Geothermal District Heating Dr. Oddur B. Björnsson Fjarhitun hf, Reykjavík, Iceland SUMMARY In geothermal heating maximum effective temperature drop should be strived at across the house heating systems. This is commensurate minimum flow rate, optimal pumping requirements and minimal fluid extraction from the geothermal reservoir. This requires the adoption of: Large and effective radiators. Double pipe heating system. Thermostatic control on each radiator. In certain cases modification of existing house heating systems, e.g. conversion from a single pipe to a double pipe system or installation of larger radiators, may not be feasible. In such cases cascaded flow of the geothermal fluid through a combination of heating systems of different temperature levels may be the solution. Direct use of the geothermal water in the house heating systems is preferred where the chemical characteristics of the geothermal fluid are suitable. Otherwise heat exchangers of resistant materials are necessary to isolate the geothermal fluid from the heating fluid where corrosion or scaling of the piping and radiator system are to be expected. Such heat exchangers must be designed for maximum temperature drop of the geothermal fluid. The paper describes the commonly used heating system configurations in Iceland and elsewhere. It outlines moreover the characteristics of geothermal heating systems, their automatic control systems and recommended geothermal field management and monitoring systems. KEYWORDS: Geothermal, direct use, cascaded use, control. INTRODUCTION Icelandic homes were converted from oil and coal fired heating to geothermal heating in the fourties, fifties and sixties. They have therefore undergone the necessary changes from a conventional fuel heated house system to a geothermal one. Experience shows that this was a wise decision. The economy of a properly designed and operated geothermal district heating system is far better than that of a conventional fossil fuel system: Heating cost is from 20% to 80% of the cost of heating with oil. Cost of heating-energy to the customer in Reykjavik is 1,5 US cents/kWh. The most expensive district heating in Iceland charges 3 US cents/kWh. Reduced tariff is offered for recreation facilities such as for swimming pools and heating of football fields. Maximum effective temperature drop in the house heating systems - hence minimum flow rate - is of fundamental importance in geothermal systems calling for: Large and effective radiators Double pipe heating system. Thermostatic control on each radiator. Where modification of existing house heating systems, e.g. conversion from a single pipe to a double pipe system or the installation of larger radiators, is not deemed feasible, cascaded flow of 1/19 the geothermal fluid through a combination of heating systems of different temperature levels may be the solution. If chemistry of the geothermal fluid permits, direct use in the house heating systems is preferred. Heat exchangers of resistant materials are necessary where corrosion or scaling of the piping system may be expected. The heat exchangers shall be designed for maximum temperature drop of the geothermal fluid. Low cost of operation and usually high cost of initial investment is what characterises geothermal district heating systems. The high initial cost involves the exploration and the drilling of the geothermal wells, as well as the production field development. The cost of investment and operation is directly related to the quantity of the geothermal fluid in motion, i.e. the number of wells, pumping from the well, and transmission to the market area and distributing to the customer. Therefore: The supply temperature to the customer is kept constant throughout the heating season and as high as legally permitted (normally 80°C in Iceland). The return temperature is made as low as possible (35°C or lower in Iceland). All hot water is metered by volume. The tariff system provides an incentive to the client to use the heat from the purchased water efficiently. Due to high initial investment the geothermal energy shall be used as base load (see the typical load duration diagram below) Load duration curves for a) Reykjavík, Iceland b) Central Eastern Europe (typical) 100% Peak load 90% fossil fuel 80% Capacity of geothermal wells 70% 60% 50% Base a) load 40% geo- Demand ratio thermal 30% b) 20% 10% Heating season (in CEE) Hot tap water only 0% 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Hours pr. annum Figure 1 Typical load duration curves. Curve a) depicts a district heating system with a high load factor and b) system with a low load factor (no space heating during summer season.) 2/19 1 DISTRICT HEATING SYSTEMS 1.1 Types of district heating systems Geothermal district heating systems may be divided into two main groups depending on whether the geothermal water is used directly in the house heating systems (secondary system) or indirectly by transferring the geothermal heat to the secondary system via heat exchangers. In the latter case the geothermal water is confined to the primary system. Figure 1 depicts a few examples of direct and indirect use of geothermal water for district heating. Temperature values shown are typical values under design conditions (e.g. -15°C outdoor temperature in Iceland.) Figure 2 Examples of district heating network with geothermal water Direct use If the chemistry of the geothermal fluid permits the water may be used directly in the house heating systems without any fluid separation by heat exchangers (example: Reykjavik, the capital of Iceland.) In such cases the following technical solutions have been used: a) Single pipe system: This single-pass or once-through system uses the geothermal fluid (temperature below 100°C) directly for heating in radiators, floor coils etc.. The spent fluid from the radiators is discharged to waste. Due to safety limits to maximum temperature of hot tap water, usually 55 to 60°C (38°C for baths), heat exchangers or automatic mixing valves are employed. In the former case cold water is heated with geothermal water through a heat 3/19 exchanger to the required temperature and in the latter the geothermal water is mixed directly with cold water. b) Dual pipe system: A part of the spent return water from the house system (temperature 30°C to 40°C) is collected and mixed with the geothermal supply water (temperature 100°C - 130°C) to obtain a constant supply temperature (e.g. 80°C) irrespective of weather conditions. The excess return water is discharged to waste or injected back into the reservoir (re-injected.) Geothermal District Heating Direct use of low temperature water Peak load boiler plant Single Gas Hot tap water distribution separator system Drain Temperature sensorTemperature 80°C Heating Supply Pump system pipline Hot tap Control water Double 130°C valve distribution Well 80°C Gas separator system Mixing pump Deep well Pump Pump 35°C Drain Return pipeline 35°C Well 130°C Deep well Pump Figure 3 Process diagram for direct use of geothermal water in a single pipe network Figure 3 shows a process diagram for direct use of geothermal water at temperatures ranging from 80°-130°C. This is in reality a simplified process diagram for Reykjavík Energy as it has been operated for over 60 years. Still today the main part of the district heating system is operated in this way. The wells are 1000 – 3000 m deep. Line shaft driven deep well pumps immersed to a depth of up to 200 m pump the water to a gas separator. The geothermal water is 80° to 130°C and the supply temperature to the consumers is 80°C. Part of the distribution network is a dual pipe system. The return water from that part is mixed together with the water from the wells to produce 80°C supply temperature. The excess return water from the double distribution system is discharged to drain. Part of the distribution network is single-pipe from which the return water is discharged from each user into the sewage system or to the rainwater discharge system. Most of the consumers receive hot tap water directly from the distribution system. The volumetric flow rate to the users is measured and the tariff is based on cubic meter of hot water. Now the charge in Reykjavík is ca. 80 UScent per m3 of hot water. The users can utilise the heat energy from the water at free will and return it to the system or discharge it to drain at as low temperature as possible. With conventional radiator system 25°C – 40°C return temperature is common, but with other heating systems such as floor heating, heated fresh air and even combined with snow melting, the return temperature can be 4/19 as low as 10°C - 20°C. This means that each m3 of hot water yields from 45 and up to 80 kWh and the corresponding energy price thus varies from 1,0 – 1,7 U Scent/kWh. There is no doubt that a district heating system of this type has the lowest energy production cost. The single pipe distribution system is less costly and has lower heat loss than a closed loop (dual pipe) system and the pumping cost is also less. It is possible to install a peak load boiler in this system, even if the well temperature is 80°C and the whole system is a single one. Then the supply temperature will be varied according to the load demand, let´s say between 80°C and 130°C. In this case, the building heating systems have to be of indirect closed loop type and the charge for energy would probably be via energy meter. The direct use process shown in Figure 3 is only possible where the chemistry of the geothermal water is such that there is no danger of mineral scaling and corrosion of ordinary carbon steel pipes and heating equipment.