Air-Conditioning System Using Clathrate Hydrate Slurry†
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JFE TECHNICAL REPORT No. 3 (July 2004) Air-Conditioning System Using Clathrate Hydrate Slurry† OGOSHI Hidemasa*1 TAKAO Shingo*2 Abstract: (approx. 5–12°C), a substantial energy-saving effect can JFE Engineering has developed clathrate hydrate be expected in air-conditioning systems if the cooling slurry (CHS) for use in next-generation energy-saving medium has a high thermal density (high unit cooling air-conditioning systems. Clathrate hydrate slurry is storage capacity) and is suitable for both cooling storage suitable for cooling applications as it possesses latent and pumping. heat in the range of 5–12°C, has a cooling storage The cooling medium developed in this work is a fluid capacity 2–3 times as large as that of the conventional of a mixed solid-liquid phase type, consisting of fine chilled water, and displays excellent pumpability. particles and an aqueous solution of clathrate hydrate Because CHS is a cooling medium with high thermal slurry (CHS). This new cooling storage medium is well- density, air-conditioning systems using CHS are suited to air-conditioning, as it has a high thermal den- expected to substantially reduce cooling medium trans- sity in the temperature range of 5–12°C and excellent portation costs and make an important contribution to properties for pumping and transportation. energy savings, with attendant benefits in reducing CO2 This paper describes the features of CHS and the emissions. This paper describes the properties of CHS results of a trial calculation for an air-conditioning sys- and the results of a trial calculation of the energy-sav- tem using this new cooling medium.1–3) ing effect of an office building air-conditioning system using CHS. 2. Features and Properties of Clathrate Hydrate Slurry 1. Introduction Although gaseous clathrate hydrates use gases such Energy consumption for air-conditioning in general as flon or methane as the guest molecule, the hydrate in private and public sectors has increased year by year. CHS is a kind of liquid clathrate hydrate with tetra-n- Thus, from the viewpoints of both energy conservation butylammonium bromide (TBAB) as the guest molecule. and reduced CO2 emissions, further energy-saving mea- When an aqueous solution of TBAB which has been dis- sures are necessary. Moreover, because heating/cooling solved in water is cooled while flowing, hydrate parti- loads are concentrated in the daytime hours, technical cles of 10–100 µm in size form in the solution, produc- development to enable load leveling in electric power ing a fluid hydrate slurry, as shown in Photo 1. consumption is also desirable. Tetra-n-butylammonium bromide is a registered In response to these needs, regenerative air- chemical under the Law for Regulation of Examination conditioning systems using water or ice as a cooling and Manufacture, Etc. of Chemical Substances (Chem- storage medium have been widely adopted. With cooling ical Examination Law), and therefore does not come storage using chilled water, the refrigerator can be oper- under the provisions of the Safety and Hygiene Law, ated at a high coefficient of performance (COP), but Poison Control Law, or Fire Services Act. Table 1 shows using a storage tank of the same capacity, the amount of the results of acute toxicity test. This hydrate has excel- cooling storage is smaller than with ice. On the other lent long-term stability and does not show changes in hand, with ice, power consumption is high due to the low thermal properties after repeated use. COP of the refrigerator. The concept of application of CHS to air- In the temperature range used in air-conditioning conditioning systems for office buildings is shown in † Originally published in JFE GIHO No. 3 (Mar. 2004), p. 1–5 *1 Principal Researcher, *2 Principal Researcher, Manager, Solution Gr., Energy Systems and Solutions Res. Dept., Energy System and Solutions Res. Dept., Engineering Research Center, Engineering Research Center, JFE Engineering JFE Engineering 1 Air-Conditioning System Using Clathrate Hydrate Slurry 10 Hydrate type II C) ° ( 5 Hydrate type I Clathrate hydrate forming temperature 0 0 10 20 30 40 50 Photo 1 CHS (clathrate hydrate slurry) Mass concentration of solution (mass%) Fig. 2 Formation of TBAB hydrate Table 1 Results of acute toxicity test Male : 1 414 mg/kg LD50 enabling direct transportation of CHS in the same (Rat, Oral) Female : 1 542 mg/kg manner as chilled water. LC 50 3 340 mg/l The following sections describe the forming, trans- (Cyprinodont, 96 hrs) portation, and heat transfer properties of CHS. Refrigerator 2.1 Forming Properties of TBAB Hydrate CHS piping Figure 2 shows the relationship between the forming Cooling tower temperature of TBAB hydrate and the concentration of CHS Storage generator the aqueous solution. tank Air/CHS heat When the ratio of TBAB in the hydrate relative exchanger hydrates as a whole is the same as the concentration of the aqueous solution, the concentration of the aque- Fig. 1 Air-conditioning system using CHS ous solution becomes constant when the solution is cooled, even if the hydrate content of the CHS increases. Fig. 1. The cooling source section of this CHS air- In experiments, when the concentration of the aque- conditioning system includes a heat exchanger for CHS ous solution was 40.5 mass%, the concentration became production and CHS storage tank, which are required in constant at about 11.8°C. From this, the hydration num- addition to the conventional system equipment. In the ber was estimated at approximately 26. This hydrate is secondary side equipment, which is located indoors, the called “Type I hydrate.” medium for transporting chilled water is replaced with When an aqueous solution with the concentration of CHS. It is possible to use piping, pumps, and heat less than 40.5 mass% is cooled, the concentration of the exchanger equipments with the same specifications as in solution in the slurry decreases as the hydrate content of the conventional chilled water systems. the CHS increases. This means that the hydrate forming The features and effects of the CHS air-conditioning temperature decreases along the hydrate forming line. system are as follows: From experiments to date, when the temperature of CHS (1) Maximum 80% reduction in pumping power con- is approximately 8°C or lower, the hydrate shifts to one sumption is possible. with a hydration number of approximately 36, referred CHS has a high thermal density of 10–17 Mcal/m3, to as “Type II hydrate” in the following: Figure 2 shows or 2–3 times that of chilled water, making it possi- 2 hydrate forming lines at 8°C and under. However, in ble to reduce the flow rate by one-half in comparison spite of the fact that Type I hydrate is formed temporar- with chilled water. As a result, power consumption ily in the cooling process, because Type II hydrate is sta- for pumping the cooling medium can be substantially ble, the final product is Type II hydrate. reduced. 2.2 Properties of TBAB Hydrate (2) 40% of energy is saved in comparison with ice pro- and Clathrate Hydrate Slurry duction. The forming temperature of CHS is in the same Table 2 shows the measured values of density, spe- range as that of chilled water (5–12°C), making it cific heat, and latent heat of the Type I and Type II possible to operate the refrigerator with higher effi- hydrates. ciency than in ice production. Figure 3 shows the results of an investigation of (3) Direct transportation to indoor air-conditioners is the relationship between solution temperature and den- possible. sity of solutions of different densities.4) With the respec- Because CHS is non-cohesive, there is no dan- tive solution densities, the solutions whose temperature ger of blockage in piping or indoor air-conditioners, is lower than the hydrate forming temperature are in a 2 JFE TECHNICAL REPORT No. 3 (July 2004) Air-Conditioning System Using Clathrate Hydrate Slurry Table 2 Thermophysical properties of TBAB hydrate 0 5 TBAB hydrate Thermophysical property C) 10 ° Type I Type II 15 Density (kg/m3) 1.08 103 1.03 103 20 25 (Base : 12 Specific heat (kJ/kgK) 2.22 – ic enthalpy (kcal/kg) 30 Concentration : 20 mass% Latent heat (kJ/kg) 193 205 Specif 35 5 6 7 8 9 10 11 12 13 Concentration (mass%) Temperature of CHS (°C) 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0 Fig. 5 Specific enthalpy of CHS vs. temperature 30.0 40.6 45.0 1 050 ) 3 1 040 an aqueous solution concentration of 20 mass% (specific 1 030 enthalpy is expressed assuming 12°C is 0). At approx- 1 020 imately 7.6°C, CHS with a specific enthalpy (thermal Density (kg/m 1 010 density) twice that of chilled water is obtained (at a tem- 1 000 perature difference of 7°C). 0 5.0 10.0 15.0 20.0 25.0 30.0 Temperature (°C) 2.3 Transportation Properties Fig. 3 Density vs. temperature of aqueous solution of Clathrate Hydrate Slurry An important feature of CHS is the fact that it can be 1 036 used not only as a cooling storage medium but also as Measured 1 032 Estimated a high density cooling transportation medium. Because ) 3 1 028 CHS is a fluid with the consistency of soft ice cream, the 20.2 mass% : CHS type I flow is stable and does not cohere and block piping or 1 024 20.2 mass% : CHS type II heat exchangers, valves, etc of air-conditioners. 1 020 Density (kg/m Figure 6 shows a comparison of transportation power 1 016 15.0 mass% : CHS type II consumption with CHS and chilled water for the ther- 1 012 mal density (specific enthalpy) of CHS at a fixed cool- 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 ing transfer rate (115 kW), based on experimental data Temperature (°C) for transportation with 50 A piping.6) With CHS, slurry Fig.