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Long-distance heat transport system using a hydrogen compressor
Vol. 23,No. 10,PP.91l-919, 1998 ICJ‘1998InternationalAssociationfor HydrogenEnergy ElsevierScienceLtd All rightsreserved.Printedin Great Britain PII: SO360-31!N(97)0012%6 036s3199/98 $19.00~0.00 ht. J. Hydrogen Energy,
Pergamon
LONG-DISTANCE
HEAT TRANSPORT SYSTEM USING A HYDROGEN COMPRESSOR K. NASAKO,*Tf Y. ITO* and M. OSUMIf
*Department of Fundamental Energy Science,Graduate School of Energy Science,Kyoto University, Yoshida, Sakyo-ku, Kyoto, Japan tMechatronics Research Center, Sanyo Electric Co. Ltd., I-18-13, Hashiridani, Hirakata City, Osaka, Japan
Abstract-A heat transport system using hydrogen absorbing alloys offers several advantages, such as enabling the use of more compact long-distance transport pipes, maintaining a constant level of efficiency regardless of transport distance, and enabling rapid transport. On the other hand, the system supplies heat to a load through a heat pump with a 20 K difference.The system therefore cannot be usedfor heat loads that require a higher temperature difference. Then, in order to increase heat load applications, we have proposed a hybrid heat transport system that uses a hydrogen compressor. We also developed a system structure and achieved a heat pump with a 40 K difference by using a 0.2-kW experimental system. Furthermore, we focused on using the same structure on the heat source side for supplying various heat loads, and have proposed a basic concept for a wide-area heat utilization system that transports heat to various loads from centralized heat sources. 0 1998 International Association for Hydrogen Energy
spheric CO2 levels as well as reductions in the amount of fossil fuels we consume. These require more efficient A Surface of heat transfer (m’) energy equipment, utilization of renewable energy, and C Heat capacity of a reaction vessel(J K-‘) efficient use of waste energy. In particular, the potentially K Overall coefficient of heat transfer (W me2 K- ‘) vast stores of renewable and waste energy are becoming n Amount of transferred H, (mol) increasingly important. The reason why we make so little Time of heat transport process (s) f,., use of this energy is that the heat sources and loads LiH, Reaction heat of heat source side alloy (J mol are not conveniently located, and the heat sources are H; ‘1 unstable. AH! Reaction heat of heat utilization side alloy (J The authors have proposed a concept for a new longmol H; ‘) distance heat transport systemusing hydrogen absorbing Ratio of sensible heat recovery at heat source alloys [ 1,2]. So far, hydrogen absorbing alloys have conside (-) ventionally received attention only with regard to their Ratio of sensible heat recovery at heat uti- energy conversion function, and have been researchedin lization side the area of conventional heat utilization systems, such Heat transport efficiency (-) v as heat storage [3, 41, heat pumps [5-71 and cold heat Temperature of heat source (K) 7-l generation [8,9]. The proposed system, however, utilizes Temperature of cooling water (K) T2 both of the merits of hydrogen gas, i.e., easy transport Temperature of heat recovery medium (K) T, over long distances and the energy conversion function Temperature of waste heat (K) T4 of hydrogen absorbing alloys. And by a subsequent Atmospheric temperature (K) T” analysis of the new system [lo], it was found that compared to conventional heat transport systems using heated water 1) there was no drop in transport efficiency 1. INTRODUCTION even at long distances, 2) the diameter of the long-disMeasures aimed at the problem of global climatic tance transport pipeline could be decreasedto l/3, and change, i.e., global warming, call for reductions in atmo- 3) intermittent heat could also be transported with the new system.This system,however, supplies heat to a load on the utilization side through a heat pump with a 20 K difference. This meant that the system could not be used SAuthor to whom all correspondence should be addressed. for loads that require a higher temperature difference. NOMENCLATURE
911
912
K. NASAKO et al. Solar collector
(Heat source side)
j ~~~~~~~~~~
l/T -w
l/l
--t
Fig. 1.Principleof the heattransportsystemusinghydrogenabsorbingalloys.
This paper describesa proposal of a hybrid heat transport system that usesa hydrogen compressor in order to increaseheat load applications, and a proposal of a widearea heat utilization system (thermal network) capable of transporting heat to various loads from centralized heat sources. 2. HYBRID HEAT TRANSPORT SYSTEM USING A HYDROGEN COMPRESSOR 2.1. Heat transport system using hydrogen absorbing alloys
The heat transport system is structured with hydrogen absorbing alloys having different characteristics installed at the heat generating site and the heat utilization site some distance away and connected by a hydrogen gas pipeline as shown in Fig. 1, In the structure, the hydrogen absorbing alloy MHl on the heat source side absorbs low-pressure hydrogen (cooled by a low-temperature (T,) heat source), and releaseshigh-pressure hydrogen (heated by a high-temperature (T,) heat source). Heat is pumped up on the utilization side by hydrogen pressure to supply the heat load. For example, a 90°C heat source and 20°C cooling water used on the heat source side generate pressure at a compression ratio of about 1: 5 (150 kPa pressure is compressed to 750 kPa pressure). After longdistance transport by hydrogen, this pressure difference can drive a heat pump with respect to the reference temperature (for example, waste heat or air source). This new heat transport system does not need to raise the temperature of the long-distance pipeline and has an extremely short rise time (about 1 min), so the system is applicable to transport the heat of an intermittent heat source such as solar energy and plant waste energy. From the result of a feasibility study [lo], it was found that small-scale heat (2-kW), which cannot be transported by heated water, can be transported with high efficiency over
long distances (2 km, 60%) by hydrogen, and that this heat transport system is also more efficient even with large-scale (lOO-kW) heat transport at distances over 2 km. The temperature rise range (temperature fall range as well), however, is limited by the energy of the pressure difference, and is about 20 K in the above case of a 1: 5 compression ratio. Despite this temperature range, the systemis still suitable for a variety of applications, including heating, cooling and hot water supply as well as industrial plant processing heat. Even so, a system with a higher temperature range is neededin order to increase the applicable heat loads. 2.2. Hybrid cycle using a compressor
A higher temperature rise range can be achieved by raising the compression ratio of the hydrogen pressure. One means of doing this is to raise the heat source temperature, but it is hard to imagine a renewable or waste energy heat source achieving temperatures higher than 90°C. Another possibility is to use a hydrogen compressor (electrical). The hydrogen compressor can be placed in one of four locations, such as in the heat cycle on the heat source or utilization sides, on the high-pressure line where pressure is further increased, or on the low-pressure line where pressure is further decreased.The compressor is better placed on the heat utilization side becausea high compression ratio is not needed for all heat demands. If the compressor were placed on the high-pressure line where pressureis further increased,then we have to significantly change the high pressure-proof structure of the reaction vessels.The best location for the hydrogen compressor then is on the low-pressure line of the utilization side. Figure 2 shows the hybrid heat transport cycle using a hydrogen compressor on the heat utilization side. From the figure it is found that a compressor ratio of 1: 20 can
LONG-DISTANCE
913
HEAT TRANSPORT SYSTEM 400 m +
300
22 3 I $ 200
Waste heat
5 0-J 2 3 100
2.0
3.0 2.5 Reciprocal temperature, l/l / 1 Oe3K-’
-I
3.5
Fig, 2. Hybrid heat cycle using a hydrogen compressor on a van7 Hoff plot (heat utilization side).
be achieved with a hydrogen compression ratio of 1 : 5 using source heat, and a compression ratio of 1 : 4 using the electrically driven compressor. If, for example, highpressure hydrogen at 750 kPa was releasedfrom the heat source, and low-pressure hydrogen at 150 kPa was absorbed, then the hybrid cycle with a hydrogen compressor achievesa heat pump with a 40 K difference (9O’C heat is recovered from 50°C waste heat) in contrast to a 20 K heat pump in a heat-driven system (90°C heat is recovered from 70°C waste heat). 3. EXPERIMENTAL SYSTEM FOR THE HYBRID HEAT TRANSPORT 3.1. System structure The experimental system we constructed was one that transported 90°C heat over a long distance, and regenerated it at the heat utilization side as 90°C heat. Our purpose for this systemwas to achieve waste heat at 50°C with the hybrid cycle as opposed to the 70°C waste heat required in heat-driven transport systems. First, an oil-Iess diaphram-type compressor was selected to eliminate any effects from lubricating oil. As can be seen from Fig. 3 showing the characteristics of the compressor, a 1 : 4 compression ratio is achieved at a flow rate of 6 x 10e4mol s-’ (50 kPa of input pressure,25°C). The scale of the system was set at 0.1-0.2 kW from this flow rate. The experimental system simulated pressure lossesin the long-distance hydrogen pipeline by using orifices that generated a lo-kPa pressureloss at the averagehydrogen flow rate, and was structured using four hydrogen absorbing alloy reaction vessels(A, A’, B, B’) as shown in Fig. 4. Here, hydrogen gas is released from hydride A using heat from a 90°C heat source, and is transported to B using the pressure difference. Heat is recovered at B
Inlet pressure : 50 kPa
L
0 0
2
4
6
8
10
Hydrogen flow rate I 1Oe4mol as-’ Fig. 3. Characteristics of the compressor used in the experimental system.
through a hydrogenation reaction, and the 90°C heat is supplied to the heat load (heat transport process). After all the hydrogen absorbed in A is released, the waste heat of 50°C is supplied to vessel B and cooling water is supplied to vesselA to return the hydrogen (regeneration process). Becausethe output is intermittent in the above operations, two pairs of alloy vessels (A-B, A’-B’) are installed in order to provide continuous output by switching the two processes. Figure 5 shows a van? Hoff heat cycle. We see from this figure that hydrogen is transported at 750 kPa by the heat transport process, and that the 40-kPa pressure of the hydrogen releasedfrom the hydrogen absorbing alloy (MH2) is raised by the compressor to 150 kPa and then the hydrogen is transported by the regeneration process. 3.2. Reaction vessels
The heat transfer performance of the reaction vesselsis extremely important becauseof the low effective thermal conductivity of the hydrogen absorbing alloy in powdered form. For the experimental system, we adopted a basic stacked fin structure that keeps the alloy packing layer extremely thin, and we produced the fin structure by overlaying four rolled fins (fin pitch of 1 mm) made of aluminum as shown in Fig. 6. For the alloys, we used LmNi4.45Mno.2sCoo.zAlo 1 (MHl) and LmNi,,eMn, 2 Co,,Snr,, (MH2) which matched the temperature-pressure characteristics of the heat cycle as shown in Fig. 5 (Lm : La rich mischmetal). The heat transfer rate between the alloy bed and the heat medium was 45 W/K, the heat transfer rate between the alloy bed and the outside air was 1.4 W/K, and the heat capacity of the vesselswas 2.0 kJ/K. 3.3. Practical operation
Figure 7 shows the results of operating the hybrid heat transport system using a hydrogen compressor. The
914
K. NASAKO ef al. Heat source
Cooling water
Heat recoverv mediuk
Waste heat
(a) Heat transport process
.B cB MH2
85’C
25’C Fig. 4. Structure of the experimental hybrid heat transport system.
MHl MH2
, Heat source 90-c
a” 1000 Y E .i_.
500 i lter .
I
I
I
3.5 3.0 2.5 Reciprocal temperature, l/r / 10” K-’ Fig. 5. Hybrid heat cycle of the experimental heat transport system.
urn pips ( + 9.5 mm)
Fig. 6. Structure of the experimental reaction vessel.
LONG-DISTANCE
915
HEAT TRANSPORT SYSTEM Inlet of heat source Outlet of heat source
Inlet of heat recovery medium Inlet of waste heat Outlet of waste heal
I
I
I
I
I
I
0
600
1200
1600
2400
3000
1 3600
Time I s Fig. 7. Practical operation of the experimental hybrid heat transport system.
plotted in the diagram are the inlet and temperatures of the four reaction vessels,and were measured by inserting a CA thermocouple into the heat medium pipe near the vessels. The operating conditions consisted of heat medium flow rates of 0.6 l/min for the 90°C heat source, 0.6 l/min for the 20°C cooling water, 0.6 l/min for the 50°C waste heat, and 0.1 l/min for the heat recovery medium during a 40-min cycle period (20-min heat transport processand 20-min regeneration process). Hydrogen was transferred for 18.5min during each process,and 1.5min was needed for processswitching to shut down the hydrogen transfer and set the temperature conditions for the next process. The practical operation results showed that the temperature of the heat recovery medium supplied at a temperature of 70°C gradually increased after the start of the reaction until it reached 90°C after about six minutes. The temperature remained above 80°C from then until the end of the reaction. The amount of heat determined by multiplying the heat medium temperature difference before and after the vesselsby the flow rate was 390 kJ input from the 90°C heat source and 110 kJ recovered with the 70°C heat recovery medium every cycle, which yiekied a heat transport efficiency of 28% and a thermal power of 92 W at the utilization side. Figure 8 shows an energy flow diagram of the experimental system. In this figure, we showed the amount of heat in one cycle, and the energy necessaryat the compressor was calculated as temperatures
Heat source (390 kJ)“’
Recovery heat (110 kJ)‘*’
outlet
Heat loss (190 W)
Cooling water (330 kJ)‘*)
(w
Heat loss (120 kJ)
Waste heat (250 kJ)‘*’
~calculated from the temp. difference of the heat medium before and after the vessel. )
Fig. 8. Energy flow diagram of the experimental hybrid heat transport system.
the theoretical amount of energy which is able to compress hydrogen gas (25°C) from 40 to 150 kPa. These results indicate that the hybrid heat transport systemcan recover 90°C heat at the referencetemperature of 50°C waste heat, and that it can achieve a high temperature rise range of 40 K. We then compared the performance of the hybrid heat
916
K. NASAKO
transport system with that of heat-driven transport systems in order to evaluate its transport performance. Figure 9 shows the results of practical operation using a structure without the hydrogen compressor. The operating conditions were the same as those shown in Fig. 7. The result was a rapid increase in the temperature of the heat recovery medium as well as an increase in the amount of recovered heat compared to those of the hybrid heat transport system. The amount of heat input from the 90°C heat source was 390 kJ and the amount of heat recovered with the 70°C heat recovery medium was 130 kJ every cycle, which yielded a heat transport efficiency of 33% and a thermal power of 110 W at the utilization side. This is because of differences in the temperature rise range for the hybrid and heat-driven transport systems. With the heat-driven transport system, the amount of reaction heat at the utilization side is equal to the thermal power because the thermal power consists of all heat above 70°C (temperature of waste heat). With the hybrid system on the other hand, the thermal power consists of the amount of heat above 70°C (inlet temperature of recovery heat medium) in contrast with the reference temperature of 50°C (temperature of waste heat). Therefore, the thermal power of the hybrid system is lower than that of the heat-driven system by the 20 K difference sensible heat of the vessel.Becauseof the sensible heat recovery control which takes place during the process switching in actual operation, the sensible heat
et al.
loss was 20 kJ, which is equal to the 10 K difference vessel’ssensible heat calculated from the vessel’sthermal capacity of 2.0 kJ/K. From the above results, we can understand that the amount of heat recovered drops becauseof the high temperature rise range with the hybrid heat transport system using a hydrogen compressor, but in terms of system performance, the levels achieved are the same as those for heat-driven transport systems. 3.4. Studies on higher efjciency
The experimental hybrid heat transport system was demonstrated on a small scale of O.lLO.2kW becauseof the limit imposed by the capacity of the hydrogen compressor. As a result, the amount of heat loss from the reaction vessels(alloy weight: 2 kg) was extremely high with respect to the reaction heat of the alloys. In other words, the heat transfer rate between the alloys and the outside air was 1.4 W/K, so the heat loss was 110 kJ, which is 44% of the amount of reaction heat (250 kJ) on the utilization side. On the other hand, the heat loss drops dramatically with increased scale, as shown in Fig. 10, becauseof the increasing ratio difference between outer surface (equivalent to heat loss) and inner volume (equivalent to alloy weight). From this figure, the heat loss of a vessel(20 kg) ten times larger than the experimental vessel(2 kg) drops
Outlet of heat source / I Inlet of heat source
1 80 -
\‘Outlet of heat recovery medium Inlet of heat recovery medium
........a vk .........................” z
Outlet 0f cooling water Inlet of cooling water
0’
! 0
I
I
I
I
I
-I
600
1200
1800
2400
3000
3600
Time / s Fig. 9. Practicaloperationof the experimentalheat-drivenheattransportsystem.
LONG-DISTANCE HEAT TRANSPORT SYSTEM [ Heat transport efficiency, 7 ]
[Thermal power, Q 1
source side (a,) and on the utilization side (cI>)were both 0.6. As a result, we can understand that by increasing the scale of the vessel structure 10 times, we can achieve an efficiency of about 60% with hybrid heat transport and a high efficiency of about 70% with heat-driven transport.
4. APPLICATION s =0 ?
917
TO OTHER HEAT LOADS
0.6
Heat transport technologies based on hydrogen absorbing alloys transport heat by using the ability of z 0.6 energy conversion on the heat source side, and converting El the temperature difference between the heat source and g 0.4 the cooling water into hydrogen pressure (high-pressure hydrogen from low-pressure hydrogen). They then convert the hydrogen pressure back into a temperature 5 0.2 a difference on the heat utilization side. ii t This enables various applications to heat loads, such 0 as heating, cooling and plant processing heat on the heat 40 20 30 50 10 utilization side. Packed alloy weight I kg As shown in Fig. 12, for example, a 1 : 5 pressure ratio Fig. IO.RelationshipbetweenheatlossK. A and packingalloy enables a 20 K temperature difference that can be used weightof a vessel. for cooling (25°C to 5°C) and for plant processing heat (90 C to 11O’C steam regeneration). When a hydrogen compressor is introduced, we have a 1 : 20 pressure ratio to I, 10, and this keeps the heat loss under 5% of the that enables a 40 K temperature difference which can be used for freezing (25°C to - 15°C) and for heating (10°C reaction heat. Figure I1 shows the effect of the heat loss from the to 50°C). Here, the hydrogen absorbing alloy installed vessel on the heat transport efficiency. Heat transport on the heat utilization side must match the temperature efficiency q is calculated with equation (1) based on the level of the heat load. On the other hand, the same alloy can be used and amount of reacting heat, the amount of heat required to raise and lower the vessel temperature, and the amount only one vessel is needed regardless of the heat load on of heat loss from the vessel.Here the amount of reaction the heat source side, and it is possible to network using hydrogen n was 7 mol, the reaction heat of alloy/H, on centralized heat sources as well becausepressure is supthe source side and on the utilization side was AH, = 32, plied at a ratio of 1 : 5. In contrast to the large-capacity AH2 = 36 kJ/mol respectively, the heat capacity C was high- and low-pressure lines and synchronized control 2.0 kJ/K and the ratio of sensible heat recovery on the for the two different alloy vesselsused with conventional heat pumps, the new system offers greater freedom because the heat source side and heat utilization side operate independently toward the constant pressure of 0.8 the long-distance pipeline. Figure 13 shows an overall view of a wide-area heat + Experimental utilization (thermal network) system based on this concept which supplies heat to various loads using a group of solar heat collectors and waste heat from a large-scale 0.6 industrial plant as the heat source. The system not only makes efficient use of heat such as solar heat over several Heat-driven cycle t? dozen kilometers, as well as industrial plant waste heat, but also effectively counters the global warming phenomenon in cities becauseit centralizes heat sources. Hereafter, for the purpose of developing this system for practical use, studies from an economical point of view are very important. Especially concerning hydriding alloys requiring a relatively large scale, it is important that the alloy composition is evaluted with respectto cost 0.2 0.4 0.6 0.8 ’ and the estimated amount of deposits. K-A / W* K-f (per I kg-alloy) The high- and low-pressure hydrogen pipelines are Fig. 11. Effect of heat loss from the vesselon heat transport feasible today, and the system holds great promise in efficiency,7. terms of the environment for tomorrow. t This study
0”
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K. NASAKO et al. Output 50%
Reciprocal temperature, l/T / 1Om3 K-l Fig. 12. Hybrid heat cycle of a wide-area heat utilization system (heat utilization side).
Solar collector group
Heat transport unit
MH vessel AA&&& Heating & hot water supply for apartments, hotels (5-5Osc)
n Waste heat
7
Processing heat of -plants (50-110°C)
Cooling, heating & hot water supply for houses (5-50 ‘C)
Fig. 13. Basic concept for a wide-area heat utilization system (thermalnetwork system).
5. CONCLUSIONS (1) A hybrid heat transport systemthat usesa hydrogen compressor was newly proposed, and shown to be superior to a heat-driven transport system in respect of the applicable heat load. (2) An experimental hybrid heat transport system
achieved twice the temperature rise range (40 K) of
that provided by a heat-driven transport system (20 K). (3) A thermal efficiency of 28% was attained with a 0.2kW experimental hybrid transport system,and it was found that 60% thermal efficiency can be achieved by increasing the scale of transport heat 10 times.
LONG-DISTANCE
HEAT TRANSPORT SYSTEM
(4) A basic concept for a wide-area heat utilization sys-
tem (thermal network) composed of both hybrid and heat-driven heat transport systems was newly proposed, which is capable of supplying heat to various loads using a group of solar heat collectors and large-scale waste heat over several dozen kilometers as heat sources.
Acknowledgements-This work was supported by NED0 (New Energy and Industrial Technology Development Organization) as a part of the Sunshine Project under the Ministry of International Trade and Industry of Japan.
REFERENCES 1. Nasako, K., Yonezu, I., Honda, N. and Furukawa, N.. Jupan Patent 4-41271 (B2), July 7, 1992. 2. Nasako, K., Yonesaki, T., Yonezu, I., Fujitani, S., Saito.
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T., Moroto, M., Osumi, M. and Furukawa, N., Proc. ISES Solar World Congress, 1990,2, 1343. Libowitz, G. G. and Blank, Z., Proc. 11th IECEC, p. 673,
3. 1976. 4, Yonezu, I., Nasako, K., Honda, N. and Sakai, T., J. LessCommon Metals, 1993,89, 351.
5, Gruen, D. M., Mendelsohn, M. H., Sheft, 1. and Lamich, G. J., Proc. 2nd WHEC, Zurich, Switzerland, p. 1931, Aug. 1978. 6. Gorman, R. and Moritz, P. S., Proc. AIAA Solar Energ.v Meeting, Phoenix, Arizona, p, I, Nov. 1978. 7, Yanoma, A., Yoneta, M., Nitta, T. and Okuda, K., Proc. ASME/JSME Thermal Engineering Joint Coyf:, p. 431. 1987. Jpn, 1996.71. 8. Murai, M. and Sato, Y., REFRIGERATION 826, p. 590. 9. Nasako, K., Hiro, N., Fukushima, K., Fujitani, S., Yonezu, 1. and Osumi, M., Solar Engineering. ASME, G00837, p. 373, 1994. 10. Nasako, K., Ito, Y. andosumi, M., ht. J. Hydrogen Energy, to be published.
Pergamon
LONG-DISTANCE
HEAT TRANSPORT SYSTEM USING A HYDROGEN COMPRESSOR K. NASAKO,*Tf Y. ITO* and M. OSUMIf
*Department of Fundamental Energy Science,Graduate School of Energy Science,Kyoto University, Yoshida, Sakyo-ku, Kyoto, Japan tMechatronics Research Center, Sanyo Electric Co. Ltd., I-18-13, Hashiridani, Hirakata City, Osaka, Japan
Abstract-A heat transport system using hydrogen absorbing alloys offers several advantages, such as enabling the use of more compact long-distance transport pipes, maintaining a constant level of efficiency regardless of transport distance, and enabling rapid transport. On the other hand, the system supplies heat to a load through a heat pump with a 20 K difference.The system therefore cannot be usedfor heat loads that require a higher temperature difference. Then, in order to increase heat load applications, we have proposed a hybrid heat transport system that uses a hydrogen compressor. We also developed a system structure and achieved a heat pump with a 40 K difference by using a 0.2-kW experimental system. Furthermore, we focused on using the same structure on the heat source side for supplying various heat loads, and have proposed a basic concept for a wide-area heat utilization system that transports heat to various loads from centralized heat sources. 0 1998 International Association for Hydrogen Energy
spheric CO2 levels as well as reductions in the amount of fossil fuels we consume. These require more efficient A Surface of heat transfer (m’) energy equipment, utilization of renewable energy, and C Heat capacity of a reaction vessel(J K-‘) efficient use of waste energy. In particular, the potentially K Overall coefficient of heat transfer (W me2 K- ‘) vast stores of renewable and waste energy are becoming n Amount of transferred H, (mol) increasingly important. The reason why we make so little Time of heat transport process (s) f,., use of this energy is that the heat sources and loads LiH, Reaction heat of heat source side alloy (J mol are not conveniently located, and the heat sources are H; ‘1 unstable. AH! Reaction heat of heat utilization side alloy (J The authors have proposed a concept for a new longmol H; ‘) distance heat transport systemusing hydrogen absorbing Ratio of sensible heat recovery at heat source alloys [ 1,2]. So far, hydrogen absorbing alloys have conside (-) ventionally received attention only with regard to their Ratio of sensible heat recovery at heat uti- energy conversion function, and have been researchedin lization side the area of conventional heat utilization systems, such Heat transport efficiency (-) v as heat storage [3, 41, heat pumps [5-71 and cold heat Temperature of heat source (K) 7-l generation [8,9]. The proposed system, however, utilizes Temperature of cooling water (K) T2 both of the merits of hydrogen gas, i.e., easy transport Temperature of heat recovery medium (K) T, over long distances and the energy conversion function Temperature of waste heat (K) T4 of hydrogen absorbing alloys. And by a subsequent Atmospheric temperature (K) T” analysis of the new system [lo], it was found that compared to conventional heat transport systems using heated water 1) there was no drop in transport efficiency 1. INTRODUCTION even at long distances, 2) the diameter of the long-disMeasures aimed at the problem of global climatic tance transport pipeline could be decreasedto l/3, and change, i.e., global warming, call for reductions in atmo- 3) intermittent heat could also be transported with the new system.This system,however, supplies heat to a load on the utilization side through a heat pump with a 20 K difference. This meant that the system could not be used SAuthor to whom all correspondence should be addressed. for loads that require a higher temperature difference. NOMENCLATURE
911
912
K. NASAKO et al. Solar collector
(Heat source side)
j ~~~~~~~~~~
l/T -w
l/l
--t
Fig. 1.Principleof the heattransportsystemusinghydrogenabsorbingalloys.
This paper describesa proposal of a hybrid heat transport system that usesa hydrogen compressor in order to increaseheat load applications, and a proposal of a widearea heat utilization system (thermal network) capable of transporting heat to various loads from centralized heat sources. 2. HYBRID HEAT TRANSPORT SYSTEM USING A HYDROGEN COMPRESSOR 2.1. Heat transport system using hydrogen absorbing alloys
The heat transport system is structured with hydrogen absorbing alloys having different characteristics installed at the heat generating site and the heat utilization site some distance away and connected by a hydrogen gas pipeline as shown in Fig. 1, In the structure, the hydrogen absorbing alloy MHl on the heat source side absorbs low-pressure hydrogen (cooled by a low-temperature (T,) heat source), and releaseshigh-pressure hydrogen (heated by a high-temperature (T,) heat source). Heat is pumped up on the utilization side by hydrogen pressure to supply the heat load. For example, a 90°C heat source and 20°C cooling water used on the heat source side generate pressure at a compression ratio of about 1: 5 (150 kPa pressure is compressed to 750 kPa pressure). After longdistance transport by hydrogen, this pressure difference can drive a heat pump with respect to the reference temperature (for example, waste heat or air source). This new heat transport system does not need to raise the temperature of the long-distance pipeline and has an extremely short rise time (about 1 min), so the system is applicable to transport the heat of an intermittent heat source such as solar energy and plant waste energy. From the result of a feasibility study [lo], it was found that small-scale heat (2-kW), which cannot be transported by heated water, can be transported with high efficiency over
long distances (2 km, 60%) by hydrogen, and that this heat transport system is also more efficient even with large-scale (lOO-kW) heat transport at distances over 2 km. The temperature rise range (temperature fall range as well), however, is limited by the energy of the pressure difference, and is about 20 K in the above case of a 1: 5 compression ratio. Despite this temperature range, the systemis still suitable for a variety of applications, including heating, cooling and hot water supply as well as industrial plant processing heat. Even so, a system with a higher temperature range is neededin order to increase the applicable heat loads. 2.2. Hybrid cycle using a compressor
A higher temperature rise range can be achieved by raising the compression ratio of the hydrogen pressure. One means of doing this is to raise the heat source temperature, but it is hard to imagine a renewable or waste energy heat source achieving temperatures higher than 90°C. Another possibility is to use a hydrogen compressor (electrical). The hydrogen compressor can be placed in one of four locations, such as in the heat cycle on the heat source or utilization sides, on the high-pressure line where pressure is further increased, or on the low-pressure line where pressure is further decreased.The compressor is better placed on the heat utilization side becausea high compression ratio is not needed for all heat demands. If the compressor were placed on the high-pressure line where pressureis further increased,then we have to significantly change the high pressure-proof structure of the reaction vessels.The best location for the hydrogen compressor then is on the low-pressure line of the utilization side. Figure 2 shows the hybrid heat transport cycle using a hydrogen compressor on the heat utilization side. From the figure it is found that a compressor ratio of 1: 20 can
LONG-DISTANCE
913
HEAT TRANSPORT SYSTEM 400 m +
300
22 3 I $ 200
Waste heat
5 0-J 2 3 100
2.0
3.0 2.5 Reciprocal temperature, l/l / 1 Oe3K-’
-I
3.5
Fig, 2. Hybrid heat cycle using a hydrogen compressor on a van7 Hoff plot (heat utilization side).
be achieved with a hydrogen compression ratio of 1 : 5 using source heat, and a compression ratio of 1 : 4 using the electrically driven compressor. If, for example, highpressure hydrogen at 750 kPa was releasedfrom the heat source, and low-pressure hydrogen at 150 kPa was absorbed, then the hybrid cycle with a hydrogen compressor achievesa heat pump with a 40 K difference (9O’C heat is recovered from 50°C waste heat) in contrast to a 20 K heat pump in a heat-driven system (90°C heat is recovered from 70°C waste heat). 3. EXPERIMENTAL SYSTEM FOR THE HYBRID HEAT TRANSPORT 3.1. System structure The experimental system we constructed was one that transported 90°C heat over a long distance, and regenerated it at the heat utilization side as 90°C heat. Our purpose for this systemwas to achieve waste heat at 50°C with the hybrid cycle as opposed to the 70°C waste heat required in heat-driven transport systems. First, an oil-Iess diaphram-type compressor was selected to eliminate any effects from lubricating oil. As can be seen from Fig. 3 showing the characteristics of the compressor, a 1 : 4 compression ratio is achieved at a flow rate of 6 x 10e4mol s-’ (50 kPa of input pressure,25°C). The scale of the system was set at 0.1-0.2 kW from this flow rate. The experimental system simulated pressure lossesin the long-distance hydrogen pipeline by using orifices that generated a lo-kPa pressureloss at the averagehydrogen flow rate, and was structured using four hydrogen absorbing alloy reaction vessels(A, A’, B, B’) as shown in Fig. 4. Here, hydrogen gas is released from hydride A using heat from a 90°C heat source, and is transported to B using the pressure difference. Heat is recovered at B
Inlet pressure : 50 kPa
L
0 0
2
4
6
8
10
Hydrogen flow rate I 1Oe4mol as-’ Fig. 3. Characteristics of the compressor used in the experimental system.
through a hydrogenation reaction, and the 90°C heat is supplied to the heat load (heat transport process). After all the hydrogen absorbed in A is released, the waste heat of 50°C is supplied to vessel B and cooling water is supplied to vesselA to return the hydrogen (regeneration process). Becausethe output is intermittent in the above operations, two pairs of alloy vessels (A-B, A’-B’) are installed in order to provide continuous output by switching the two processes. Figure 5 shows a van? Hoff heat cycle. We see from this figure that hydrogen is transported at 750 kPa by the heat transport process, and that the 40-kPa pressure of the hydrogen releasedfrom the hydrogen absorbing alloy (MH2) is raised by the compressor to 150 kPa and then the hydrogen is transported by the regeneration process. 3.2. Reaction vessels
The heat transfer performance of the reaction vesselsis extremely important becauseof the low effective thermal conductivity of the hydrogen absorbing alloy in powdered form. For the experimental system, we adopted a basic stacked fin structure that keeps the alloy packing layer extremely thin, and we produced the fin structure by overlaying four rolled fins (fin pitch of 1 mm) made of aluminum as shown in Fig. 6. For the alloys, we used LmNi4.45Mno.2sCoo.zAlo 1 (MHl) and LmNi,,eMn, 2 Co,,Snr,, (MH2) which matched the temperature-pressure characteristics of the heat cycle as shown in Fig. 5 (Lm : La rich mischmetal). The heat transfer rate between the alloy bed and the heat medium was 45 W/K, the heat transfer rate between the alloy bed and the outside air was 1.4 W/K, and the heat capacity of the vesselswas 2.0 kJ/K. 3.3. Practical operation
Figure 7 shows the results of operating the hybrid heat transport system using a hydrogen compressor. The
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K. NASAKO ef al. Heat source
Cooling water
Heat recoverv mediuk
Waste heat
(a) Heat transport process
.B cB MH2
85’C
25’C Fig. 4. Structure of the experimental hybrid heat transport system.
MHl MH2
, Heat source 90-c
a” 1000 Y E .i_.
500 i lter .
I
I
I
3.5 3.0 2.5 Reciprocal temperature, l/r / 10” K-’ Fig. 5. Hybrid heat cycle of the experimental heat transport system.
urn pips ( + 9.5 mm)
Fig. 6. Structure of the experimental reaction vessel.
LONG-DISTANCE
915
HEAT TRANSPORT SYSTEM Inlet of heat source Outlet of heat source
Inlet of heat recovery medium Inlet of waste heat Outlet of waste heal
I
I
I
I
I
I
0
600
1200
1600
2400
3000
1 3600
Time I s Fig. 7. Practical operation of the experimental hybrid heat transport system.
plotted in the diagram are the inlet and temperatures of the four reaction vessels,and were measured by inserting a CA thermocouple into the heat medium pipe near the vessels. The operating conditions consisted of heat medium flow rates of 0.6 l/min for the 90°C heat source, 0.6 l/min for the 20°C cooling water, 0.6 l/min for the 50°C waste heat, and 0.1 l/min for the heat recovery medium during a 40-min cycle period (20-min heat transport processand 20-min regeneration process). Hydrogen was transferred for 18.5min during each process,and 1.5min was needed for processswitching to shut down the hydrogen transfer and set the temperature conditions for the next process. The practical operation results showed that the temperature of the heat recovery medium supplied at a temperature of 70°C gradually increased after the start of the reaction until it reached 90°C after about six minutes. The temperature remained above 80°C from then until the end of the reaction. The amount of heat determined by multiplying the heat medium temperature difference before and after the vesselsby the flow rate was 390 kJ input from the 90°C heat source and 110 kJ recovered with the 70°C heat recovery medium every cycle, which yiekied a heat transport efficiency of 28% and a thermal power of 92 W at the utilization side. Figure 8 shows an energy flow diagram of the experimental system. In this figure, we showed the amount of heat in one cycle, and the energy necessaryat the compressor was calculated as temperatures
Heat source (390 kJ)“’
Recovery heat (110 kJ)‘*’
outlet
Heat loss (190 W)
Cooling water (330 kJ)‘*)
(w
Heat loss (120 kJ)
Waste heat (250 kJ)‘*’
~calculated from the temp. difference of the heat medium before and after the vessel. )
Fig. 8. Energy flow diagram of the experimental hybrid heat transport system.
the theoretical amount of energy which is able to compress hydrogen gas (25°C) from 40 to 150 kPa. These results indicate that the hybrid heat transport systemcan recover 90°C heat at the referencetemperature of 50°C waste heat, and that it can achieve a high temperature rise range of 40 K. We then compared the performance of the hybrid heat
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K. NASAKO
transport system with that of heat-driven transport systems in order to evaluate its transport performance. Figure 9 shows the results of practical operation using a structure without the hydrogen compressor. The operating conditions were the same as those shown in Fig. 7. The result was a rapid increase in the temperature of the heat recovery medium as well as an increase in the amount of recovered heat compared to those of the hybrid heat transport system. The amount of heat input from the 90°C heat source was 390 kJ and the amount of heat recovered with the 70°C heat recovery medium was 130 kJ every cycle, which yielded a heat transport efficiency of 33% and a thermal power of 110 W at the utilization side. This is because of differences in the temperature rise range for the hybrid and heat-driven transport systems. With the heat-driven transport system, the amount of reaction heat at the utilization side is equal to the thermal power because the thermal power consists of all heat above 70°C (temperature of waste heat). With the hybrid system on the other hand, the thermal power consists of the amount of heat above 70°C (inlet temperature of recovery heat medium) in contrast with the reference temperature of 50°C (temperature of waste heat). Therefore, the thermal power of the hybrid system is lower than that of the heat-driven system by the 20 K difference sensible heat of the vessel.Becauseof the sensible heat recovery control which takes place during the process switching in actual operation, the sensible heat
et al.
loss was 20 kJ, which is equal to the 10 K difference vessel’ssensible heat calculated from the vessel’sthermal capacity of 2.0 kJ/K. From the above results, we can understand that the amount of heat recovered drops becauseof the high temperature rise range with the hybrid heat transport system using a hydrogen compressor, but in terms of system performance, the levels achieved are the same as those for heat-driven transport systems. 3.4. Studies on higher efjciency
The experimental hybrid heat transport system was demonstrated on a small scale of O.lLO.2kW becauseof the limit imposed by the capacity of the hydrogen compressor. As a result, the amount of heat loss from the reaction vessels(alloy weight: 2 kg) was extremely high with respect to the reaction heat of the alloys. In other words, the heat transfer rate between the alloys and the outside air was 1.4 W/K, so the heat loss was 110 kJ, which is 44% of the amount of reaction heat (250 kJ) on the utilization side. On the other hand, the heat loss drops dramatically with increased scale, as shown in Fig. 10, becauseof the increasing ratio difference between outer surface (equivalent to heat loss) and inner volume (equivalent to alloy weight). From this figure, the heat loss of a vessel(20 kg) ten times larger than the experimental vessel(2 kg) drops
Outlet of heat source / I Inlet of heat source
1 80 -
\‘Outlet of heat recovery medium Inlet of heat recovery medium
........a vk .........................” z
Outlet 0f cooling water Inlet of cooling water
0’
! 0
I
I
I
I
I
-I
600
1200
1800
2400
3000
3600
Time / s Fig. 9. Practicaloperationof the experimentalheat-drivenheattransportsystem.
LONG-DISTANCE HEAT TRANSPORT SYSTEM [ Heat transport efficiency, 7 ]
[Thermal power, Q 1
source side (a,) and on the utilization side (cI>)were both 0.6. As a result, we can understand that by increasing the scale of the vessel structure 10 times, we can achieve an efficiency of about 60% with hybrid heat transport and a high efficiency of about 70% with heat-driven transport.
4. APPLICATION s =0 ?
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TO OTHER HEAT LOADS
0.6
Heat transport technologies based on hydrogen absorbing alloys transport heat by using the ability of z 0.6 energy conversion on the heat source side, and converting El the temperature difference between the heat source and g 0.4 the cooling water into hydrogen pressure (high-pressure hydrogen from low-pressure hydrogen). They then convert the hydrogen pressure back into a temperature 5 0.2 a difference on the heat utilization side. ii t This enables various applications to heat loads, such 0 as heating, cooling and plant processing heat on the heat 40 20 30 50 10 utilization side. Packed alloy weight I kg As shown in Fig. 12, for example, a 1 : 5 pressure ratio Fig. IO.RelationshipbetweenheatlossK. A and packingalloy enables a 20 K temperature difference that can be used weightof a vessel. for cooling (25°C to 5°C) and for plant processing heat (90 C to 11O’C steam regeneration). When a hydrogen compressor is introduced, we have a 1 : 20 pressure ratio to I, 10, and this keeps the heat loss under 5% of the that enables a 40 K temperature difference which can be used for freezing (25°C to - 15°C) and for heating (10°C reaction heat. Figure I1 shows the effect of the heat loss from the to 50°C). Here, the hydrogen absorbing alloy installed vessel on the heat transport efficiency. Heat transport on the heat utilization side must match the temperature efficiency q is calculated with equation (1) based on the level of the heat load. On the other hand, the same alloy can be used and amount of reacting heat, the amount of heat required to raise and lower the vessel temperature, and the amount only one vessel is needed regardless of the heat load on of heat loss from the vessel.Here the amount of reaction the heat source side, and it is possible to network using hydrogen n was 7 mol, the reaction heat of alloy/H, on centralized heat sources as well becausepressure is supthe source side and on the utilization side was AH, = 32, plied at a ratio of 1 : 5. In contrast to the large-capacity AH2 = 36 kJ/mol respectively, the heat capacity C was high- and low-pressure lines and synchronized control 2.0 kJ/K and the ratio of sensible heat recovery on the for the two different alloy vesselsused with conventional heat pumps, the new system offers greater freedom because the heat source side and heat utilization side operate independently toward the constant pressure of 0.8 the long-distance pipeline. Figure 13 shows an overall view of a wide-area heat + Experimental utilization (thermal network) system based on this concept which supplies heat to various loads using a group of solar heat collectors and waste heat from a large-scale 0.6 industrial plant as the heat source. The system not only makes efficient use of heat such as solar heat over several Heat-driven cycle t? dozen kilometers, as well as industrial plant waste heat, but also effectively counters the global warming phenomenon in cities becauseit centralizes heat sources. Hereafter, for the purpose of developing this system for practical use, studies from an economical point of view are very important. Especially concerning hydriding alloys requiring a relatively large scale, it is important that the alloy composition is evaluted with respectto cost 0.2 0.4 0.6 0.8 ’ and the estimated amount of deposits. K-A / W* K-f (per I kg-alloy) The high- and low-pressure hydrogen pipelines are Fig. 11. Effect of heat loss from the vesselon heat transport feasible today, and the system holds great promise in efficiency,7. terms of the environment for tomorrow. t This study
0”
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K. NASAKO et al. Output 50%
Reciprocal temperature, l/T / 1Om3 K-l Fig. 12. Hybrid heat cycle of a wide-area heat utilization system (heat utilization side).
Solar collector group
Heat transport unit
MH vessel AA&&& Heating & hot water supply for apartments, hotels (5-5Osc)
n Waste heat
7
Processing heat of -plants (50-110°C)
Cooling, heating & hot water supply for houses (5-50 ‘C)
Fig. 13. Basic concept for a wide-area heat utilization system (thermalnetwork system).
5. CONCLUSIONS (1) A hybrid heat transport systemthat usesa hydrogen compressor was newly proposed, and shown to be superior to a heat-driven transport system in respect of the applicable heat load. (2) An experimental hybrid heat transport system
achieved twice the temperature rise range (40 K) of
that provided by a heat-driven transport system (20 K). (3) A thermal efficiency of 28% was attained with a 0.2kW experimental hybrid transport system,and it was found that 60% thermal efficiency can be achieved by increasing the scale of transport heat 10 times.
LONG-DISTANCE
HEAT TRANSPORT SYSTEM
(4) A basic concept for a wide-area heat utilization sys-
tem (thermal network) composed of both hybrid and heat-driven heat transport systems was newly proposed, which is capable of supplying heat to various loads using a group of solar heat collectors and large-scale waste heat over several dozen kilometers as heat sources.
Acknowledgements-This work was supported by NED0 (New Energy and Industrial Technology Development Organization) as a part of the Sunshine Project under the Ministry of International Trade and Industry of Japan.
REFERENCES 1. Nasako, K., Yonezu, I., Honda, N. and Furukawa, N.. Jupan Patent 4-41271 (B2), July 7, 1992. 2. Nasako, K., Yonesaki, T., Yonezu, I., Fujitani, S., Saito.
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T., Moroto, M., Osumi, M. and Furukawa, N., Proc. ISES Solar World Congress, 1990,2, 1343. Libowitz, G. G. and Blank, Z., Proc. 11th IECEC, p. 673,
3. 1976. 4, Yonezu, I., Nasako, K., Honda, N. and Sakai, T., J. LessCommon Metals, 1993,89, 351.
5, Gruen, D. M., Mendelsohn, M. H., Sheft, 1. and Lamich, G. J., Proc. 2nd WHEC, Zurich, Switzerland, p. 1931, Aug. 1978. 6. Gorman, R. and Moritz, P. S., Proc. AIAA Solar Energ.v Meeting, Phoenix, Arizona, p, I, Nov. 1978. 7, Yanoma, A., Yoneta, M., Nitta, T. and Okuda, K., Proc. ASME/JSME Thermal Engineering Joint Coyf:, p. 431. 1987. Jpn, 1996.71. 8. Murai, M. and Sato, Y., REFRIGERATION 826, p. 590. 9. Nasako, K., Hiro, N., Fukushima, K., Fujitani, S., Yonezu, 1. and Osumi, M., Solar Engineering. ASME, G00837, p. 373, 1994. 10. Nasako, K., Ito, Y. andosumi, M., ht. J. Hydrogen Energy, to be published.