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water chemical heat pump for cogeneration
Applied Thermal Engineering 21 (2001) 1067±1081
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Thermal analysis of a magnesium oxide/water chemical heat pump for cogeneration Yukitaka Kato *, Fu-uta Takahashi, Akihiko Watanabe, Yoshio Yoshizawa Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 3 June 2000; accepted 2 October 2000
Abstract A chemical heat pump is examined experimentally as a chemical heat storage system in order to evaluate the contribution of the chemical heat pump to decentralised cogeneration. A new system that combines cogeneration with a chemical heat pump that uses a magnesium oxide/water reaction is proposed, and the feasibility of the combined system is discussed. A packed bed reactor of a magnesium oxide/water chemical heat pump was examined experimentally under various operation conditions. Thermal performance of the heat pump was analysed using the experimental results. The heat pump containing the reactor is expected to enhance the energy utilisation eciency of the cogeneration system by storing and utilising surplus exhaust heat generated by the cogeneration system. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Chemical heat pump; Cogeneration; Magnesium oxide; Water; Experiments
1. Introduction Heat storage is one of the key technologies for energy utilisation and the reduction of global carbon dioxide emissions. A chemical heat pump, which achieves heat transformation via a chemical reaction, is one type of heat storage and utilisation system. The present study attempts to show the feasibility of using a magnesium oxide/water chemical heat pump for eective heat utilisation. In particular, as a practical application, a decentralised cogeneration system using the
*
Corresponding author. Tel./fax: +81-3-5734-2963. E-mail address: [email protected] (Y. Kato).
1359-4311/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 1 0 3 - 4
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Nomenclature C (kJ/kg K) apparent speci®c heat of coolant DH (kJ/mol) enthalpy change of a reaction Li (kg) initial mass of magnesium hydroxide M (kmol/kg) molecular weight m (kg) material weight P (kPa) reaction pressure q (kJ/kg) heat output per 1 kg of initially charged Mg(OH)2 qrcv (kJ/kg) total recovered heat in the hydration operation by the coolant per 1 kg of initial charged Mg(OH)2 rp heat output ratio heat recovery ratio rr heat source ratio rs T (°C) temperature t (s) reaction time 3 v (m /s) ¯ow rate of the coolant Wele (kW) electric output rate from the diesel engine generator Wh (kW) total heat output rate from the reactor at the hydration operation wh (W/kg) reactor heat output rate in the hydration operation per 1 kg of initial charged Mg(OH)2
wrcv (W/kg) heat recovery rate in the hydration operation by the coolant per 1 kg of initial charged Mg(OH)2 Wstd (kW) standard heat output rate from the exhaust gas boiler x mole reacted fraction Dm (kg) weight change of the reactor Dx change in mole reacted fraction q (kg/m3 ) coolant density Subscripts c coolant at the entrance of the heating tube cd condensation d dehydration of Mg(OH)2 e evaporation h hydration of MgO H2 O water MgO magnesium oxide out coolant at the exit of the heating tube r reaction s sensible heat sd the saturated reacted fraction of the former dehydration t total
heat pump is discussed. A common scheme for cogeneration involves using the shaft work and exhaust heat of a diesel, gas engine or micro gas turbine for electrical and heat output, respectively. However, since the demand for the electrical output is generally inconsistent with that for the heat output, a large amount of surplus heat output is occasionally discharged into the atmosphere. The present study examines the application of a chemical heat pump as a means of utilising the surplus heat and enhancing the actual energy eciency of the cogeneration. A magnesium oxide/water chemical heat pump is used as the heat pump in the present study. The heat pump is operated batchwise and can store heat and transform it to another temperature. In the combined system of the heat pump and an engine for cogeneration, the heat pump is used to store the surplus heat output of the engine during periods of low heat demand and to output transformed heat during peak demand periods. The combined system is expected to contribute to load levelling and to the utilisation of what was previously considered to be waste thermal energy.
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The feasibility of the combined system is discussed, based on the results of the packed bed reactor experiment in the present study. 1.1. Chemical heat pump A chemical heat pump that uses a reversible magnesium oxide/water reaction system has been examined to promote thermal energy utilisation by Bhatti and Dollimore [1], Ervin [2] and Kato et al. [3]. This heat pump is based on the following equilibria: MgO s H2 O g Mg OH2 s; H2 O g H2 O l;
DH20
DH10
81 kJ=mol
40 kJ=mol
1 2
This heat pump enables thermal energy to be stored via the dehydration of magnesium hydroxide (Eq. (1)) and releases the stored energy on demand via the hydration of magnesium oxide. The principle of this heat pump is shown in Fig. 1. The heat pump consists of a magnesium oxide reactor and a water reservoir. The heat pump has two operation modes: a heat storage mode and a heat output mode. In the heat storage mode (Fig. 1(a)), magnesium hydroxide (Mg(OH)2 ) is dehydrated by surplus heat at Td . Generated vapour is condensed at the reservoir at Tcd , and the latent heat of condensation of the vapour is utilised. In the heat output mode (Fig. 1(b)), water in
Fig. 1. Principle of the chemical heat pump.
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Fig. 2. Combined system of the heat pump and a diesel engine.
the reservoir is reheated by heat at a low temperature, Te . The hydration of magnesium oxide proceeds in the reactor by introducing the vapour, and hydration heat output at Th is generated. Thermal drivability, which does not require mechanical work, is one of advantages of the heat pump. The environmentally friendly and economical nature of the reactants is also advantageous. In addition, a new reactant made from an ultra®ne powder of MgO and puri®ed water was found to demonstrate durability to repetitive reaction [4,5]. 1.2. Combined system The proposed combined system is shown in Fig. 2. The system consists of a heat pump and a diesel engine used for a cogeneration. The diesel engine generates electricity and thermal energy simultaneously. The high-temperature exhaust gas of the engine is generally used to raise steam at a steam boiler, and warm water from the jacket is exhausted to the atmosphere. The combined system is intended to use both the exhaust gas and the jacket water in the heat pump. The combined system is operated between heat storage mode (a) during low heat demand periods, and heat output mode (b) during high heat demand periods. The following temperature values for each operation were derived from a previous kinetic study [6]. In the heat storage mode, the dehydration proceeds by consuming the exhaust gas heat generated from by engine at over 350°C thereby generating magnesium oxide and water vapour. The vapour is condensed in the water vessel at 50°C. In the heat output mode, the condensed water is heated up to around 80°C by consuming the jacket water heat of the engine, and the vapour produced reacts with magnesium oxide in the reactor. The heat of hydration is released at up to 150°C. A packed bed reactor experiment was necessary in order to evaluate the practical performance of the heat pump and the subsequent operation of the combined system. The thermobalance analysis is insucient for a practical reactor design, because in the practical reactor, complex phenomena are induced by chemical reactions, vapour diusion and thermal conduction in the particle bed. Therefore, a packed bed reactor of a magnesium oxide/water heat pump in which a heating tube is installed, was examined in order to evaluate the practical performance of the heat pump reactor [7]. A prototype heat pump had previously demonstrated a heat output performance at a temperature of 85°C [8,9]. In the present study, thermal output performance for an
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output above 100°C was examined in order to extend the applicability of the system. The thermal performance of the combined system was estimated using the experimental results.
2. Experiment A schematic diagram of the laboratory-scale chemical heat pump is shown in Fig. 3. A plate type packed bed reactor (6) was suspended from a load cell attached to the ceiling of the chamber. The chamber was maintained at constant temperature and pressure by an oil circulating system equipped with a thermostat, and a vacuum system. The weight change of the reactor during reactions was measured directly by the load cell. Fig. 4 shows the packed bed reactor. The reactor plate was made of stainless steel of 3 mm thickness, the outside dimensions of which are 360 240 50 mm3 . A total of 1.8 kg of particle reactant of Mg(OH)2 (average diameter of 1.5 mm) is charged in the reactor. The temperature at the bottom centre of the bed was used as the representative bed temperature, and was monitored to provide bed temperature control. A heating tube constructed of a rectangular alumina tube having a cross-section of 5 50 mm2 and a length of 3600 mm was installed in the plate. The thermocouples were installed at the entrance and exit of the heating tube in order to measure coolant temperature change. The magnesium
Fig. 3. Experimental apparatus: (1) evaporator, (2) reactor chamber, (3) load cell, (4) reactor plate, (5) heating tube, (6) reactor bed, (7) radiation heater, (8) water cooler, (9) vacuum pump, (10) circulator of cooling water, (11) controller, (12) thermocouple, (13) pressure controller, (14) electric valve, (15) data logger, (16) PC and (17) circulator.
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Fig. 4. Reactor bed: (a) top view, (b) cross-section view (A±A0 ), (1) reactor bed and (2) heating tube. The solid circles show thermocouple positions.
hydroxide of the initial reactant was produced from an ultra®ne magnesium oxide powder (average particle diameter: 10 nm, UBE Material Industries, Ltd.) and water. The ultra®ne oxide powder was hydrated with puri®ed water in a ball mill. After hydration, the pasty product was dried and the resulting ¯akes were sieved. 2.1. Experimental procedure The following one-cycle operation was carried out for each experiment. Mg OH2
Heat storage
Heat output
Dehydration
Hydration
!
MgO
!
Mg OH2
3
The hydration operation described in Eq. (3) controls all operational processes of the heat pump [3]. Thus, the dependence of the hydration on the reaction condition was the principle object of the present study. In heat storage (dehydration) mode, the dehydration of Mg(OH)2 in the bed
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proceeded under the same conditions for each run: a temperature of 430°C achieved by using a radiation heater and a pressure of 14.7 kPa. In heat output mode, an oil coolant was circulated in the heating tube as a preliminary step. After the bed reactor and the chamber attained the steady state, steam generated by the evaporator was introduced into the chamber for the hydration. The magnesium oxide reacted with the steam and generated a heat output. The steam pressures of the hydration, Ph , were 31.2, 47.4 and 70.1 kPa, for the dew points, Ts , of 70°C, 80°C and 90°C, respectively. Ts corresponds to the temperature of evaporation (Te ) in the heat pump shown in Fig. 1. The chamber and coolant temperature were maintained at 5°C higher than the dew point of each run, because 5°C above Ts is high enough to avoid steam condensation and low enough to allow the hydration measurement (about 50°C above Ts ). Silicone oil coolant was circulated in the heating tube at a ¯ow rate of 400 cm3 /min. The hydration output was calculated based on the ¯ow rate and the temperature dierence between the exit and entrance of the heating tube. Although the temperature of the coolant was raised to about 7°C higher than the temperature at the entrance of the tube, the coolant entrance temperature, Tc , is employed as the measure of the hydration output. The results were analysed using the following values. 2.2. Measured value de®nitions 2.2.1. Reacted fraction The reactor's weight change due to the reaction, Dm, was caused by the movement of water. Thus, the mole reacted fraction, x, is de®ned as follows: 4 x 1 Dm=MH2 O = mMgO =MMgO where mMgO is the weight of magnesium oxide in the reactor bed. The hydration experiments started from the dehydrated state. The dehydration of each sample did not proceed to x 0 due to the existence of structural water in the reactant [6], and the sample saturated at around x 0:18. In order to obtain an objective comparison of the hydration reactivity, the change in mole reacted fraction, Dx, is de®ned as follows. Dx xh
xsd
5
where xh and xsd are the reacted fraction of hydration and saturated reacted fraction of the former dehydration, respectively. 2.2.2. Heat recovery rate The hydration heat output per unit weight of the initial charged Mg(OH)2 that is recovered by the coolant circulated in the heating tube, is de®ned as heat recovery rate, wrcv (W/kg). wrcv Tout
Tc vqC=Li
6
2.2.3. Total recovered heat Integrated hydration heat output rate recovered by the coolant is de®ned as total recovered heat, qrcv (kJ/kg). Z 7 qrcv wrcv dth
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2.2.4. Heat recovery ratio The ratio of the recovered heat output to the generated hydration heat, qh (kJ/kg), is de®ned as the heat recovery ratio rr . rr qrcv =qh
8
3. Experimental results The performance of the packed bed reactor was analysed based on the hydration experiments. The eect of hydration pressure, Ph , on the hydration rate at a coolant temperature, Tc , of 110°C is shown in Fig. 5. The coolant temperature corresponds to the heat output temperature of the heat pump. Fig. 5(a) shows the reacted fraction change, Dx, and the heat recovery ratio, rr . Dx increases faster at a higher hydration pressure, because the hydration reactivity becomes more active at higher pressures. The ratio becomes lower as the pressure rises, because a heat loss from the bed to the inner chamber atmosphere increases as the pressure
Fig. 5. Eect of hydration pressure on high-temperature output experiment: (a) temporal changes in reacted fraction amount and heat recovery ratio and (b) changes in heat recovery rate and total recovered heat.
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Fig. 6. Eect of heat output temperature on the high-temperature output experiment: (a) temporal changes in reacted fraction amount and heat recovery ratio and (b) changes in heat recovery rate and total recovered heat.
rises. Fig. 5(b) shows the change in the heat recovery rate, wrcv , and the total recovered heat, qrcv . Both wrcv and qrcv increase as the pressure rises. Fig. 6 shows the eect of Tc on the hydration under a hydration pressure of 47.4 kPa. Dx increases faster at lower Tc , because the exothermic hydration is enhanced at lower reaction temperatures realised by a lower Tc . The recovery ratio is enhanced at a lower Tc , because the temperature dierence between the bed and the coolant increases as the Tc decreases, thus the eciency of heat recovery is enhanced. wrcv and qrcv in Fig. 6(b) increase as Tc decreases, because the exothermic hydration reactivity increases.
4. Performance estimation of a combined system The cogeneration system depicted in Fig. 2 is evaluated from a thermal performance standpoint using the above-described experimental results of the packed bed reactor.
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4.1. Conditions for the estimate A diesel engine having 500 kW electrical output (S12A2-PTA, Meidensha Corporation), which is used in practical cogeneration systems, is employed as the subject of the estimation. The speci®cations of the diesel engine are shown in Table 1. The pre-conditions of the heat pump operation for the combined system evaluation are shown in Table 2. Each dehydration of Mg(OH)2 is presumed to operate under the same conditions. The dehydration proceeded at a temperature of 350°C under a pressure of 12.3 kPa for 6 h. This pressure corresponds to a pressure of saturated water at 50°C, and this temperature of 50°C corresponds to the condensation temperature of the storage vessel, Tcd . The dehydration at below 350°C under a pressure of Table 1 Speci®cations of the diesel engine Electricity output Operation load Fuel consumption Exhaust gas temperature Exhaust gas output Exhaust gas ¯ow rate Jacket water temperature Jacket water output Jacket water ¯ow rate
500 kW 100% 1353 kW 424°C 426 kW 1.004 kg/s 90°C 331 kW 15.8 kg/s
Table 2 Preconditions for the heat pump operation for the performance estimation (a) Heat storage mode Dehydration of Mg(OH)2 Dehydration temperature Dehydration pressure Dehydration period Exhaust gas temperature at reactor entrance Exhaust gas temperature at reactor exit Heat input to reactor from exhaust gas Heat input amount to reactor from exit gas
350°C 12.3 kPa 6h 424°C 360°C 68.9 kW 1488 MJ
Condensation of vapour Temperature of storage water Pressure of storage water
50°C 12.3 kPa
(b) Heat output mode Evaporation Evaporation temperature Evaporation pressure Jacket water temperature at entrance of evaporator Jacket water temperature at exit of evaporator Practicable heat input to evaporator
80°C 47.4 kPa 90°C 85°C 332 kW
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12.3 kPa was measured in a previous thermobalance study [4]. Although the reactor bed in the present study required 430°C for dehydration due to heat losses, dehydration at 350°C appears to be achievable in the packed bed reactor by enhancement of the thermal insulation around the reactor and of heat conduction in the reactor bed. For the dehydration, exhaust gas heat is used as it is cooled from 424°C to 360°C. In the heat output mode, heat given up by the jacket water as it cools from 90°C to 85°C is used to evaporate water at 80°C. From the evaporation temperature of 80°C, the eect of the hydration on the thermal performance of the heat pump system was evaluated, based on the hydration results in Fig. 6 that are at the same evaporation temperature. 4.2. Results of the estimation 4.2.1. Reactor performance The total heat output (qt (kJ/kg)) from the heat pump reactor in the combined system comprises the sensible heat of the reactor bed (qs (kJ/kg)) and the hydration heat of MgO (qh (kJ/kg)). qt qs qh
9
qs is obtained from the temperature dierence between the dehydration temperature and the hydration temperature. The total reactor heat output at the hydration operation per unit weight of Mg(OH)2 , wh (W/kg), is de®ned as follows: wh qt =th
10
The result for wh are shown in Fig. 7 under the hydration conditions shown in Fig. 6. The lines accompanying solid circles show the outputs based on the measured recovered heat output in the experiment, and the plain lines show the case of 100% heat recovery. The output is based on the assumption that the sensible heat is independent of the hydration time and could be used instantaneously. Thus, wh shows the maximum averaged output that is obtained during a hydration
Fig. 7. Eect of coolant temperature on heat output rate per unit weight of the reactant.
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time. For the case based on the measured value, 100 W/kg of the output is expected in 54 min for 110°C output. For the case of 100% heat recovery, 142 W/kg is anticipated under the same conditions. This enhancement of the heat recovery ratio is obviously desirable for heat output maximisation. The ratio of the hydration heat, qh , to the sensible heat, qs , is de®ned as the heat source ratio, rs . rs qh =qs
11
The source ratio under the conditions of Fig. 7 is shown in Fig. 8. The ratio increases as the hydration time increases. The ratio increment demonstrates that the chemical (hydration) heat output becomes more eective at longer operation periods. On the other hand, the sensible heat of reactants provides a higher proportion of the total output for shorter output periods. Larger ratios are obtained at lower output temperatures, because as the temperature decreases the hydration activity becomes higher and the eect of sensible heat becomes smaller compared to the hydration heat. The heat source ratios for the case of 100% heat recovery are about double the ratios for the case based on the measured heat recovery ratio. The enhancement of the heat recovery ratio is important because the enhancement improves the heat source ratio and thus the eective utilisation of the hydration heat output. 4.2.2. System evaluation Fig. 9 shows the volume of initial charged particle Mg(OH)2 reactant, Vr (m3 ), and water volume, Vw (m3 ), for each operation. The estimation is based on a total heat input to the reactor from the exhaust gas of 1488 MJ for the dehydration (Table 2). The volume includes that of the voids between particles, but not that of the heat exchanger or vapour connecting volumes. A longer reaction period requires a smaller reactant amount, because the reaction conversion achieved is larger. The higher the output temperature, the larger the reactant volume, because the
Fig. 8. Temporal change in the heat source ratio.
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Fig. 9. Reactant volumes of magnesium hydroxide and water for a diesel engine having an electric output of 500 kW.
reaction rate becomes slower at higher temperatures. When Th 110°C, a reactant volume of 8.2 m3 and a water volume of 0.12 m3 is required for the output of th 30 min. 4.2.3. Heat output ratio The ratio of the total output from the heat pump reactor to the standard output from the exhaust gas boiler, is de®ned as the heat output ratio, rp , and is a measure of the eectiveness of the heat pump operation at peak demand periods of the cogeneration system. rp
total output from the heat pump reactor; Wh kW standard output from the exhaust gas boiler; Wstd kW
12
Both outputs in Eq. (12) are depicted in Fig. 2(b). The total output from the heat pump reactor comprises the sensible and hydration heat of the packed bed reactor. The standard output is generated from the exhaust gas boiler in Fig. 2(b) by using the whole exhaust gas output. This standard output assumes that the exhaust gas heat is utilised from its original temperature of 424°C down to a hydration temperature (Tc ) as a heat source for the boiler input. The eective output at peak demand periods comprises the standard output from the exhaust gas and the total output from the heat pump reactor. Fig. 10 shows the heat output ratios for Th 110°C in the cases of the experimental heat recovery ratio and rr 100%. The standard and electrical outputs (Wstd and Wele (kW)) from the engine are also shown. At 30 min output, the experimental heat recovery ratio, rp was 1.9; when rr 100%, rp was 2.3. In other words, the eective outputs using both the heat pump and exhaust gas outputs at peak demand periods are 2.9 and 3.3 times the standard output, respectively. Thus, when combined with a diesel engine type cogeneration system, the heat pump may contribute to load levelling. A summary of the results of this estimation is given in Table 3. Further evaluation would require the heat performance to be compared to the construction cost of the combined system.
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Fig. 10. Temporal change in the heat output ratio and output rates of unit operations in a cogeneration system.
Table 3 Summary of the estimation results Operation conditions Dehydration temperature Dehydration pressure Dehydration period Condensation temperature
350°C 12 kPa 6h 50°C
Hydration output temperature Hydration pressure Hydration period Evaporation temperature
110°C 47 kPa 0.5 h 80°C
Estimation results Heat output from the pump Heat output amount Amount of reactant Mg(OH)2 Volume of reactant of Mg(OH)2 Volume of reactant of water Peak output per ton of Mg(OH)2 Eective output of exhaust gas Heat output ratio
627 kW 1.13 GJ 4.56 ton 8.29 m3 0.12 m3 138 kW/ton 345 kW 1.82
5. Conclusions A magnesium oxide/water chemical heat pump was expected to be applicable to load levelling in a common cogeneration system by storing chemically surplus heat during low heat demand and supplying heat during peak load periods. A combined system comprising the heat pump and a diesel engine was proposed. By storing surplus exhaust gas heat of the engine in the heat pump
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during low demand periods and supplying the heat pump output during peak demand periods, the heat output from the combined system during the peak periods is expected to be several times that of a common cogeneration heat output using an exhaust gas boiler. The possibility of applying a chemical heat pump as a heat storage system was demonstrated experimentally.
Acknowledgements The authors greatly thank to MEIDENSHA Corporation for their ®nancial support and useful cooperation.
References [1] A.S. Bhatti, D. Dollimore, The rates of hydration of sea water magnesias, Surf. Technol. 22 (1984) 181±188. [2] G. Ervin, Solar heat storage using chemical reactions, J. Solid State Chem. 22 (1977) 51±61. [3] Y. Kato, N. Yamashita, Y. Yoshizawa, Study of chemical heat pump with reaction system of magnesium oxide/ water, Kagaku Kogaku Ronbunshu 19 (1993) 1213±1216 (in Japanese). [4] Y. Kato, K. Kobayashi, Y. Yoshizawa, Durability to repetitive reaction of magnesium oxide/water reaction system for a heat pump, Appl. Therm. Engng. 18 (1998) 85±92. [5] Y. Kato, J. Nakahata, Y. Yoshizawa, Durability characteristics of the hydration of magnesium oxide under repetitive reaction, J. Mater. Sci. 34 (1999) 475±780. [6] Y. Kato, N. Yamashita, K. Kobayashi, Y. Yoshizawa, Kinetic study of the hydration of magnesium oxide for a chemical heat pump, Appl. Therm. Engng. 16 (1996) 853±862. [7] Y. Kato, S. Inoue, W. Nanmoku, Y. Yoshizawa, Kinetic study of the hydration of magnesium oxide/water chemical heat pump, Proc. 30th Intersoc. Energy Conver., vol. 3, Orlando, USA, 1995, pp. 17±22. [8] Y. Kato, F. Takahashi, W. Nanmoku, Y. Yoshizawa, Heat output performance study of magnesium oxide/water chemical heat pump, Kagaku Kogaku Ronbunsyu 24 (6) (1998) 840±844 (in Japanese). [9] Y. Kato, W. Nanmoku, F. Takahashi, Y. Yoshizawa, Reaction durability of a magnesium oxide/water chemical heat pump, Proceedings of 32nd Intersociety of Energy Conversion Engineering Conference, vol. 2, Honolulu, USA, 1997, pp. 1246±1251.
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Thermal analysis of a magnesium oxide/water chemical heat pump for cogeneration Yukitaka Kato *, Fu-uta Takahashi, Akihiko Watanabe, Yoshio Yoshizawa Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan Received 3 June 2000; accepted 2 October 2000
Abstract A chemical heat pump is examined experimentally as a chemical heat storage system in order to evaluate the contribution of the chemical heat pump to decentralised cogeneration. A new system that combines cogeneration with a chemical heat pump that uses a magnesium oxide/water reaction is proposed, and the feasibility of the combined system is discussed. A packed bed reactor of a magnesium oxide/water chemical heat pump was examined experimentally under various operation conditions. Thermal performance of the heat pump was analysed using the experimental results. The heat pump containing the reactor is expected to enhance the energy utilisation eciency of the cogeneration system by storing and utilising surplus exhaust heat generated by the cogeneration system. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Chemical heat pump; Cogeneration; Magnesium oxide; Water; Experiments
1. Introduction Heat storage is one of the key technologies for energy utilisation and the reduction of global carbon dioxide emissions. A chemical heat pump, which achieves heat transformation via a chemical reaction, is one type of heat storage and utilisation system. The present study attempts to show the feasibility of using a magnesium oxide/water chemical heat pump for eective heat utilisation. In particular, as a practical application, a decentralised cogeneration system using the
*
Corresponding author. Tel./fax: +81-3-5734-2963. E-mail address: [email protected] (Y. Kato).
1359-4311/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 1 0 3 - 4
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Nomenclature C (kJ/kg K) apparent speci®c heat of coolant DH (kJ/mol) enthalpy change of a reaction Li (kg) initial mass of magnesium hydroxide M (kmol/kg) molecular weight m (kg) material weight P (kPa) reaction pressure q (kJ/kg) heat output per 1 kg of initially charged Mg(OH)2 qrcv (kJ/kg) total recovered heat in the hydration operation by the coolant per 1 kg of initial charged Mg(OH)2 rp heat output ratio heat recovery ratio rr heat source ratio rs T (°C) temperature t (s) reaction time 3 v (m /s) ¯ow rate of the coolant Wele (kW) electric output rate from the diesel engine generator Wh (kW) total heat output rate from the reactor at the hydration operation wh (W/kg) reactor heat output rate in the hydration operation per 1 kg of initial charged Mg(OH)2
wrcv (W/kg) heat recovery rate in the hydration operation by the coolant per 1 kg of initial charged Mg(OH)2 Wstd (kW) standard heat output rate from the exhaust gas boiler x mole reacted fraction Dm (kg) weight change of the reactor Dx change in mole reacted fraction q (kg/m3 ) coolant density Subscripts c coolant at the entrance of the heating tube cd condensation d dehydration of Mg(OH)2 e evaporation h hydration of MgO H2 O water MgO magnesium oxide out coolant at the exit of the heating tube r reaction s sensible heat sd the saturated reacted fraction of the former dehydration t total
heat pump is discussed. A common scheme for cogeneration involves using the shaft work and exhaust heat of a diesel, gas engine or micro gas turbine for electrical and heat output, respectively. However, since the demand for the electrical output is generally inconsistent with that for the heat output, a large amount of surplus heat output is occasionally discharged into the atmosphere. The present study examines the application of a chemical heat pump as a means of utilising the surplus heat and enhancing the actual energy eciency of the cogeneration. A magnesium oxide/water chemical heat pump is used as the heat pump in the present study. The heat pump is operated batchwise and can store heat and transform it to another temperature. In the combined system of the heat pump and an engine for cogeneration, the heat pump is used to store the surplus heat output of the engine during periods of low heat demand and to output transformed heat during peak demand periods. The combined system is expected to contribute to load levelling and to the utilisation of what was previously considered to be waste thermal energy.
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The feasibility of the combined system is discussed, based on the results of the packed bed reactor experiment in the present study. 1.1. Chemical heat pump A chemical heat pump that uses a reversible magnesium oxide/water reaction system has been examined to promote thermal energy utilisation by Bhatti and Dollimore [1], Ervin [2] and Kato et al. [3]. This heat pump is based on the following equilibria: MgO s H2 O g Mg OH2 s; H2 O g H2 O l;
DH20
DH10
81 kJ=mol
40 kJ=mol
1 2
This heat pump enables thermal energy to be stored via the dehydration of magnesium hydroxide (Eq. (1)) and releases the stored energy on demand via the hydration of magnesium oxide. The principle of this heat pump is shown in Fig. 1. The heat pump consists of a magnesium oxide reactor and a water reservoir. The heat pump has two operation modes: a heat storage mode and a heat output mode. In the heat storage mode (Fig. 1(a)), magnesium hydroxide (Mg(OH)2 ) is dehydrated by surplus heat at Td . Generated vapour is condensed at the reservoir at Tcd , and the latent heat of condensation of the vapour is utilised. In the heat output mode (Fig. 1(b)), water in
Fig. 1. Principle of the chemical heat pump.
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Fig. 2. Combined system of the heat pump and a diesel engine.
the reservoir is reheated by heat at a low temperature, Te . The hydration of magnesium oxide proceeds in the reactor by introducing the vapour, and hydration heat output at Th is generated. Thermal drivability, which does not require mechanical work, is one of advantages of the heat pump. The environmentally friendly and economical nature of the reactants is also advantageous. In addition, a new reactant made from an ultra®ne powder of MgO and puri®ed water was found to demonstrate durability to repetitive reaction [4,5]. 1.2. Combined system The proposed combined system is shown in Fig. 2. The system consists of a heat pump and a diesel engine used for a cogeneration. The diesel engine generates electricity and thermal energy simultaneously. The high-temperature exhaust gas of the engine is generally used to raise steam at a steam boiler, and warm water from the jacket is exhausted to the atmosphere. The combined system is intended to use both the exhaust gas and the jacket water in the heat pump. The combined system is operated between heat storage mode (a) during low heat demand periods, and heat output mode (b) during high heat demand periods. The following temperature values for each operation were derived from a previous kinetic study [6]. In the heat storage mode, the dehydration proceeds by consuming the exhaust gas heat generated from by engine at over 350°C thereby generating magnesium oxide and water vapour. The vapour is condensed in the water vessel at 50°C. In the heat output mode, the condensed water is heated up to around 80°C by consuming the jacket water heat of the engine, and the vapour produced reacts with magnesium oxide in the reactor. The heat of hydration is released at up to 150°C. A packed bed reactor experiment was necessary in order to evaluate the practical performance of the heat pump and the subsequent operation of the combined system. The thermobalance analysis is insucient for a practical reactor design, because in the practical reactor, complex phenomena are induced by chemical reactions, vapour diusion and thermal conduction in the particle bed. Therefore, a packed bed reactor of a magnesium oxide/water heat pump in which a heating tube is installed, was examined in order to evaluate the practical performance of the heat pump reactor [7]. A prototype heat pump had previously demonstrated a heat output performance at a temperature of 85°C [8,9]. In the present study, thermal output performance for an
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output above 100°C was examined in order to extend the applicability of the system. The thermal performance of the combined system was estimated using the experimental results.
2. Experiment A schematic diagram of the laboratory-scale chemical heat pump is shown in Fig. 3. A plate type packed bed reactor (6) was suspended from a load cell attached to the ceiling of the chamber. The chamber was maintained at constant temperature and pressure by an oil circulating system equipped with a thermostat, and a vacuum system. The weight change of the reactor during reactions was measured directly by the load cell. Fig. 4 shows the packed bed reactor. The reactor plate was made of stainless steel of 3 mm thickness, the outside dimensions of which are 360 240 50 mm3 . A total of 1.8 kg of particle reactant of Mg(OH)2 (average diameter of 1.5 mm) is charged in the reactor. The temperature at the bottom centre of the bed was used as the representative bed temperature, and was monitored to provide bed temperature control. A heating tube constructed of a rectangular alumina tube having a cross-section of 5 50 mm2 and a length of 3600 mm was installed in the plate. The thermocouples were installed at the entrance and exit of the heating tube in order to measure coolant temperature change. The magnesium
Fig. 3. Experimental apparatus: (1) evaporator, (2) reactor chamber, (3) load cell, (4) reactor plate, (5) heating tube, (6) reactor bed, (7) radiation heater, (8) water cooler, (9) vacuum pump, (10) circulator of cooling water, (11) controller, (12) thermocouple, (13) pressure controller, (14) electric valve, (15) data logger, (16) PC and (17) circulator.
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Fig. 4. Reactor bed: (a) top view, (b) cross-section view (A±A0 ), (1) reactor bed and (2) heating tube. The solid circles show thermocouple positions.
hydroxide of the initial reactant was produced from an ultra®ne magnesium oxide powder (average particle diameter: 10 nm, UBE Material Industries, Ltd.) and water. The ultra®ne oxide powder was hydrated with puri®ed water in a ball mill. After hydration, the pasty product was dried and the resulting ¯akes were sieved. 2.1. Experimental procedure The following one-cycle operation was carried out for each experiment. Mg OH2
Heat storage
Heat output
Dehydration
Hydration
!
MgO
!
Mg OH2
3
The hydration operation described in Eq. (3) controls all operational processes of the heat pump [3]. Thus, the dependence of the hydration on the reaction condition was the principle object of the present study. In heat storage (dehydration) mode, the dehydration of Mg(OH)2 in the bed
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proceeded under the same conditions for each run: a temperature of 430°C achieved by using a radiation heater and a pressure of 14.7 kPa. In heat output mode, an oil coolant was circulated in the heating tube as a preliminary step. After the bed reactor and the chamber attained the steady state, steam generated by the evaporator was introduced into the chamber for the hydration. The magnesium oxide reacted with the steam and generated a heat output. The steam pressures of the hydration, Ph , were 31.2, 47.4 and 70.1 kPa, for the dew points, Ts , of 70°C, 80°C and 90°C, respectively. Ts corresponds to the temperature of evaporation (Te ) in the heat pump shown in Fig. 1. The chamber and coolant temperature were maintained at 5°C higher than the dew point of each run, because 5°C above Ts is high enough to avoid steam condensation and low enough to allow the hydration measurement (about 50°C above Ts ). Silicone oil coolant was circulated in the heating tube at a ¯ow rate of 400 cm3 /min. The hydration output was calculated based on the ¯ow rate and the temperature dierence between the exit and entrance of the heating tube. Although the temperature of the coolant was raised to about 7°C higher than the temperature at the entrance of the tube, the coolant entrance temperature, Tc , is employed as the measure of the hydration output. The results were analysed using the following values. 2.2. Measured value de®nitions 2.2.1. Reacted fraction The reactor's weight change due to the reaction, Dm, was caused by the movement of water. Thus, the mole reacted fraction, x, is de®ned as follows: 4 x 1 Dm=MH2 O = mMgO =MMgO where mMgO is the weight of magnesium oxide in the reactor bed. The hydration experiments started from the dehydrated state. The dehydration of each sample did not proceed to x 0 due to the existence of structural water in the reactant [6], and the sample saturated at around x 0:18. In order to obtain an objective comparison of the hydration reactivity, the change in mole reacted fraction, Dx, is de®ned as follows. Dx xh
xsd
5
where xh and xsd are the reacted fraction of hydration and saturated reacted fraction of the former dehydration, respectively. 2.2.2. Heat recovery rate The hydration heat output per unit weight of the initial charged Mg(OH)2 that is recovered by the coolant circulated in the heating tube, is de®ned as heat recovery rate, wrcv (W/kg). wrcv Tout
Tc vqC=Li
6
2.2.3. Total recovered heat Integrated hydration heat output rate recovered by the coolant is de®ned as total recovered heat, qrcv (kJ/kg). Z 7 qrcv wrcv dth
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2.2.4. Heat recovery ratio The ratio of the recovered heat output to the generated hydration heat, qh (kJ/kg), is de®ned as the heat recovery ratio rr . rr qrcv =qh
8
3. Experimental results The performance of the packed bed reactor was analysed based on the hydration experiments. The eect of hydration pressure, Ph , on the hydration rate at a coolant temperature, Tc , of 110°C is shown in Fig. 5. The coolant temperature corresponds to the heat output temperature of the heat pump. Fig. 5(a) shows the reacted fraction change, Dx, and the heat recovery ratio, rr . Dx increases faster at a higher hydration pressure, because the hydration reactivity becomes more active at higher pressures. The ratio becomes lower as the pressure rises, because a heat loss from the bed to the inner chamber atmosphere increases as the pressure
Fig. 5. Eect of hydration pressure on high-temperature output experiment: (a) temporal changes in reacted fraction amount and heat recovery ratio and (b) changes in heat recovery rate and total recovered heat.
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Fig. 6. Eect of heat output temperature on the high-temperature output experiment: (a) temporal changes in reacted fraction amount and heat recovery ratio and (b) changes in heat recovery rate and total recovered heat.
rises. Fig. 5(b) shows the change in the heat recovery rate, wrcv , and the total recovered heat, qrcv . Both wrcv and qrcv increase as the pressure rises. Fig. 6 shows the eect of Tc on the hydration under a hydration pressure of 47.4 kPa. Dx increases faster at lower Tc , because the exothermic hydration is enhanced at lower reaction temperatures realised by a lower Tc . The recovery ratio is enhanced at a lower Tc , because the temperature dierence between the bed and the coolant increases as the Tc decreases, thus the eciency of heat recovery is enhanced. wrcv and qrcv in Fig. 6(b) increase as Tc decreases, because the exothermic hydration reactivity increases.
4. Performance estimation of a combined system The cogeneration system depicted in Fig. 2 is evaluated from a thermal performance standpoint using the above-described experimental results of the packed bed reactor.
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4.1. Conditions for the estimate A diesel engine having 500 kW electrical output (S12A2-PTA, Meidensha Corporation), which is used in practical cogeneration systems, is employed as the subject of the estimation. The speci®cations of the diesel engine are shown in Table 1. The pre-conditions of the heat pump operation for the combined system evaluation are shown in Table 2. Each dehydration of Mg(OH)2 is presumed to operate under the same conditions. The dehydration proceeded at a temperature of 350°C under a pressure of 12.3 kPa for 6 h. This pressure corresponds to a pressure of saturated water at 50°C, and this temperature of 50°C corresponds to the condensation temperature of the storage vessel, Tcd . The dehydration at below 350°C under a pressure of Table 1 Speci®cations of the diesel engine Electricity output Operation load Fuel consumption Exhaust gas temperature Exhaust gas output Exhaust gas ¯ow rate Jacket water temperature Jacket water output Jacket water ¯ow rate
500 kW 100% 1353 kW 424°C 426 kW 1.004 kg/s 90°C 331 kW 15.8 kg/s
Table 2 Preconditions for the heat pump operation for the performance estimation (a) Heat storage mode Dehydration of Mg(OH)2 Dehydration temperature Dehydration pressure Dehydration period Exhaust gas temperature at reactor entrance Exhaust gas temperature at reactor exit Heat input to reactor from exhaust gas Heat input amount to reactor from exit gas
350°C 12.3 kPa 6h 424°C 360°C 68.9 kW 1488 MJ
Condensation of vapour Temperature of storage water Pressure of storage water
50°C 12.3 kPa
(b) Heat output mode Evaporation Evaporation temperature Evaporation pressure Jacket water temperature at entrance of evaporator Jacket water temperature at exit of evaporator Practicable heat input to evaporator
80°C 47.4 kPa 90°C 85°C 332 kW
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12.3 kPa was measured in a previous thermobalance study [4]. Although the reactor bed in the present study required 430°C for dehydration due to heat losses, dehydration at 350°C appears to be achievable in the packed bed reactor by enhancement of the thermal insulation around the reactor and of heat conduction in the reactor bed. For the dehydration, exhaust gas heat is used as it is cooled from 424°C to 360°C. In the heat output mode, heat given up by the jacket water as it cools from 90°C to 85°C is used to evaporate water at 80°C. From the evaporation temperature of 80°C, the eect of the hydration on the thermal performance of the heat pump system was evaluated, based on the hydration results in Fig. 6 that are at the same evaporation temperature. 4.2. Results of the estimation 4.2.1. Reactor performance The total heat output (qt (kJ/kg)) from the heat pump reactor in the combined system comprises the sensible heat of the reactor bed (qs (kJ/kg)) and the hydration heat of MgO (qh (kJ/kg)). qt qs qh
9
qs is obtained from the temperature dierence between the dehydration temperature and the hydration temperature. The total reactor heat output at the hydration operation per unit weight of Mg(OH)2 , wh (W/kg), is de®ned as follows: wh qt =th
10
The result for wh are shown in Fig. 7 under the hydration conditions shown in Fig. 6. The lines accompanying solid circles show the outputs based on the measured recovered heat output in the experiment, and the plain lines show the case of 100% heat recovery. The output is based on the assumption that the sensible heat is independent of the hydration time and could be used instantaneously. Thus, wh shows the maximum averaged output that is obtained during a hydration
Fig. 7. Eect of coolant temperature on heat output rate per unit weight of the reactant.
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time. For the case based on the measured value, 100 W/kg of the output is expected in 54 min for 110°C output. For the case of 100% heat recovery, 142 W/kg is anticipated under the same conditions. This enhancement of the heat recovery ratio is obviously desirable for heat output maximisation. The ratio of the hydration heat, qh , to the sensible heat, qs , is de®ned as the heat source ratio, rs . rs qh =qs
11
The source ratio under the conditions of Fig. 7 is shown in Fig. 8. The ratio increases as the hydration time increases. The ratio increment demonstrates that the chemical (hydration) heat output becomes more eective at longer operation periods. On the other hand, the sensible heat of reactants provides a higher proportion of the total output for shorter output periods. Larger ratios are obtained at lower output temperatures, because as the temperature decreases the hydration activity becomes higher and the eect of sensible heat becomes smaller compared to the hydration heat. The heat source ratios for the case of 100% heat recovery are about double the ratios for the case based on the measured heat recovery ratio. The enhancement of the heat recovery ratio is important because the enhancement improves the heat source ratio and thus the eective utilisation of the hydration heat output. 4.2.2. System evaluation Fig. 9 shows the volume of initial charged particle Mg(OH)2 reactant, Vr (m3 ), and water volume, Vw (m3 ), for each operation. The estimation is based on a total heat input to the reactor from the exhaust gas of 1488 MJ for the dehydration (Table 2). The volume includes that of the voids between particles, but not that of the heat exchanger or vapour connecting volumes. A longer reaction period requires a smaller reactant amount, because the reaction conversion achieved is larger. The higher the output temperature, the larger the reactant volume, because the
Fig. 8. Temporal change in the heat source ratio.
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Fig. 9. Reactant volumes of magnesium hydroxide and water for a diesel engine having an electric output of 500 kW.
reaction rate becomes slower at higher temperatures. When Th 110°C, a reactant volume of 8.2 m3 and a water volume of 0.12 m3 is required for the output of th 30 min. 4.2.3. Heat output ratio The ratio of the total output from the heat pump reactor to the standard output from the exhaust gas boiler, is de®ned as the heat output ratio, rp , and is a measure of the eectiveness of the heat pump operation at peak demand periods of the cogeneration system. rp
total output from the heat pump reactor; Wh kW standard output from the exhaust gas boiler; Wstd kW
12
Both outputs in Eq. (12) are depicted in Fig. 2(b). The total output from the heat pump reactor comprises the sensible and hydration heat of the packed bed reactor. The standard output is generated from the exhaust gas boiler in Fig. 2(b) by using the whole exhaust gas output. This standard output assumes that the exhaust gas heat is utilised from its original temperature of 424°C down to a hydration temperature (Tc ) as a heat source for the boiler input. The eective output at peak demand periods comprises the standard output from the exhaust gas and the total output from the heat pump reactor. Fig. 10 shows the heat output ratios for Th 110°C in the cases of the experimental heat recovery ratio and rr 100%. The standard and electrical outputs (Wstd and Wele (kW)) from the engine are also shown. At 30 min output, the experimental heat recovery ratio, rp was 1.9; when rr 100%, rp was 2.3. In other words, the eective outputs using both the heat pump and exhaust gas outputs at peak demand periods are 2.9 and 3.3 times the standard output, respectively. Thus, when combined with a diesel engine type cogeneration system, the heat pump may contribute to load levelling. A summary of the results of this estimation is given in Table 3. Further evaluation would require the heat performance to be compared to the construction cost of the combined system.
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Fig. 10. Temporal change in the heat output ratio and output rates of unit operations in a cogeneration system.
Table 3 Summary of the estimation results Operation conditions Dehydration temperature Dehydration pressure Dehydration period Condensation temperature
350°C 12 kPa 6h 50°C
Hydration output temperature Hydration pressure Hydration period Evaporation temperature
110°C 47 kPa 0.5 h 80°C
Estimation results Heat output from the pump Heat output amount Amount of reactant Mg(OH)2 Volume of reactant of Mg(OH)2 Volume of reactant of water Peak output per ton of Mg(OH)2 Eective output of exhaust gas Heat output ratio
627 kW 1.13 GJ 4.56 ton 8.29 m3 0.12 m3 138 kW/ton 345 kW 1.82
5. Conclusions A magnesium oxide/water chemical heat pump was expected to be applicable to load levelling in a common cogeneration system by storing chemically surplus heat during low heat demand and supplying heat during peak load periods. A combined system comprising the heat pump and a diesel engine was proposed. By storing surplus exhaust gas heat of the engine in the heat pump
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during low demand periods and supplying the heat pump output during peak demand periods, the heat output from the combined system during the peak periods is expected to be several times that of a common cogeneration heat output using an exhaust gas boiler. The possibility of applying a chemical heat pump as a heat storage system was demonstrated experimentally.
Acknowledgements The authors greatly thank to MEIDENSHA Corporation for their ®nancial support and useful cooperation.
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