Geothermal energy is a domestic energy resource with cost, reliability and environmental advantages over conventional energy sources. It contributes both to energy supply, with electrical power generation and direct-heat uses. For generation of electricity, hot water is brought from the underground reservoir to the surface through production wells, and is flashed to steam in special vessels by release of pressure. The steam is separated from the liquid and fed to a turbine engine, which turns a generator. Spent geothermal fluid is injected back into peripheral parts of the reservoir to help maintain reservoir pressure.

In the absence of steam, heat from hot water is extracted through a secondary fluid and the high-pressure vapor from the secondary fluid is utilized to run the turbine.
If the reservoir is to be used for direct-heat application, the geothermal water is usually fed to a heat exchanger and the heat thus extracted is used for wide variety of small-scale industries. Hot water at temperatures less than 120 degrees Celsius can be used for this purpose. Further, the spent hot water, after generating electricity can also be used for direct application.

Geothermal plants are modular, and can be installed in increments as needed. Because they are modular, these can be transported conveniently to any site. Both baseline and peaking power can be generated. Construction time can be as little as 6 months for plants in the range 0.5 to 10 MW and as little as 2 years for clusters of plants totalling 250 MW or more.

World Geothermal Energy Status
The USA, Philippines, Italy, Mexico, Iceland Indonesia, Japan and New Zealand are the largest users of geothermal energy resources (both direct and indirect). Other countries with less than 20 MW generations are: Argentina, Australia Ethiopia, France (Guadeloupe), Greece, Portugal (Azores), Russia and Thailand. Currently geothermal resources in over 30 countries provide directly used heat capacity of over 12,000 MW. These countries include: Algeria, Austria, Belgium, Bulgaria, China, Denmark, England, France, Georgia, Germany, Greece, Hungary, Iceland, Indonesia, Ireland, Italy, Japan, Latvia, Nicaragua, Philippines, Poland, Portugal, Romania, Russia, Slovakia, Sweden, Switzerland, Thailand and Turkey.
The majority of the earlier geothermal plants were funded and operated by National Power agencies around the world with the exception of California where the development of the Geysers geothermal field was carried out by privately funded utility companies. With the recent international trend towards de-regulation in the power industry, private developers have become more directly involved in both resource assessment and development. This has been particularly so in Indonesia and the Philippines.

The world map illustrates where geothermal resources are currently being utilised. The red dots indicate power generation, which includes small scale, rural development as well as large scale power plants. Direct use is also applicable to the red dots. The blue dots indicate both high temperature and medium to low temperature resources where direct use of the geothermal heat is being applied. The yellow dots indicate World Bank funded projects, and the green dots indicate Hot Dry Rock (HDR) research initiative.



Flash steam plants totally dominate the marketplace, but over the past ten years many smaller scale binary cycle plants have been installed while several combined (flash steam/binary plants) have been installed. The majority of the World's geothermal power stations are base load stations meaning that they operate 24 hours a day for 365 days with a load factor of about 80%.

Technology & Resource Type
Geothermal resources vary in temperature from 30-350 degrees Celsius, and can either be dry, mainly steam, a mixture of steam and water or just liquid water. In order to extract geothermal heat from the earth, water is the transfer medium. Naturally occurring groundwater is available for this task in most places but more recently technologies are being developed to even extract the energy from hot dry rock resources. The temperature of the resource is a major determinant of the type of technologies required to extract the heat and the uses to which it can be put.

Flash Steam Power Plant
This is the most common type of geothermal power plant. The steam, once it has been separated from the water, is piped to the powerhouse where it is used to drive the steam turbine. The steam is condensed after leaving the turbine, creating a partial vacuum and thereby maximizing the power generated by the turbine-generator. The steam is usually condensed either in a direct contact condenser, or a heat exchanger type condenser. In a direct contact condenser the cooling water from the cooling tower is sprayed onto and mixes with the steam. The condensed steam then forms part of the cooling water circuit, and a substantial portion is subsequently evaporated and is dispersed into the atmosphere through the cooling tower. Excess cooling water called blow down is often disposed of in shallow injection wells. As an alternative to direct contact condensers shell and tube type condensers are sometimes used. In this type of plant, the condensed steam does not come into contact with the cooling water, and is disposed of in injection wells.

Typically, flash condensing geothermal power plants vary in size from 5 MW to over 100 MW. Depending on the steam characteristics, gas content, pressures, and power plant design, between 6000 kg and 9000 kg of steam each hour is required to produce each MW of electrical power. Small power plants (less than 10 MW) are often called wellhead units as they only require the steam of one well and are located adjacent to the well on the drilling pad in order to reduce pipeline costs.

Binary Cycle Power Plants
In reservoirs where temperatures are typically less than 220 degrees Celsius. but greater than 100 degrees C binary cycle plants are often utilised. The reservoir fluid (either steam or water or both) is passed through a heat exchanger, which heats a secondary working fluid (organic) which has a boiling point lower than 100 degrees C. This is typically an organic fluid such as Isopentane, which is vaporized and is used to drive the turbine. The organic fluid is then condensed in a similar manner to the steam in the flash power plant described above, except that a shell and tube type condenser rather than direct contact is used. The fluid in a binary plant is recycled back to the heat exchanger and forms a closed loop. The cooled reservoir fluid is again re-injected back into the reservoir.

Binary cycle type plants are usually between 7 and 12 % efficient, depending on the temperature of the primary (geothermal) fluid. Binary Cycle plants typically vary in size from 500 kW to 10 MW.

Combined Cycle (Flash & Binary)
Combined Cycle power plants are a combination of conventional steam turbine technology and binary cycle technology. By combining both technologies, higher overall utilization efficiencies can be gained, as the conventional steam turbine is more efficient at generation of power from high temperature steam, and the binary cycle from the lower temperature separated water. In addition, by replacing the condenser-cooling tower cooling system in a conventional plant by a binary plant, the heat available from condensing the spent steam after it has left the steam turbine can be utilized to produce more power.

The type of technology selected for utilizing geothermal heat for direct use applications is dependent on the nature of the geothermal fluid and the type of direct use planned. In many direct use applications, the geothermal fluid cannot be used directly, such as in drying processes or where clean steam or hot water is necessary, as geothermal fluid often contains chemical contaminants. In such cases heat exchangers are utilized to extract the heat from the hot geothermal fluid and transfer it to either clean water, or in the case of drying processes, to air.

There are two main types of heat exchangers commonly used. They are plate heat exchangers and shell and tube. The heat exchanger technology employed in the geothermal industry is the same as is commonly used over a wide range of industries where heat exchangers are utilized. Commonly used heat pump technology can also be employed in order to utilize geothermal heat for air conditioning and refrigeration applications.

Technological Issues with Geothermal Developments
Geothermal reservoirs that are close enough to the surface to be reached by drilling can occur in places where geologic process have allowed magma to rise up through the crust, near to the surface or where it flows out as lava. The crust of the earth is made up of huge plates, which are in constant but very slow motion relative to one another. Magma can reach near the surface in three main geologic areas;
• Where earth's large oceanic and crustal plates collide and one slides beneath another, called a subduction zone
• Spreading centers, where these plates are sliding apart
• Places called hot spots- fixed points in the mantle that continually produce magma to the surface. Because the plate is continually moving across the hot spot, strings of volcanoes are formed.

Whether geothermal energy is utilized for power production or for direct use applications, there are issues in geothermal utilization that often have technical implications.

Geothermal fluids often contain significant quantities of gases such as hydrogen sulphide as well as dissolved chemicals and can sometimes be acidic. Because of this, corrosion, erosion and chemical deposition may be issues, which require attention at the design stage and during operation of the geothermal project. Well casings and pipelines can suffer corrosion and /or scale deposition, and turbines, especially blades can suffer damage leading to higher maintenance costs and reduced power output.

However, provided careful consideration of such potential problems is made at the design stage, there are a number of technological solutions available. Such potential problems can be normally overcome by a combination of utilising corrosion resistant materials, careful control of brine temperatures, the use of steam scrubbers and occasionally using corrosion inhibitors.

Provided such readily available solutions are employed, geothermal projects generally have a very good history of operational reliability. Geothermal power plants for example, can boast of high capacity factors (typically 85-95%)
With all projects of significant size, geothermal projects are developed through a series of logical stages; Decisions to proceed to the next stage are normally made progressively through out the project.

Reconnaissance and Exploration Geothermal resources are usually located and defined by a progressively more intensive (and expensive) exploration program that starts at a regional level and eventually results in a drilling program to positively delineate the resource. Reconnaissance surveys will identify the most suitable prospect areas by recognition of favorable geological settings and locating any hot springs or other surface thermal discharge. Reconnaissance studies involve mapping any hot springs or other surface thermal features and the identification of favorable geological structures. The chemical composition of the discharging fluids reveals information about the deeper reservoir, including temperature and fluid characteristics. Geological studies provide information about the probable distribution and extent of aquifers, as well as the likely heat source and heat flow regime. Areas identified as having high potential or that are favored because of proximity to an energy use center, will be explored by more comprehensive scientific survey methods.

In addition to more detailed geological and geochemical studies, a range of geophysical techniques may be used including gravity, magnetic and resistivity surveys. Resistivity surveys in particular can locate anomalies that are directly related to the presence of geothermal fluids. Interpretation of these integrated geoscientific studies leads to prioritization of targets for exploration drilling programs. The application of sound scientific method and analysis during these early phases increases the probability of success with subsequent drilling and development. If these surveys provide very good indications for the presence of a useful geothermal reservoir, the resource is tested by the drilling of exploration wells so that actual subsurface temperatures can be measured and reservoir productivity tested. The exploration program should therefore be designed to suit the type of resource expected, the amount of energy expected to be produced from the project and the timeframe for the development. Geothermal energy when not found in optimum conditions may still be utilized through the help of heat engines. It is therefore appropriate to describe the use of Heat pumps with reference to geothermal energy Heat pumps move heat from one place to another and may be considered as an integral part of using geothermal energy. In case of using geothermal energy in heating or cooling house, many innovative ideas are in place All heat pumps have an outdoor unit (called a condenser) and an indoor unit (an evaporator coil). A substance called a refrigerant carries the heat from one area to another. When compressed, it is a high temperature, high-pressure liquid. If it is allowed to expand, it turns into a low temperature, low pressure gas. The gas then absorbs heat. The normal heat pump system extracts heat from outdoor air and transfers it inside where it is circulated through home's ductwork by a fan.

Geothermal heat pumps are similar to ordinary heat pumps, but instead of using heat found in outside air, they rely on the stable, even heat of the earth to provide heating, air conditioning and, in most cases, hot water. A geothermal heat pump doesn't create heat by burning fuel, like a furnace does. Instead, in winter it collects the Earth's natural heat through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. Fluid circulates through the loop and carries the heat to the house. There, an electrically driven compressor and a heat exchanger concentrate the Earth's energy and release it inside the home at a higher temperature. Ductwork distributes the heat to different rooms. In summer, the process is reversed. The underground loop draws excess heat from the house and allows it to be absorbed by the Earth. There are two major types of loops (a) horizontal and (b) vertical, as described below:

Horizontal Ground Closed Loops
This type is usually the most cost effective when trenches are easy to dig and the size of the yard is adequate. Workers use trenchers or backhoes to dig the trenches three to six feet below the ground in which they lay a series of parallel plastic pipes. They backfill the trench, taking care not to allow sharp rocks or debris to damage the pipes. Fluid runs through the pipe in a closed system. A typical horizontal loop will be 400 to 600 feet long for each ton of heating and cooling.

Vertical Ground Closed Loops
This type of loop is used where there is little yard space, when surface rocks make digging impractical, or when it is desired to disrupt the landscape as little as possible. Vertical holes 150 to 450 feet deep - much like wells - are bored in the ground, and a single loop of pipe with a U-bend at the bottom is inserted before the hole is backfilled. Each vertical pipe is then connected to a horizontal underground pipe that carries fluid in a closed system to and from the indoor exchange unit.. Another type of loop design referred as Pond closed loop may be the most economical when a home is near a body of water such as a shallow pond or lake. Fluid circulates underwater through polyethylene piping in a closed system, just as it does through ground loops. The pipes may be coiled in a slinky shape to fit more of it into a given amount of space.

Regional Picture on Geothermal Energy

Nepal
Geothermal manifestations occur in more than 28 localities in Nepal, mainly scattered along the main Central Thrust and the main Boundary Fault. The surface temperatures of the thermal waters vary between 23 and 73C. Hot springs throughout the country used for tourist and religious purposes have long been popular. Preliminary analyses of the waters and isotopic studies indicate that there should be a large reservoir in western Nepal. Drilling activities for large-scale utilization of geothermal water have been limited by road access.

India
In India there are nearly 400 springs distributed over seven geothermal provinces Exploration and pre-feasibility studies on promising geothermal provinces have demonstrated that commercial exploitation of these reserves need to be initiated. Tattapani geothermal field is reported to have the potential for using the effluent from a 300 kW binary plant in a direct heat plant.

Geothermal provinces, as identified, include The Himalayas: Sohana: West coast; Cambay: Son-Narmada-Tapi (SONATA): Godavari and Mahanadi. These springs are perennial and their surface temperature range from 37 to 90 degrees C with a cumulative surface discharge of over 1000 l/m. These provinces are associated with major rifts or subduction tectonics and registered high heat flow and high geothermal gradient

China
China has more than 2700 thermal springs and, if thermal wells and mine outflows are counted, more than 3200 thermal features. Reports are that 255 high- temperature geothermal systems have been identified and, of these, more than 50 have been studied and assessed. Since 1977, a few moderate-to-high temperature resources have been used to generate power. For the last five years, the emphasis has been on the development of low and medium enthalpy resources and this has progressed so rapidly that there are now more than 1620 direct use sites in operation.

Pakistan
Pakistan is situated over the junctions of the tectonic plates of the sub-continents and is rich in geothermal resources. Three parts of Pakistan i.e. Kashmir, NWFP and Balochistan are the potential zones where geothermal resources are located. So far, no work has been done to mark the seismic zones of the country with particular reference to geothermal resources. However, hot springs maps prepared by the Geology department of Pakistan are available.

Hot Spring of Manghopir (Karachi) is one of the known sources of hot water. The temperature of these springs varies from 50-55°C.

Hot-spring of Drig Road (Karachi) is similar to the Hot Spring of Manghopir. The temperature of this spring varies from 40 to 45°C.

Hot Spring of Chitral earlier owned by the ex-ruler of Chitral is another potential geothermal source in Pakistan. Water of this hot spring was simply flowing into nearby valley and then draining out in the nearby River

Myanmar
Myanmar is one of the countries with abundant geothermal resources. A total of 93 geothermal locations have been identified throughout the country. Out of the 93 geothermal sites, in 43 locations investigations had been made by Myanmar Oil and Gas Enterprise ("MOGE") of the Ministry of Energy. Myanmar has five distinctive igneous alignments related to geographical features of the country, which stretches from North to South. The igneous activity appeared to be more intense during late Tertiary and Quaternary; although the activity ranged from Cretaceous to as late as Recent.

Widespread occurrences of hot springs had been known to exist not only in the younger volcanic regions but also in non-volcanic and metamorphosed areas where ground water heated at depths have ascended through faults, fractures and fissures.

Hot springs are found in Kachin State, Shan State, Kayah State, the Southern Part of Rakhine State in Kyaukphyu, Central Myanmar Area, Shwebo-Monywa Area and especially in Mon State and Taninthayi Division.

Bangladesh
Some features, which were encountered during implementation of other projects related to earth science, if analyzed carefully could establish the fact that the geothermal energy resource may be an issue to be considered with due seriousness in Bangladesh,
During early 60s when Jaipurhat Limestone mining project was being considered for implementation, Fried Krupp rhostoffe of Germany recommended for two vertical shafts up to a depth of 1600 ft underground. During detailed geological investigations in 1980 by Cementation Mining of U.K. it was observed, through shaft site drill hole, that the actual temperature of the strata in and around the limestone bed was above 45 degree centigrade which would alter the design parameters of freezing technology and also the shaft lining with adverse impact on lime stone mining itself. One of the major reasons for shelving the limestone project was the higher temperature underground

Similar phenomenon was observed during Barapukuria coal mining and Maddhyapara granite mining project implementations, when higher temperature in underground became a matter of concern.

The issue of abnormal high temperature became prominent when hot water was encountered in the under ground development works in Barapukuria coal mine itself. In this underground mine water was recorded to flow 10-20 meter cube per hour at -430 m level with temperature ranging from 47- 52 degree Celsius, Though the source of water could not be located it was presumed that the coal seam must have some contact with the basement rock with higher temperature wherefrom the water was flowing to the mine.

There is a known hot saltwater spring, known as Labanakhya in Bangladesh at 5 kilometers to the north of Sitakund (40 kilometers from Chittagong). A number of hydrocarbon exploratory wells in the country met high-pressure hot water; data on which are not properly available. There are some dry wells or abandoned wells, which the well site geologists have mentioned to contain hot water under pressure. As the basement rock is close to the surface and the area, which the Indian geologists are marking as high prospect Himalayan zone, also fall within the northern tip of Bangladesh, it is expected that given due attention; Bangladesh might discover a good source of Geothermal energy within its territory.

Water temperature underground in Barapukuria coal mine
Water inflow point--------- Water temperature(c)--------------- Water inflow
-431 m------------------------- 47 ----------------------------------------10 m3/hour
-428----------------------------- 52---------------------------------------- 3
-350----------------------------- 49 ----------------------------------------20
-372----------------------------- 49---------------------------------------- 20
-425 ----------------------------48 -----------------------------------------23
-429---------------------------- 50 -----------------------------------------1
-430---------------------------- 51 -----------------------------------------16
-425 ----------------------------52------------------------------------------ 5
-430---------------------------- 49------------------------------------------ 7

Interesting temperature variations have been observed in some of the exploratory drill holes in Bangladesh. Some figures are given below.

EXPLORATORY WELL DATA
Locations---------- Temperature (F)---------- Depth (m) ----------Bottom Hole Pressure (psig)
Bakhrabad -------------187 -----------------------9314 -------------------------3500
Beanibazar -------------195 ----------------------12500 ------------------------4900
Bogra ---------------------187 ----------------------4300 -------------------------6900
Fenchuganj -------------185----------------------- 5000------------------------- 2900
Feni -----------------------138----------------------- 3000 -------------------------3200
Habiganj -----------------137----------------------- 3500 -------------------------2000
Kailashtilla ---------------170 ----------------------11600------------------------ 4300
Lalmai ---------------------216 ----------------------11000------------------------ -
Patharia ------------------163 -----------------------4370 -------------------------10000
Rashidpur ---------------148 ------------------------7100 -------------------------3500
Salda Nadi --------------137------------------------ 2400 -------------------------3200
Shahbazpur -------------239------------------------ 3367 -------------------------5000
Sylhet ---------------------145 ------------------------7136------------------------- 2800
Titas -----------------------190 ------------------------7500 -------------------------4400
Atgram --------------------251 -----------------------16000 -----------------------12000
Jaldi --------------------------------------------------------- - --------------------------8000
Kamta ---------------------220------------------------- 3600------------------------- -
Kuchma -------------------200------------------------- 8900 ----------------------3000
Shalbanhat --------------182 -------------------------2513 -------------------------
Semutang -----------------91 -------------------------3000------------------------- -

It should be emphasized that the exploratory wells have been focused on data related to oil and gas, as such water issues have not been appropriately recorded. It is necessary that the available data of exploratory wells should be re-examined when some interesting correlation may emerge there from.

Geothermal Energy presently being considered in many parts of the world as a major source of energy. Though Bangladesh does not have proven record of geothermal energy in commercial perspective , yet indications are there that this energy resource may be available within the country. It is of utmost importance that a program is chalked out to collect all available information on geothermal sources . A specialized section in Petrobangla or Bapex may be assigned to look after the geothermal issues in the country which may work out ,among others, as follows;-
• Review the past activities and reports including the well logs of the exploratory drill holes for oil and gas with special reference to water temperature and pressure.
• Examine and follow up the Geothermal program of the neighboring countries
• Draw up a geophysical survey program giving due emphasis on the exploratory drill holes aiming to locate the sources of geothermal energy.
• A pilot project may be taken up to exploit the available geothermal energy.
• All future drilling programs in Bangladesh must incorporate program for testing of high temperature and pressure water presence along with tests for hydrocarbon.

With the present global trend in managing the impending energy crisis, it goes without saying that all possible avenues must be explored to add new items to the energy resource base in Bangladesh and geothermal energy may appear reasonably promising in this respect.

Md. Mosharraf Hossain: Energy Expert.