began implementing a self-described “Energy Revolution.” Part of this new strategy was to reduce energy consumption by replacing inefficient appliances and light bulbs and to increase energy supply by deploying renewable energy technologies. The following year, a group of energy professionals was convened to study strategies for increasing the supply of renewable energy. One of the primary findings of this working group was that Cuba had significant biogas potential that could be effectively developed in the near term.

Biogas Digesters Powering Rural Cuba
Cuba has a long history of using biogas, with the earliest installation dating to the 1940s. Digesters in Cuba fall under two design categories – fixed-bell and moving or floating-bell systems (called such because the units are shaped like a bell). The fixed-bell, as the name implies, does not have any moving parts and is more suitable for residential applications since it is easier to build and maintain. The bell is partially underground to provide support to the structure as pressure builds up inside.
In the moving bell, the sides and the cap of the bell are not attached, so the cap can move up and down with the level of gas produced. Moving-bell systems require more maintenance and more expensive materials (steel and clay as opposed to the fixed-bell design made of clay and sand).
Cuban digesters are typically designed to use a variety of feedstocks such as manure or crop waste. Common digester substrates include pig and cow manure, and agricultural waste from sugarcane, sweet pea, cassava and beans. The typical biogas digester is small, on the order of 12 to 24 m3 with energy conversion in the range of 0.5m3 to 0.8 m3 of biogas required per kWh of electricity generated. Biogas is typically used to power gas stoves and refrigerators (in some cases in locations where citizens previously did not have either appliance prior to installing a digester).

Digestion Process
The digestion process starts with manure and/or vegetal waste being added to the digester along with water in a ratio of about two parts water to one part organic waste. As the organic matter decomposes, gas builds up in the bell, creating pressure that pushes water from the bell into a holding tank. The increased pressure in the bell forces the processed organic material through piping that extends from the bottom of the bell to the surface. Processed organic waste can be used as fertilizer. The gas is removed as needed from valve-regulated piping. When gas is removed the pressure decreases, which causes water from the holding tank to flow back into the bell. The entire process is continuous, with new substrate added as necessary. This simple design does not require sophisticated temperature or pressure controls or extensive maintenance – a critical feature in remote rural communities.
In some cases, the organic material can be pretreated with microorganisms to accelerate the fermentation process. According to Dr. Jose Antonio Guardado Chacon, a Cuban biogas expert, the literature indicates it takes about 30 days from the start of the process before gas is produced. In Cuba, however, experience has been it takes 21 days due to the tropical climate. With the experimental moving-bell design, gas was produced in just 15 days.

Agricultural School Installation
The 9th International CubaSolar Conference was held in the beginning of April in Bayamo, Granma Province. CubaSolar is a nongovernmental agency focused on promoting renewables in Cuba. The conference included site visits to renewable energy installations, including ones at several schools. Throughout Cuba, children attend agricultural schools to learn animal husbandry and crop management. All students are required to spend time on a farm with the goal of instilling an appreciation for food production in the young citizens.
In Granma Province, one school is building a new fixed-bell biogas digester using clay bricks. The unit is being built by the school’s staff and community members with guidance from CubaSolar. The school has around 600 students and 150 staff. They raise pigs and grow numerous crops for feeding the school children. All of the materials to be fed into the digester will be produced on-site.
This digester has a capacity of 65 m3 and is expected to produce around 25 m3 of biogas per day, and is projected to be operational for 25 years. The school plans to collect solids from the bottom of the digester every two to three days, and use them to fertilize the crops. Biogas will be used for cooking.

Future Of Biogas In Cuba
In many ways, Cuba has been forced into self-sufficiency. As a country with limited resources and subject to sweeping economic restrictions under the U.S. embargo, Cuba has learned how to make do with what it has. Biogas digesters represent another way in which Cubans can use the materials and resources readily available to them to meet their needs.
Biogas also offers many social, economic and environmental benefits that fit with Cuba’s focus on sustainable development:
Social: Biogas provides rural communities with power that may not otherwise be available to them from the power grid. It also benefits the entire country as it moves toward energy independence.
Environmental: Solids produced during digestion can be used in place of agricultural chemicals. Research has shown that biogas-based fertilizer increases yields by 9.7 to 11.8 percent when applied to rice paddies, compared with a 5.6 to 8.2 percent increase in yield from urea. Furthermore, spraying plants with digester effluent reduces insect infestations by 55 percent, making it a natural insecticide.
Economic: Biogas digesters are being built across the country by the end users themselves. Engineers and technicians with experience in building digesters instruct individuals and communities on how to build their own digesters, making the system much more economic for both the end user and the government who would otherwise need to build new transmission lines or install other distributed generation such as PV systems.
Still, biogas does have an image problem in Cuba. Many people see biogas as a “second rate” technology, and would rather be connected to the grid. According to Guardado, as many as 1,000 biogas units have been installed across the island, though approximately half are not operational (the official state estimate of biogas digesters is only 198, however it is likely that this number does not include the small-scale units in rural areas). Some of these units are not functioning because they have encountered a technical problem that the owner does not know how to fix; in other cases the owner has lost interest in running the digester because they have substituted it with another power source.
CubaSolar is working to bring the nonfunctioning units back into operation via technical assistance and education. However, the ones that have been replaced with grid power will be difficult to reinstate because the Cuban government still subsidizes electricity. Therefore, even with the very low wages that Cubans earn, the cost of electricity is often not high enough to make people feel the need to use biogas.
Public perception of biogas is changing, though. The Cuban government is trying to encourage energy conservation by increasing electricity rates. Also, there is a growing consciousness in the country to treat manure. Both of these trends will likely lead to more demand for biogas units. Even though Cuba is a small country with a population of around 11 million, Guardado estimates there is potential for at least 20,000 biogas plants generating millions of cubic meters of biogas.

Hilary Flynn is with Meister Consultants Group in Boston, Massachusetts (hilary.flynn@mc-group.com). She specializes in renewable energy market assessments and policy analysis and recently returned from a research tour on renewable energy and energy efficiency in Cuba. The author would like to thank Dr. Jose Guardado of CubaSolar and Professor Mario Alberto Arrastia Avila of CubaEnergia for information presented in this article.

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