INTERNATIONAL SUMMER SCHOOL on Direct Application of Geothermal Energy

Under the auspice of the Division of Earth Sciences

DIRECT USE APPLICATIONS OF GEOTHERMAL ENERGY IN

Trevor M. Hunt1 & John W. Lund2

1 Institute of Geological & Nuclear Sciences, Private Bag 2000, Taupo, New Zealand ([email protected]) 2 Geo-Heat Center, Oregon Institute of Technology, 3201 Campus Drive, Klamath Falls, Oregon 97601, USA. ([email protected])

ABSTRACT temperature geothermal resources in a triangular shaped area called the Taupo Direct use of geothermal energy in Volcanic Zone (Fig. 1). In some other New Zealand is relatively small (approx. parts of the low enthalpy 7000 TJ/yr) because of cheap electricity, springs occur associated with deep circu- but is increasing. At present, the greatest lation of meteoric waters; these springs use is as industrial process heat in a generally emerge at active tectonic faults. timber mill at where fluid from a In the a narrow zone of low high-temperature geothermal re-servoir is enthalpy springs occur; these are also as- used directly to heat combustion air and sociated with major tectonic faults (Fig. 1). shatter sprays in chemical recovery boi- Because of the abundance of high-en- lers, heat black liquor, and to pre-heat thalpy geothermal resources, and the boiler water. Clean steam, obtained using wide-spread availability of cheap hydro- a heat exchanger, is used to dry paper, generated electricity (US3c/kWhr, heat boiler water, and generate electricity wholesale) and natural gas, little effort has for the mill. Other significant uses are in been made to explore and develop low- space heating, timber drying, fish farming enthalpy geothermal resources except for and orchid growing. bathing. In addition, most of New Zealand has a moderate winter climate in which INTRODUCTION temperatures at night rarely fall below Ð5 New Zealand lies astride the junction oC and during the day are about 10-15 oC. of the Pacific and Australian lithospheric Most other direct uses are associated with plates. Active volcanism associated with the wastewater from geothermal power subduction of the Pacific plate beneath the stations. North Island re-sults in abundant high-

Table 1. Direct heat use in New Zealand, at end of 1999; taken from Thain and Dunstall (2000).

Use Location Installed Capacity Annual Use (MWt) (TJ/yr) Process heat Kawerau 210 5500 Agricultural drying Kawerau, Reporoa 29.3 >253 Space heating , Taupo 722 700 Fish farming Wairakei 18.6 363 Heating greenhouses Kawerau, Taupo Total: >307.9 >7081

This paper has been compiled and con- Foster (1998), Lund & Klein (1998), densed from Anderson (1998), Dunstall & McLachlan (1998), Scott & Lund (1998).

- 80 - INDUSTRIAL PROCESS HEATING from a single heat ex-changer; the other three use geothermal fluid and have their A large timber pulp and paper mill is own, smaller heat exchanger. This is a situated on the Kawerau Geothermal relatively early example of a scheme that Field, and geothermal steam is used di- distributes secondary water rather than re- rectly for heating combustion air and shat- circulating geothermal fluid. The heat is ter sprays in chemical recovery boilers, used for space heating, domestic hot wa- heating black liquor, and to pre-heat boiler ter heating, and heating small swimming water. Clean steam, obtained using a pools. This system has been successful heat exchanger is used to dry paper, heat but there have been a number of design boiler water, and generate electri-city for weaknesses which have contributed to the mill. Engineering details of the piping corrosion, internal and external, and equipment are given by Hotson (1994). difficulty in getting adequate circulation to The use of geothermal steam and other some users. fuels in the mill are controlled to optimise costs using a sophisticated computerised Institutions system (Hotson & Everett, 2000). At the Queen Elizabeth Hospital in DOMESTIC AND COMMERCIAL Rotorua, geothermal fluid is used for spa- HEATING & BATHING (Anderson, 1998) ce heating, and for domestic hot water heating. In addition, some of the partially Beneath Rotorua City are several cooled geothermal water is collected, cool- shallow aquifers containing near-boiling ed further and de-gassed in a concrete water associ-ated with outflows from a vat, then piped to the Hydrotherapy De- deeper, high-tempe-rature geothermal partment for use in the treat-ment of skin system. These outflows ori-ginate from an ailments and arthritis. upflow region that manifests itself at the surface in the Whakarewarewa Thermal Hotels Area where there are many natural thermal features (Allis & Lumb, 1992). Many hotels and motels in Rotorua The earliest uses of the geothermal and Taupo use hot geothermal water for re-sources in Rotorua, and which pre-date space heating and for spa pools. For Euro-pean settlement (1830 A.D.), were example, the Millennium Hotel in Rotorua for bathing, washing and cooking in these uses geothermal water for space heating, thermal features by the native Maori domestic hot water heating, pool heating people. Today, geothermal is applied to a (a medium-sized swimming pool and a variety of domestic and com-mercial number of spa pools), all using fresh heating to provide “mineral” water for water. bathing. Increased usage since 1950 has cau-sed a decline in natural activity at Swimming Pools Whakare-warewa. To counteract this, the This is one of the major uses of geo- government implemented in the 1980s a thermal in Rotorua and Taupo, both public control program that included closure of all and private. Pools contain either town geothermal wells within 1.5 km of the water which is heated by geothermal, or centre of Whakarewarewa and payment geothermal water ("mineral" pools). for use of geothermal fluid (O’Shaugh- The Aquatic Centre in Rotorua is a nessy, 2000). These measures ap-pear to relatively large scale public swimming pool have been successful in stopping the facility. This complex has two indoor swim- decline and many thermal features have ming pools (Fig. 2) and a larger outdoor been rejuvenated (Scott & Cody, 2000). pool; all three are geothermally heated. In addition, geothermal energy is used for Domestic Group Heating Schemes space heating and for domestic hot water There is a housing development with a heating. All of the geothermal fluid is pas- geothermal group heating scheme. There sed through a single plate-type heat ex- are 13 units on this site, which share a changer, which heats the secondary wa- well with six other dwellings on adjoining ter, which is a circulating system of town properties. Six-teen of the total are sup- water. Because of the high capacity of the plied with a circulating supply of hot water geothermal heating system, the supply for showers and hand basins is heated on an

- 81 - as-required basis, avoiding the cost of pool at Whakarewarewa is also used to installing a large storage calorifier. The cook food (potatoes, sweet corn) for sale cold mains water feed is passed through a to tourists. small heat exchanger fitted with an automatic temperature control. There is a GEOTHERMAL GREENHOUSES small buffer tank in the circuit, sized to (Dunstall & Foster, 1998) absorb the variations in temperature in the Waste geothermal steam available hot water that occur when there is a big from the Kawerau geothermal field is used change in the demand. to heat greenhouses in which capsicums To minimise chlorine odour, a once- (bell peppers) are grown for both local and through ventilation system with a relatively export markets. The total greenhouse high air flow rate has been installed. No area is 5250 m2, consisting of 3600 m2 in air is re-circulated. The incoming air is an early timber frame, fan-ventilated, heated to main-tain the building's internal single plastic-covered design built in 1982, temperature. A heat pipe, heat recovery and an area of 1650 m2 in a modern, fan- unit has been incorporated to significantly ventilated, twin skin steel and aluminium reduce the high heat losses that would framed greenhouse built in 1994. Heating otherwise occur. The hot, moist, chlorine- requirements are relatively large compa- laden air being extracted is passed red to other greenhouses in New Zealand, through a heat recovery unit, which uses it due to the high minimum night and day to preheat the incoming air. This unit temperatures needed for the crop and the recovers about 75% of the heat in the cool Kawerau climate at night. exhaust air. The heating of the fresh air is The capsicum crop grows in specially completed by a heating coil through which graded pumice, and is fertilised and wa- low-temperature hot water is circulated. tered using hydroponics. The climate is The Aquatic Centre has annual computer controlled, with humidity regula- maintenance costs for its geothermal tion and a CO2 enrichment system. The ir- system of about US$10,000. If natural gas rigation system is controlled by solar sen- were used to pro-vide the energy, the gas sors, dispensing nutrients at a rate depen- cost would be around $75,000. dent on the uptake rate in the plants. Ma- ture plants grow to a height of approxi- Balneology mately 3.5 m, at which point they are dis- The Polynesian Spa in Rotorua is one carded and the growing process is restar- of the best known examples of direct ted from new seedlings. A high quality mineral pools (Fig. 3). Some of the pools crop is grown, with a yield of 20-30 kg/m2 are built around natural springs with the annually. inflow coming up through the sandy Steam for the greenhouse is supplied bottom of the pools, while other pools use from a 2-phase fluid line fed by several piped-in mineral water. deep wells. The primary separator for this The reputed therapeutic benefits for fluid is 500 m distant, so a small separator geothermal waters have long been mounted beside the 2-phase line is used. exploited. Originally it was intended that Flows to the greenhouse are small, so Rotorua would become a spa in the steam can be tapped from this small European fashion. The Bathhouse, separator and the remaining water retur- opened in 1908 complete with the latest ned to the 2-phase line. The steam supply balneological equipment and related is at 270oC and a pressure of 1.5 MPa (15 treatments then in vogue in Europe, was bar) in the 2-phase line, and a pressure built for this purpose. The Bathhouse is control valve reduces and maintains this at now closed but hydrotherapy still 0.5 MPa (5 bar) for the 50 mm greenhouse continues at Queen Elizabeth Hospital. steam supply line, which is buried in a shallow trench. Motorised valves connec- Cooking ted to temperature controllers in the green- Cooking has been a use of house then determine the flow of steam geothermal heat from pre-European times. admitted to distribution headers, which in Some house-holds in Rotorua still have turn supply a network of 25 mm diameter small, enclosed steam boxes, outdoors, heating pipes. Condensate and residual which are used for steaming food. A hot steam from the heating pipes flow to

- 82 - atmosphere. The black steel heating pipes of 800 mg/kg when the greenhouse is are installed above the crop, supplying closed and 300 - 350 mg/kg while venting. heat to the air and radiant energy to the The steam supplied to the greenhouses plants. Wet and dry bulb sensors are us- each year for heating purposes contains ed for humidity and temperature regulation almost exactly the quantity of bottled CO2 as part of the control system. used each year. With appropriate The annual energy usage is about treatment, the geothermal steam might

700,000 kW/h, at a cost of about NZ$4900 ultimately be as valuable for its CO2 (US$2200). The unit energy cost is contribution as for its heating value. therefore NZ$0.007/kWh, or about NZ$ 5.50 per tonne of steam. The steam flow TIMBER DRYING (Scott & Lund, 1998) is not metered; instead a flat rate is The New Zealand forestry industry is charged each year, based on historical based on plantation grown pine (Pinus condensate flow data. The capital cost for radiata), which is harvested when the the heating system was NZ$44,000. trees are 25-35 years old. Compared to Assuming a 20 year equipment life the many other timber species, it has very discounted cost (12% discount rate) is high moisture content (up to 130% by NZ$0.036/kWh ($0.029 equipment + weight) and must be dried. The main $0.007 energy). Heating a similar green- reasons for drying are to set the sap and house with electricity would cost about prevent warping. The sap usually sets at 11c/kWh (7c energy + 4c installation). 57 - 60 oC, and warping is prevented by The use of geothermal energy therefore establishing a uniform moisture content results in a saving of $38,000 per year, but through the thickness of the wood, which part of this saving is offset by extra is best achieved in a kiln. The drying rate transportation costs. varies with the species of wood and with Most heat is lost from greenhouses by thickness. Natural air-drying is slow and air leakage and wind effects due to poor expensive (because the land cannot be construction. Air leakage rates have a otherwise used); kiln drying is the quickest severe impact on economics especially and simplest way to add value to the when CO2 enrichment and humidity control timber. A timber-drying kiln is a large oven are used because these are much more in which the enclosed heated air is expensive to provide and control than circulated to draw moisture from the timber temperature. Modern plastic covered and then exhaust it to the atmosphere. greenhouses may use as little as 30% of The heat energy is generally supplied by the heat needed for an older glass hot water at high pressure and covered greenhouse. Typical heat loss temperature to heat exchangers in the 2 rates at Kawerau are 3.5 W/m K in plastic kilns. Fans distribute the hot air at velo- 2 greenhouses compared to 12 W/m K in cities of 5-8 m/s and at kiln temperatures glass covered greenhouses. The new of 80-140 oC. twin-skin plastic greenhouse achieves A geothermal drying kiln is associated these low heat losses without loss of solar with the Tasman Pulp and Paper Plant at light transmission required for plant Kawerau. This kiln uses steam at inlet growth. temperature of 180 oC, which produces a Carbon dioxide is an effective growth temperature of 150 oC in the kiln. Batches stimulant for plants, and there is an of 80-100 m3 of sawn timber, each piece opportunity to use the CO2 present in the separated by about 2 cm using spacers, geothermal fluid at Kawerau, however, the are moved into the kiln on three rail- plants would react adversely to the small mounted tracks (Fig. 4). Fans (2 m diam.) amounts of hydrogen sulphide present in produce an air flow velocity of 9 m/s the non-condensible gas. Even very low across pipe heat exchangers, and the H2S concentrations of 0.03 mg/kg can circulation direction is reversed (by have negative effects on the growth of computer) every 1.5 hours. The moisture plants (National Research Council, 1979). is reduced to 10% after about 20 hours, Purification of the available CO2 would then the timber is put through a cool-down therefore be required before it could be period for 2 hours, and finally recon- used in the greenhouses. At Kawerau, ditioned for 6-8 hours at 90-100oC which bottled carbon dioxide is used at a rate of increases the moisture content to 20% about 50 kg per day, to provide CO2 levels

- 83 - (Fig. 5). This creates uniform moisture At present, the farm produces about content throughout each piece of timber. 16 tonnes of prawns per year. The life of The typical timber dried is planks 5 x 10 the prawn starts in salt water inside a cm in size, however, wider material building in breeding tanks, and ends in the requires a lower temperature of 120oC and freshwater ponds outside where they are longer time. The dried product is stained harvested after 9 months averaging 30 to by sap on the surface, which is removed in 40 per kg (about 30 g each). The breed- the finish planing operation. Prior to using ing tanks hold 100-150 breeding stock; geothermal energy, the timber was dried one male to five females. After about 18 at 70oC for four days - a much more costly months the males are replaced. The operation. The entire procedure is moni- female prawns spawn five times each year tored and controlled by computer. The in brackish water (1/3 saltwater and 2/3 final product sells in New Zealand for freshwater), and produce between 20,000 US$150-200 per m3 thus adding 50 - to 80,000 larvae per spawning. A total of 100% to the price. Kiln drying costs about one million larvae are produced per cycle, US$20 per m 3 of which the geothermal giving total production of 9 million post- energy is about 5-10% of this cost. larvae per year. After spawning, the larvae are siphoned off into a catching FISH FARMING (Lund & Klein, 1998) bucket and placed in the larval tanks (Fig. 9). They are fed three times a day with a Malaysian Freshwater Prawns (macro- mixture of crushed mussels and scram- bra-chium rosenbergii) are farmed in bled eggs. They undergo 11 different ponds within a 10.2 ha area, near Waira- moults to metamorphose into a post-larvae kei Geothermal Power Station. There are stage in 30 days. They are then moved to nine ponds varying from 0.2 to 0.35 ha in nursery ponds at 28 oC (Fig. 10). Here area and 1.0 to 1.2 m in depth (Fig. 7). they grow from 0.01 g to 4-6 g in four The length to width ratio of the pond months, and are fed pellets containing surface is approximately 20:1. The sides fishbone and minerals three times a day. are sloped for stability and for ease of After four months in the nursery, the harvest. The bottom material is composed prawns are transferred to outdoor ponds of an impermeable volcanic ash that also for their final growth of a further five stores nutrients for the prawns. The months. The prawns are moved manually ponds are kept at a temperature of 24 (+1) using a net with mesh openings that catch- oC. A temperature probe controls the flow es only the largest ones. They are fed of water into the ponds and an aerator is pellets, but zooplankton growing in the used to obtain vertical mixing of the pond ponds provide additional food. Prawns are water. Without this vertical mixing, the voracious eaters and will turn cannibalistic pond would become temperature stratified, if undernourished. Two kilograms of food reducing the production of prawns. produces one kilogram of prawns. After Water for the ponds comes from two five months, the ponds are drained, the sources: a plate heat exchanger (at 55 oC) prawns picked up manually from the bot- (Fig. 8) and river water (at 10oC). Water is tom, and then transferred to an on-site circulated throughout the system from a hatchery for washing before they are taken storage and settling pond that is kept at fresh to a restaurant or snap frozen for 21oC. About 90% of the water is future use (Fig. 11). recirculated with the 10% makeup water The stocking rate in the outdoor coming from the river. Water is circulated growout ponds is about 30 per square through the plate heat exchanger at a rate meter or 20-30,000 prawns per pond, of 250 t/hr in summer and 400 t/hr in depending upon the size. At harvest time, winter. The primary side of the plate heat 400-500 kg are produced from the smaller exchanger takes wastewater from the ponds and 800-1000 kg produced from the Wairakei geothermal power plant; about larger ponds. Ninety percent of the 4000 t/hr of 90oC wastewater is dischar- harvested prawns are sold in the adjacent ged from the plant and flows across the restaurant; the remainder are sold to pumps supplying the plate heat exchan- gourmet restaurants. Over a recent three- ger. The geothermal water cannot be year period, the restaurant has served used directly in the ponds due to the more than 30 tonnes of prawns to more presence of sulphur, lithium and arsenic. than 45,000 visitors. The restaurant offers

- 84 - a plate of 16 prawns for US$15. The farm amount of the steam from three wells is also a commercial tourist attraction with which supply the nearby Poihipi power visitors able to have a half-hour tour of the plant. These wells are 450-500 m deep hatchery, five times each day. and each can produce 120 t/hr at 0.7 MPa. Prawn farming is labour-intensive with Some of the steam supplied to the high overheads: six hectares cost greenhouse is passed through a shell-and- US$330,000 per year to operate, but tube heat exchanger (Fig. 14). The return about US$60,000 per ha. Factors heating is computer-controlled and is which have contributed to the success of supplied to the greenhouse by large hot the farming are the availability of cheap water heaters (using fans and finned heat hot water, cheap flat land, and tourism in extractors) or by steam radiators and fans the area. (Fig. 15). The energy supplied to the greenhouses is about 200 MJ/hr for GROWING ORCHIDS (McLachlan, 1998) (peak). Tropical orchids (Phalaenopsis) are Fertiliser is applied daily through a grown in greenhouses on Wairakei series of computer-controlled electric Geothermal Field. These grow wild in the solenoids so that each plant can take both jungles of southwest Asia, with a few water and fertiliser up through the capillary species extending northwards to Taiwan watering system used on the growing and Sikkim, and southward to northern tables. This enables the plants to be fed Australia. In nature, the plants are living and watered without water lying in the leaf on the surface of trees (epiphytes) and the joints, which would cause plant rot and stems hang downward, with the leaves subsequent death. The lighting system arranged in a spiral fashion. However, produces light in an oblong pattern, so that under cultivated conditions the flower light is not wasted on the pathways. The stems are tied vertically and grow upward amount of light applied is measured by (Fig. 12). They are shade-loving plants, comparing light received by an exterior requiring a light intensity of only 1000 foot light sensor and light sensors in each candles (10,700 lx) or only 10% of full growing area. The computer turns on the sunlight. lighting system, until the amount of light The orchids are grown for cut-flower applied reaches the required level for production. The plants are grown in maximum growth. artificial monsoon conditions, similar to Air samples are collected (15 min those experi-enced by the plants in their intervals) and passed through a gas natural habitat. This enables production at analyser to assesses the level of CO2 in any time of the year to meet market each greenhouse, compared with pre-set requirements in Japan and elsewhere. levels in the computer, and if additional The company has approximately 250,000 CO2 is required, this is applied to each plants in its greenhouses (Fig. 13), orchid table though micro tubes. laboratory and quarantine area; of these, 30,000 are mature plants. Each plant REFERENCES produces on average two stems of blooms Allis, R.G., Lumb, J.T., 1992. The Rotorua each year upon reaching maturity (two geothermal field, New Zealand: its physical years old), and these blooms are specially setting, hydrology, and response to packed to produce the best possible return exploitation. Geothermics 21, 7-24. from overseas markets. An individual Dunstall, M., Foster, B., 1998. Geothermal plant-performance recording system greenhouses at Kawerau. Geo-Heat enables the company to breed selectively. Center Quarterly Bulletin 19 (3), 21-23. In addition to good breeding, parent plants Hotson, G.W., 1994. The long term use of geothermal resources at the Tasman Pulp must also have good colour, shape of & Paper Co Ltd’s mill, Kawerau, New bloom, and be prolific in their bloom Zealand. Proceedings 16 th NZ production per stem, to qualify as breeding Geothermal Workshop, 261-268. stock. Hotson, G.W., Everett, G., 2000. Energy The greenhouses cover an area of 0.8 model for an industrial plant using ha and are kept at temperatures of 26 oC geothermal steam in New Zealand. during the day and 21 oC at night. The Proceedings of the World Geothermal greenhouses are heated using a small Congress 2000, 3439-3445.

- 85 - Lund, J.W., Klein, R., 1998. Prawn Park Ð and varying management regimes. Taupo, New Zealand. Geo-Heat Center Geothermics 29, 539-556. Quarterly Bulletin 19 (3), 29-31. Scott, J.W., Lund, J.W., 1998. Timber drying at McLachlan, A., 1998. Geothermal orchids. Kawerau. Geo-Heat Center Quarterly Geo-Heat Center Quarterly Bulletin 19 (3), Bulletin 19 (3), 19-20. 32-34. Thain, I.A., Dunstall, M., 2000. 1995-2000 O’Shaughnessy, B.W. 2000. Use of economic Update report on the existing and planned instruments in management of Rotorua use of geothermal energy for electricity geothermal field, New Zealand. generation and direct use in New Zealand. Geothermics 29, 573-592. Proceedings of the World Geothermal Scott, B.J., Cody, A.D., 2000. Response of the Congress 2000, 481-489. Rotorua geothermal system to exploitation

Isolated high temperature resource North Zone of Island high temperature resources (TVZ) Rotorua Kawerau N Zones of low temperature Strongly springs Mineralised warm springs

Wairakei Australian Plate

Zone of warm springs associated with Alpine Fault Pacific Plate

South Island

Dunedin

0 400 km

Fig. 1. Map of New Zealand showing distribution of low- and high-enthalpy geothermal resources.

- 86 - Fig. 2: Indoor swimming pool at the Aquatic Centre, Rotorua. Fig. 6: Green peppers growing in a geothermally heated greenhouse at Kawerau.

Fig. 3: Mineral Pools, Polynesian Spa, Fig. 7: Geothermally heated outdoor pools o Rotorua (temperature 34-44 C). containing tropical prawns, Wairakei.

Fig. 8: Hot water intake and plate type heat Fig. 4: Timber stacked ready for geothermal exchanger at prawn farm, Wairakei. drying, Kawerau.

Fig. 5: Reconditioning of geothermally dried Fig. 9: Larval breeding tanks at Wairakei prawn farm. timber, Kawerau.

- 87 - Fig. 10: Nursery ponds at Wairakei prawn Fig. 13: Geothermally heated greenhouse farm. with potted orchid plants, Wairakei.

Fig. 11: Harvested prawns, Wairakei prawn farm. Fig. 14: Main heat exchanger at Wairakei orchid farm.

Fig. 15: Forced air heat exchanger at Wairakei orchid farm.

Fig. 12: Orchid Phalaenopsis

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