Direct Use of Geothermal Energy in New Zealand

Abstract

One of well at Mokai field which has lower pressure than Mokai Power Station system needed, is used by greenhouses to plant vegetables like tomatoes. This direct use system use shell and tube heat exchanger. This heat exchanger type is also use by Rotorua hospital, for space heating and others activities in hospital and staff resident and timber drying factory. In timber drying factory, geothermal fluid system replaces natural gas or coal system.

Plate heat exchanger is used for many places which used geothermal fluid as thermal sources. They are Rotorua Aquatic Centre, Polynesian Spa, Prawn farm. For Rotorua aquatic centre and Polynesian Spa, they have two heating systems, primary and secondary. So that water which used is not geothermal fluid directly but secondary system water which heated by primary system water. Geothermal fluid is just heating water at primary loop. For prawn farm, they use brine from Wairakei field as heat source for heating prawn ponds.

Keyword: Geothermal, Direct Use, New Zealand, Heat Exchanger

1.Introduction

Geothermal energy has been known and used for centuries. Records show that the Chinese, Romans, Turks, Japanese, Icelanders, Central European and the Maori people of New Zealand have used the resources for heating, bathing, and cooking. These uses continue today with Japan, for example using over 1,500 hot-spring resorts. Geothermal energy was also used in Indutrial context in Nothern Italy, who made decorative enamels from the boric acid deposited by steam and hot water at Lardarello. The acid was first extracted commercially in 1818. It quickly became useful and by 1835, 9 (nine) factories had been constructed in the region. The municipal heating districk of Reykjavik, Iceland, in 1928 was another early industrial application. These uses of the resource were restricted to areas where the hot water or steam was easily accessible, so while it was marvelled at, the concept of use not widespread. Other forms of energy were plentiful and inexpensive at the time.

Ilustration of Geothermal Energy Base on Lindal Diagram

The picture above shows utilization of geothermal energy base on Lindal Diagram. The utilization is depend on the temperature, and it varies from soil warming to cement and aggreagate drying. Direct use of geothermal energy use the heat exchanger to transfer heat from geothermal fluid to working fluid. Currently, there are two kind of kinds of heat exchanger like the picture below. They are shell and tube heat exchanger and plate heat exchanger.

Type of Heat Exchanger

In the New Zealand, direct use of geothermal energy has been used in several industries and had been reached the commerciality. Even in Indonesia we have used it, but the utilization is not in large scale and less commercial. Below is several industries in New Zealand which used geothermal energy directly. Some of them are applicable in Indonesia with different commodities.

2. Mokai Glasshouses

Mokai greenhouse is the greenhouse which used geothermal fluid from Mokai geothermal field. The fluid is come from one production well that is not use by power plant because its pressure is not enough to fulfill the power plant system requirement. Mokai-2 (MK-2) is the name of the well which use for this greenhouses, it has about 1,500 m depth. Wellhead pressure is about 45 barg, and 12 barg for inlet pressure heat exchanger system. Type of heat exchanger is shell and tube heat exchanger. The geothermal fluid is inside tube and fresh water which will be heated is outside the tubes (shell side). The fresh fluid can heat about 11.5 Ha (115,000 m²) of greenhouses areas. One problem in this system is scaling because silica precipitation and then blocked the tube in heat exchanger. It was because of uncontinous production about three days on and off, and now they are trying to keep the production continuously to avoid scaling problems.

Shell and tube heat exchanger (top)

and heat exchanger system in Mokai Greenhouses

This system is for keeping the greenhouses temperature, which plant tomatoes, about 16°C -22°C. CO₂ supply is provided by liquid CO₂. Currently, they are trying to separates CO₂ from the wells from H₂S. They can export it the product to Japan for all year because it is not depend on the seasons so that, it is very good business. This kind of direct use can be applied in Indonesia, for only export commodity because the price of vegetable especially tomatoes is cheap even sometime its price is higher in dry seasons. So that, it is needed more feasibility study.

Mokai Greenhouses

3. Timber Drying

This is one of industry that use geothermal as energy source replacing coal and natural gas. The purpose of industry is to increase value of product. So that price of timber will increase after this process. Final product will be lighter, stronger and durable than before it is processed. Type of tree which use for this industry is fine woods that had reached 25 years old. Market of this product is North America countries. Price depends on supply and demand. This facilities can reduce moisture of the timber from170% to just 8%. Moisture of final product depend on county which product will be sold. Product moisture is comparison between weight of water and weight of wood. Total cost for building these facilities which use geothermal fluid is about fourteen millions; twelve millions was funded by Contact Energy, the company which drill wells and provide steam to the timber drying system, and the rest by Tenon, the timber drying company. Each year, Tenon can reduce operating cost for 1.2 millions, and pays for Contact Energy about three to four millions. So that for both of them, this is a very good business because pay back period of the project is less than three years. After that, all revenue is profit.

Kiln Process System

Geothermal fluid for supplying this binary cycle system is come from TH-2 and TH-6. Geothermal fluid temperature for TH-2 is about 180°C and 210°C for TH-6. After through the heat exchanger temperature decrease to 150°C, geothermal fluid is injected to 2 well, TH-7 and TH-8. Fresh fluids, as working fluids, temperature increases from 78°C to 94°C. This fresh water will heat up kiln rooms. The humidity and temperature of drying rooms is controlled by fan, so that the temperature and humidity can be keep constant. This factory can develop in Indonesia, especially in Sumatra and Sulawesi that has geothermal potential and the forest. So that forest products have added value before they are exported or just for domestic use.

Tenon Timber Drying Factory

4.Prawn Farm

Simplified Prawn Farm Heating System

The species that develop in this farm is Malaysian prawn. This area use geothermal fluid as energy source to keep the pond at 25°C - 32°C, and divide to three kind of pond i.e. for prawn eggs hatching, baby prawn and adult prawn according to temperature of water. They use plate heat exchanger which is transferred heat from geothermal fluid to fresh water. Fresh water temperature increase from 14°C to 40°C and geothermal fluid temperature decrease from 90°C to 60°C. Geothermal fluid rate that flows through to heat exchanger is 480 tonnes per hours and fresh water about 800 tonnes per hours. The problems that faces in this area is silica deposition in pipe, so that it must clean once each two to three weeks. And open the heat exchanger once in three months.

Wairakei Prawn Farm

5.Rotorua Aquatic Centre Bores

Simplified Aquatic Centre Heating System

This aquatic centre has good position. It lies in the middle of the city of rotorua. In this area used geothermal fluid for heating fresh water in primary heating system. This system is used for bathing needed and heating secondary system. This secondary heating system water which is used for heating the pools to about 29°C-32°C. And geothermal fluid temperature is about 160°C and re-injected at 110°C. For fresh water in primary heating system is heated up from at 60°C to 85°C. Problem for this system is solid deposition, there is no scaling problem because they using two system, geothermal fluid just exist in primary heating system and full fresh water at secondary heating system.

Rotorua Aquatic Centre

6.Wairakei Terrace

Amount of brine from separator is flowed to certain area to make terraces, silica terraces. This artificial terraces have been becoming a nice place for tourist. This is brilliant idea as another application of geothermal direct use. The brine has another added value in this field.

Wairakei Terraces

7.Rotorua Hospital

Simplified Rotorua Heating System

Geothermal fluid in this hospital is used as energy source for space heating and others activities in hospital. Besides that, it is providing energy for resident of doctor, nurse and administration staff. This system generates about 40 MW in the winter and 14 MW in summer. That energy is provided by four production wells and three injection wells. Type of heat exchanger in this system is tube-shell type. It has six boxes and six shell and tubes heat exchanger in each box which joined in series. They are separated into two lines system, each connected to three boxes. They use tube-shell type heat exchanger is because in that time when they built this system plate heat exchanger was not produced yet. There is problems with calcite and silica scaling but because they use shell and tubes type, so that it is easy to be cleaned. Geothermal fluid from well has temperature 140°C and it is injected at 102°C.

8.Rotorua Polynesian Spa

Simplified Rotorua Polynesian Spa

System in Polynesian Spa is similar to aquatic centre. They have 2 heating system, primary and secondary. They have two sources fluid, one is come from production well and anoher is from the spring, Rachel Spring. In primary heating system, geothermal fluid heats up fresh water. This fresh water which had been heated up at primary system will heat up another fresh water at secondary system. Little amount of fresh water in primary system is used for clothes dryer. This dryer is maybe the first and the only clothes dryer using geothermal fluids. Fresh water that had been heated by secondary heating system will be mixed with water from spring. So that, they have water with high mineral for spa and temperature can be controlled. It is different with aquatic system which used secondary heating system water directly.

Rotorua Polynesian Spa

9. Conclusion

1. Most of direct use application use plate heat exchanger and shell and tube heat exchanger.
2. Direct use of geothermal can use as space heating e.g. Mokai Greenhouses, Rotorua Hospital and as dryer e.g. Tenon Timber Dryer, clothes dryer in Polynesian Spa.
3. Other purpose is as water heating e.g. Prawn Farm, Aquatic Centre and Polynesian spa and for tourism destination like Wairakei Terraces
4. Sources of geothermal fluid can be directly from production well like in Rotorua Hospital, Tenon Timber Dryer, Rotorua Aquatic Centre, Polynesian Spa, or from unused well by power plant e.g. Mokai Greenhouses
5. Others sources are brine from separator e.g. Prawn Farm and Wairakei Terraces and from spring like Polynesian Spa.
6. Geothermal fluid can heat up working fluid directly (single system) or can Heat up the first working fluid and then first working fluid heat up second working fluids (double system) e.g. Aquatic Centre and Polynesian Spa, working fluid in those both places is fresh water.
7. Direct use and indirect use of geothermal can be developed parallel and almost all kinds of direct use are applicable in Indonesia with different methods and commodities.

10.References

1. Lund, J.W., (2005): World Wide Direct Uses of Gothermal Energy, Proceedings World Geothermal Congress, Antalya, Turkeys

2. Nugraha, H.S., (2007): Wairakei 2^nd Field Trip Report, Final Project Report, Geothermal Institute-University of Auckland, Auckland, New Zealand

3. Risdianto, D,. (2007): Wairakei 2^nd Field Trip Report, Final Project Report, Geothermal Institute-University of Auckland, Auckland, New Zealand

4. Zarrouk, S., (2007): Direct Use of Geothermal Energy, Geothermal Institute, New Zealand

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