Volume 7 geothermal energy 7 05 – direct heat utilization of geothermal energy

Volume 7 geothermal energy 7 05 – direct heat utilization of geothermal energy Volume 7 geothermal energy 7 05 – direct heat utilization of geothermal energy Volume 7 geothermal energy 7 05 – direct heat utilization of geothermal energy Volume 7 geothermal energy 7 05 – direct heat utilization of geothermal energy Volume 7 geothermal energy 7 05 – direct heat utilization of geothermal energy Volume 7 geothermal energy 7 05 – direct heat utilization of geothermal energy Volume 7 geothermal energy 7 05 – direct heat utilization of geothermal energy

JW Lund,Geo-Heat Center, Oregon Institute of Technology, Klamath Falls, OR, USA

© 2012 Elsevier Ltd All rights reserved

7.05.1 Introduction

The direct heat utilizations of geothermal energy are traditional and well established worldwide The people of Japan have

indigenous people have been awed by geothermal phenomena considering them sacred sites and a place of refuge Nowadays there are many large-scale uses of geothermal energy Well-known examples are district heating in Iceland, greenhouse heating in Hungary, process heat with steam in New Zealand, mineral extraction in Italy, and individual residential space heating in the United States Direct heat applications of geothermal energy are also called nonelectric uses to distinguish them from electric power generation

The technology of direct uses is generally well established The various applications include: (1) space heating, including district heating systems; (2) greenhouse and covered ground heating; (3) aquaculture pond and raceway heating; (4) agricultural drying; (5) industrial applications; (6) bathing, swimming pools, and spa heating; and (7) snow melting and space cooling Many of these

the application This diagram has recently been updated by the Geothermal Education Office to reflect the temperature range

require the lowest temperatures, with geothermal fluid values from 25 to 90 °C Space heating requires resource temperatures in the

7.05.2 Current Utilization

Today, 78 countries have reported some form of direct utilization of geothermal energy with a total installed capacity of 15 358 MWt

Comprehensive Renewable Energy, Volume 7 doi:10.1016/B978-0-08-087872-0.00707-1 171

�C �F

200

Conventional Digestion in paper pulp

electric

300 Canning of food Evaporation in sugar refining

Evaporation Drying and curing of cement block

Space heating (building and greenhouses)

150 Air conditioning

Space heating

20

Fish farming

50

0

has increased 2.29 times or 5.67% annually

energy use, and capacity factor The capacity factor reflects the equivalent full-load operating hours in a year (annual energy use/ (installed capacity � 8760 h yr−1)) The higher the number, the more efficient the use of the geothermal resource (Table 3)

7.05.3 Global Distribution of Geothermal Heat Utilization

The leading users of geothermal energy for direct utilization of the heat are given in Tables 1 and 2

In terms of the contribution of geothermal direct heat utilization to the national energy budget, two countries stand out: Iceland

contributions to various countries is given in Table 4

7.05.4 Development of Direct Heat Utilization Projects

Before proceeding with a direct heat utilization project, several questions need to be investigated and answered by the potential developer: (1) What are the estimated (or known) temperature and flow rate of the resource? (2) What is the chemistry of the resource? (3) What potential markets do they have for the energy, and what would be the expected income? (4) Do they have the experience, or are you willing to hire experienced people to run the project? (5) Do they have financing and is the estimated net income enough to

resource The following sections describe in more detail some of the potential uses based on temperature and possible limitations 7.05.4.1 Spas and Pools

People have used geothermal and mineral water for bathing and their health for many thousands of years Balneology, the practice of using natural mineral water for the treatment and cure of disease, also has a long history A spa originates at a location mainly due to the

i l

700°F

Geothermal energy uses

400°F

Geothermal 204°C

Typical uses of geothermal energy at

& Minerals Recovery

350°F

177°C

Ethanol, Biofuels

149°C

Cement &

Aggregate

250°F

geothermal Power Plants

Onion &

Garlic

Evaporation

& Pulp Drying

200°F

95°C

150°F

66°C

100°F

38°C

Hydrogen Production*

Fruit &

Vegetable Drying

Mushroom Culture

Soft Drink Carbon­

ation

Drying

Food Processing

Snow

Pulp &

Paper Processing

Lumber Drying

Concrete Block Curing Green-housing

& Soil Sterilization

Fabric

Cooling

&

Water Heating

Blanching, Cooking

&

Pasteur­

ization

Biogas Process

70°F

Melting

& De-icing

Aqua­

**Cool water is added to make the temperature just right for the fish

water from a spring or well The water, with certain mineral constituents and often warm, gives the spa certain unique characteristics that will attract customers Associated with most spas is the use of muds (peoloids), which either are found at the site or are imported from special locations The use of geothermal and mineral water for drinking and bathing, and the use of muds are thought to give certain health benefits to the user Spas and pools for swimming, bathing, and soaking can use some of the lower temperature resources

temperature at 27 °C; however, this will vary from culture to culture by as much as 5 °C Spas and soaking pools (hot tubs) generally are kept at 40 °C, but this can also vary by as much as 5 °C If the geothermal water is higher in temperature, then some sort of mixing or cooling by aeration or in a holding pond is required to lower the temperature, or it can first be used for space heating and then cascaded into the pool If the geothermal water is used directly in the pool, then a flow-through process is necessary to replace the ‘used’ water on

a regular basis In many cases, the pool water must be treated with chlorine; thus, it is more economical to use a closed loop for the treated water and have the geothermal water provide heat through a heat exchanger To conserve heat, a pool may be covered by a

7.05.4.2 Space and District Heating

District heating involves the distribution of heat (hot water or steam) from a central location, through a network of pipes to individual houses or blocks of buildings The distinction between a district heating system and a space heating is that space

250,000 18,000

Year

Installed capacity

Country

Annual energy

−1

China Turkey Japan Iceland New Zealand Hungary USA Italy Brazil Mexico Slovakia Argentina

46

36

25

24

9

9

9

8

6

4

3

3

313

349

630

341

513

249

152

980

622

023

054

048

Bathing, district heating Bathing, district heating Bathing, space heating District heating Industrial Bathing, greenhouses Bathing, space heating Space heating, aquaculture Bathing

Bathing Bathing, space heating Bathing

heating usually involves one geothermal well per structure District heating system has one or more wells serving a number of buildings through a central control station and an extensive piping network An important consideration in district heating projects is the thermal load density, or the heat demand divided by the ground area of the district A high heat density, generally >1.2 GJ h−1 ha−1, or a favorability ratio of >2.5 GJ ha−1 yr−1 is recommended Often fossil fuel peaking is used to meet the coldest period, rather than drilling additional wells or pumping more fluids, as geothermal can usually meet 50% of the

Table 3 Summary of the various applications for direct use worldwide for the period

1995–2010

Capacity (MWt)

Utilization(TJ yr−1)

Capacity factor

District heating is approximately 85% of the space heating values

Iceland

Turkey

Tunisia

Japan

France

Hungary

China

Provides 89% of the country’s space heating needs through 30 urban district heating systems and 200 rural systems

Space heating has increased by 40% in the past 5 years, supplying 201 000 equivalent residences, and 30% of the country will be heated with geothermal energy in the future

Greenhouse heating has increased from 100 to 194 ha over the past 5 years

Over 2000 hot spring resorts (onsens), over 5000 public bath houses, and over 15 000 hotels, visited by 15 million guests per year Geothermal district heating supplies heat to 150 000 dwellings, mainly in the Paris and Aquitaine basins

Geothermal energy is used for a variety of applications, including heating greenhouses and animal farms, heating of spas and sports centers, for secondary oil recovery, and for district heating

Almost equal amount of geothermal energy is utilized for fish farming, heating greenhouses, agricultural crop drying, industrial process heat, district heating, and bathing and swimming The country is the largest user of geothermal energy in the world, accounting for 20% of the annual energy used

district heating systems are capital intensive: the principal liabilities are initial investment costs for production and injection wells, downhole and circulation pumps, heat exchangers, pipelines and distribution network, flow meters, valves and control

of the entire project cost Operating expenses, however, are in comparison lower and consist of pumping power, system maintenance, control, and management The typical savings to consumers range from approximately 30% to 50% per year of the cost of natural gas [9]

Percentage of peak demand

(�C)

100

75

Geothermal

50

25

0

Hours per year

−20

−15

−10

0

−5

5

10 Peaking boiler (6%)

15

8000 6000 4000 2000

0

Geothermal heat pump

(31%)

hot water

Fossil fuel

Meeting peak demand with fossil fuel

7.05.4.3 Greenhouses

A variety of commercial crops can be raised in greenhouses, making geothermal resources in cold climates particularly attractive Crops include vegetables, flowers (potted and cut), houseplants, and tree seedlings Greenhouse heating can be accomplished by several methods: finned pipe, unit heater and fan coil units delivering heat through plastic tubes in the ceiling or under benches, radiant floor systems, bare tubing, or a combination of these methods The use of geothermal energy for heating can reduce operating costs and allow operation in colder climates where commercial greenhouses would not normally be economical It is also important, for certain crops as shown in Figure 5, to keep temperatures constant to optimize growth – a task ideally suited for geothermal energy Economics of a geothermal greenhouse operation depends on many variables, such as type of crop, climate,

and a 2.0 ha facility would require 20 GJ yr−1 (5.5 MWt) of installed capacity With a load factor of 0.50, the annual energy

7.05.4.4 Aquaculture

Aquaculture involves the raising of freshwater or marine organisms in a controlled environment to enhance production rates The principal species raised are aquatic animals such as catfish, bass, tilapia, sturgeon, shrimp, tropical fish, and even alligators The application temperature in fish farming depends on the species involved, ranging from 13 to 30 °C, and the geothermal water can

be used in raceways, ponds, and tanks The benefit of a controlled rearing temperature in aquaculture operations can increase

control are important in fish farming and, thus, need to be considered when using geothermal fluids directly in the ponds

7.05.4.5 Industrial and Agricultural Drying

Industrial and agricultural drying applications mostly need higher temperature as compared to space heating, greenhouses, and aquaculture projects, which is generally >100 °C Examples of industrial operations that use geothermal energy are heap leaching operations to extract precious metals in the United States (110 °C), dehydration of vegetables in the United States (104 °C),

32 50

Temperature (°F)

100

80

60

40

20

0

Trout

Temperature (°C)

Temperature (

125

100

75

50

25

0

Cucumber

Temperature (°C)

diatomaceous earth drying in Iceland (180 °C), and pulp and paper processing in New Zealand (205 °C) Drying and dehydration may be the two most important process uses of geothermal energy A variety of vegetable and fruit products can be considered for dehydration at geothermal temperatures, such as onions, garlic, carrots, pears, apples, and dates Industrial processes also make

the cost per unit of energy used as indicated in Figure 7 (Rafferty, 2003)

7.05.5 Selecting the Equipment

It is often necessary to isolate the geothermal fluid from the user side to prevent corrosion and scaling Care must be taken to prevent oxygen from entering the system (geothermal water is normally oxygen free), and dissolved gases and minerals such as boron, arsenic, and hydrogen sulfide must be removed or isolated as they are harmful to plants and animals Hydrogen sulfide will also attack copper

Plate heat

User system

Injection wellhead equipment

Production wellhead equipment

Geothermal

180 °F

(80 °C)

140 °F (60 °C)

170 °F (75 °C)

130 °F (55 °C)

Peaking/

backup unit

Cost of energy

10

8

6

4

2

0

System load factor

and solder, in addition to being deadly to humans On the other hand, carbon dioxide, which often occurs in geothermal water, can be extracted and used for carbonated beverages or to enhance growth in greenhouses The typical equipment for a direct-use system is

distribution lines (normally insulated pipes), heat extraction equipment, peaking or backup plants (usually fossil fuel fired) to reduce

7.05.5.1 Downhole Pumps

Pumping is often necessary to bring geothermal fluids to the surface, if the well is not artesian For direct heat applications, there are two main types of downhole pumps used in producing geothermal fluids: (1) the lineshaft turbine pump and (2) the submersible pump The difference between the two is the location of the driver In a lineshaft pump, the driver, usually a vertical shaft electric motor, is mounted above the wellhead and drives the pump, which may be located as much as 600 m below the ground surface, by means of a lineshaft In pumping geothermal waters, the lineshaft usually has to be enclosed and an oil drip system used to lubricate the bearings, as hot and cold water do not lubricate the bearings In a submersible pump, the driver, a long, small-diameter electric motor, is located in the well below the surface of the fluid being pumped and below the pump itself and drives the pump through a relatively short shaft with a seal section to protect the motor from the well fluid [11]

Lineshaft pumps have three limitations: (1) they must be installed in relatively straight wells; (2) they are economically limited

to settings of <600 m, and in geothermal applications the depth setting is limited to approximately 250 m; and (3) allowance must

Motor controls

Pump

intake

Multistage

pump

Motor

Wellhead

Variable

speed drive

Lineshaft

column spacer See detail drawings

Discharge head

Shaft

Production

tubing

(Pump column)

Well casing

Open lineshaft bearing lubrication

Lineshaft Pump column

Coupling

Column spacer (Centralizer)

Lineshaft Enclosing tube

Pump column

Column spacer

Wellhead

Electric cable Check

valve

Well casing Multistage pump

Pump intake Seal section (protector)

Pothead seal

Electric motor Controls

be made for differences in thermal expansion between the column and the shaft, by either adjusting the impellers or allowing extra

however, they may be limited by the temperature of the fluid Both types may incorporate the use of a variable frequency drive,

7.05.5.2 Piping

The source of geothermal fluid for a direct-use application is often located some distance away from the user, which will require a transmission pipeline to transport the fluid Geothermal fluid for direct heat applications is usually transported in the liquid phase and the system can often be designed as a standard water distribution network However, allowance must be made for temperature, chemical composition, pipe expansion and contraction, and the use of insulation Pipes can be directly buried, with allowance made for potential external corrosion, placed in underground tunnels or vaults, or located on the surface such as is often done in Iceland For fluid temperatures above 100 °C, the pipe material is usually required to be made of metal, usually carbon steel or ductile iron, although aluminum has been used in some cases With fluid temperatures below boiling, various plastic materials can be used, such as fiberglass reinforced plastic (FRP), polyvinyl chloride (PVC), chlorinated polyvinyl chloride

temperature and chemical quality of the geothermal fluid, in addition to cost, usually determine the type of pipeline material used Generally, the higher the temperature rating, the more expensive the pipe material PEX piping has gained popularity in recent years due to its high temperature and pressure rating (maximum 93 °C at 550 kPa); however, at present, it is only available

in sizes below 10 cm in diameter

Pipelines are often insulated with either polyurethane foam or rock wool covered by a jacket of PE, PVC, or fiberglass The insulation may increase the cost of the piping material by as much as 50%; however, this additional cost may be justified to reduce

the ambient air or soil Temperature loss (°C m−1) is the result of heat loss due to the flow of water in the line The greater the flow in

a particular diameter pipe, the lower the temperature loss At flowing conditions, the temperature loss in insulated pipelines is in the

Transmission length (miles)

300

140

260

Soil temperature

5 ft s–1 (1.5 m s–1) 1.6 ft s–1 (0.5 m s–1)

100

180

140

0

Transmission length (km)

29 km-long and 80 and 90 cm-diameter line (with 10 cm of rock wool insulation) from Nesjavellir to Reykjavik in Iceland The flow rate is around 560 l s−1 and takes 7 h to cover the distance

7.05.5.3 Heat Exchangers

Geothermal water, due to its high temperature, may contain a variety of dissolved chemicals that can be corrosive to various metals used in the heating system Thus, it is usually advisable to isolate the geothermal water from the secondary heating system flowing through the various equipment components To transfer the heat from the geothermal water to a secondary system (water or antifreeze fluid), a heat exchanger is used The heat exchanger can be of the shell-and-tube type or a plate-and-frame type The shell­ and-tube types are not normally used in geothermal systems due to their large size compared to the plate-and-frame types and the difficulty of cleaning the tubes The plate-and-frame types are the most common type of heat exchangers used in geothermal systems due to the superior thermal performance (i.e., low temperature loss between the geothermal and secondary fluid, called the

can easily be taken apart and the individual plates cleaned or replaced as necessary), the expandability (plates can easily be

Downhole heat exchangers (DHEs) are the third type of heat exchangers used in geothermal systems, which eliminate the problem of disposal of geothermal fluid, since only heat is taken from the well However, their use is limited to small heating loads (usually <1 MWt) such as heating of individual homes The exchanger consists of a system of pipes suspended in a well through

Two primary temperature differences govern the feasibility, flow requirements, and design of a heat exchanger: (1) the difference

entering and leaving geothermal fluid temperature, which determines the flow rate The temperature of the entering geothermal fluid must be sufficiently above the process (secondary fluid or air) temperature to be reasonable to size the heat exchanger