Energy efficiency performance of multi-energy district heating and hot water supply system

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  • J. Cent. South Univ. (2012) 19: 13771382 DOI: 10.1007/s11771-012-1153-8

    Energy efficiency performance of multi-energy district heating and hot water supply system

    JIN Nan(), ZHAO Jing(), ZHU Neng()

    School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China

    Central South University Press and Springer-Verlag Berlin Heidelberg 2012

    Abstract: A district heating and hot water supply system is presented which synthetically utilizes geothermal energy, solar thermal energy and natural gas thermal energy. The multi-energy utilization system has been set at the new campus of Tianjin Polytechnic University (TPU). A couple of deep geothermal wells which are 2 300 m in depth were dug. Deep geothermal energy cascade utilization is achieved by two stages of plate heat exchangers (PHE) and two stages of water source heat pumps (WSHP). Shallow geothermal energy is used in assistant heating by two ground coupled heat pumps (GCHPs) with 580 vertical ground wells which are 120 m in depth. Solar thermal energy collected by vacuum tube arrays (VTAs) and geothermal energy are complementarily utilized to make domestic hot water. Superfluous solar energy can be stored in shallow soil for the GCHP utilization. The system can use fossil fuel thermal energy by two natural gas boilers (NGB) to assist in heating and making hot water. The heating energy efficiency was measured in the winter of 20102011. The coefficients of performance (COP) under different heating conditions are discussed. The performance of hot water production is tested in a local typical winter day and the solar thermal energy utilization factor is presented. The rusults show that the average system COP is 5.75 or 4.96 under different working conditions, and the typical solar energy utilization factor is 0.324. Key words: geothermal energy; solar thermal energy; district heating; hot water supply 1 Introduction

    Building energy consumption takes about 23% in the total energy consumption in China at present and it is in growth trend [1]. Heating loads, cooling loads and hot water loads are the main items of building energy consumption. Renewable energy has been widely used in China for recent years. Geothermal energy and solar thermal energy are most commonly used in buildings. Independent utilization of single renewable energy is popular, but integrated utilization of several kinds of renewable energy is relatively scarce.

    A district heating and hot water supply system was designed for the new campus of Tianjin Polytechnic University, China. In this system, geothermal plate heat exchanger (PHE), water source heat pump (WSHP), ground coupled heat pump (GCHP) and natural gas boiler (NGB) are used synthetically to heat the campus buildings; geothermal PHE, vacuum tube array (VTA) and NGB are used complementarily to make domestic hot water.

    There are some case studies on geothermal district heating in Turkey in resent years [27]. Some researches focused on energy analysis of geothermal heating systems [815]. In these researches, deep geothermal

    energy is the only source of heating. This system uses multi-energy in heating, including deep geothermal energy, shallow geothermal energy and natural gas thermal energy. The system uses solar thermal energy, deep geothermal energy and natural gas thermal energy to make dwells hot water. Though there was previous research on solar energy used in heating and cooling [16], solar hot water used directly as dwells hot water is a direct and highly efficient way to utilize solar thermal energy. The coefficients of performance (COPs) under different heating conditions are discussed according the actual measurement data in the winter of 20102011. The performance of solar hot water production in a local typical sunshine winter day is presented. The design idea of this system can be referred in other multi-energy building system designs.

    2 Systems and operation strategy

    The geothermal, solar thermal and natural gas thermal energy coupling utilization system for district heating and central hot water supply of TPUs new campus consists of two parts: multi-energy district heating system (MEDH) (Fig. 1) and multi-energy central hot water supply system (MECHWS) (Fig. 2).

    Foundation item: Project(2010DFA72740-06) supported by International Science & Technology Cooperation Program of China Received date: 20110726; Accepted date: 20111114 Corresponding author: ZHAO Jing, PhD; Tel: +862227409188; E-mail: zhaojing@tju.edu.cn

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    Fig. 1 Schematic diagram of MEDHs

    Fig. 2 Schematic of MECHWSs 2.1 MEDH

    The MEDH utilizes both geothermal energy of deep layer and shallow layer. Natural gas thermal energy is used as an auxiliary and assistant heating source. A couple of geothermal wells that reach the Ordovician in depth of 2 300 m were dug. Each well was set in a well house and equipped with well mouth equipments. Groundwater is drawn up by a variable frequency submersible pump (VFSP). Then, the groundwater is pumped to well water heat exchangers (WWHE) and the hot water heat exchangers (HWHE) of the MECHWSs.

    The control valve (CV) in Fig. 1 is only opened when the MECHWS needs geothermal water. The WWHEs are divided into two stages. The groundwater is pumped through the hot side of the first stage of the WWHE (WWHE-I). The cold side of the WWHE-I is connected to the water feeder and collector of the

    MEDHs. The circular flow pumped by the pump P1 through the cold side of the WWHE-I is a part of the heating flow. The groundwater is divided into two flows after it comes out the hot side of the WWHE-I by an electric control valve (ECV). One part is sent to the hot side of the second stage of the WWHE (WWHE-II) and the flow rate is set according to the outlet water temperature of the cold side of WWHE-II. The outlet water of the WWHE-II is pumped to the evaporators of the WSHP. When the temperature exceeds 24 C, the ECV will decrease the groundwater flow rate to the WWHE-II until the temperature is below 24 C. When the temperature is below 20 C, the ECV will do the opposite operation. This is because exorbitant evaporator water temperature would cause the compressor lubricating oil to be carbonized and the low evaporator water temperature would lead to an inferior COP of the WSHP.

    The groundwater flows are combined and sent to the inverted well after cascade utilization, as shown in Fig. 1. The groundwater can be inverted to 100% due to the local geological structure and there is no need for pressure inverted pumps.

    The WSHPs are divided into two stages too, WSHP-I and WSHP-II. The cold side circular flow of the WWHE-II pumped by P2 is just the heat source of the WSHP. The circular flow is sent into the evaporator of WSHP-I and then the evaporator of the WSHP-II gives off heat in sequence. The condensers of the WSHP are paralleled and connected to the water feeder and collector of the MEDHs. The circular flow through the condensers pumped by the pump P3 is another part of the heating flow. The WSHP are also used as freezers in summer with the cold source of cooling towers.

    Two GCHPs are paralleled to heat and cool the buildings too. 580 vertical double U-tube ground heat exchangers (VDUGHE) are used to absorb and release heat. The holes of U-tubes are 120 m in depth. The GCHPs are connected to the water feeder and water collector of the MEDHs to supply heating and cooling flows.

    Two parallel NGBs are used to assist in heating when the geothermal energy cannot fulfill the peak heat load or the geothermal energy equipments are broken. 2.2 MECHWSs

    The MECHWSs utilize solar thermal energy, geothermal energy and natural gas thermal energy in a complementary way. Two parallel VTAs are used to collect solar thermal energy. Each of the VTAs consists of 120 collectors. Each collector has 50 vacuum tubes in parallel connection. Every 5 collectors are set in series and 24 such series are paralleled each other to constitute an array. The total absorber area of two VTAs is 1 500 m2.

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    The solar hot water is used as hot water of dwells directly. The water is circulated by the pump P4 between the VTA and a solar hot water tank (SHWT) which is 20 t in capacity. The water temperatures in the middle height of the tank and the outlet of the VTA are tested by platinum resistance thermometers. The temperature signals are sent to an electric control casing (ECC-1) which controls the water circulation between the VTA and the SHWT.

    The water in the SHWT can be pumped by the pump P5 to a central hot water tank (CHWT) which is 200 t in capacity and the water in the CHWT can be pumped to the SHWT by the pump P6. The water temperature in the CHWT is tested by a platinum resistance thermometer. This temperature signal and the SHWT water temperature signal are sent to the second electric control casing (ECC-2). The water circulation between the SHWT and the CHWT is controlled by the ECC-2.

    If the collected solar thermal energy cannot meet the energy requirement to make enough hot water of dwells, geothermal energy will be used to heat water through the HWEC. Natural gas thermal energy is also used as an auxiliary and assistant source to heat the water in the CHWT through the HWEC. The water of the CHWT will be circulated by the pump P7 when it needs to be heated through the HWEC. The hot water circulation between the CHWT and the campus building ends is pumped by the pump P8.

    The solar thermal energy collected by the VTA cannot be consumed in summer vacation and needs to be stored. The