Applicability of Tri-generation Energy Production for Air-conditioning Systems in Czech Republic

Transcription

1 Proceedings of the 5th IASM/WSAS Int. Conference on eat Transfer, Thermal ngineering and nvironment, Athens, Greece, August 25-27, Alicability of Tri-generation nergy Production for Air-conditioning Systems in Czech Reublic J. POSPISIL, J. FIDLR nergy Institute, Deartment of Power ngineering Faculty of Mechanical ngineering Brno University of Technology Technicka 2896/2, Brno CZC RPUBLIC Abstract: This contribution resents the analysis of ossible utilizing of a tri-generation system for effective electricity, heat and cool roduction. A tri-generation offers more effective utilizing of rimary energy sources in comarison with searate roduction of different kinds of energy. Natural gas is considered as the most common rimary energy source utilized in tri-generation units. This analysis resects the legislation and technical directives valid in the Czech Reublic in year The analysis synthesizes technical requirements for roduction of heat during a heating eriod and roduction of cool during a cooling eriod. The studied tri-generation system consists from a cogeneration unit, based on a stroke engine technology, connected with a convenient absortion machine for roduction of cool water. The mixture of water and LiBr is considered as aroriate working-air medium oerating in the absortion cycle. This aer focuses on environmental benefit and economical assessment of a tri-generation technology in residences and mid-size buildings. Key-Words: oly-generation, tri-generation, heating, cooling, absortion cycle, cogeneration 1 Introduction Poly-generation of energy is the nowadays trend in an energy suly of buildings. Poly-generation offers significant advantages in comarison with searate energy roduction. lectricity is commonly roduced in ower lants. A rimary chemical energy of fuel is transformed to electricity with the efficiency about 38 %. The rest of the rimary energy leaves the ower lants in form of a waste heat without utilizing. On the second side, the heat energy for heating of buildings is roduced commonly in boilers with the efficiency close to 90 %. Cool for air-conditioning systems is the most frequently roduced with utilizing of vaor cooling cycles driven by electricity. The cool roduction reresents a technology with significant consumtion of energy. The second ossible way of the cool roduction is to utilize an absortion cycle technology driven by heat energy from different sources. Poly-generation of energy enables to utilize a waste heat released during ower lant generation of electricity. So, oly-generation significantly increases effectiveness of rimary energy sources utilizing and decreases roduction of CO 2. The oly-generation reresents effective roduction of energy close to a osition of consumtion. A short connection between the osition of the generation and the osition of consumtion is an imortant advantage that enables to utilize majority of the generated energy. A combined roduction of electricity, heat and cool is called the tri-generation. Residences and mid-size buildings are commonly connected with significant consumtion of electricity, heat (during winter) and cool (during summer). Cogeneration units reresent the most common olygeneration systems roducing electricity and heat. Small size cogeneration units are the most frequently based on the stroke engine basis. Another considered olygeneration system is a tri-generation. The trigeneration consists from a cogeneration unit connected with a convenient absortion cycle for roduction of chill water. ot water or steam serves as a convenient source of a driving heat for an absortion cycle. The terminal cooling caacity of the absortion system deends on the temerature and quantity of the driving heat. Vaor-comression cycles rovide high COP values for a small temerature difference between the evaorator temerature and the condenser temerature in comarison with the absortion cycle. The temerature difference increase causes decrease of the COP value. An absortion cycle COP value is not so sensitive to the temerature difference between an evaorator and a condenser. But there are hysical limitations of the working temerature range. Utilizing of oly-generation requires simultaneity in consumtion of all kinds of roduced energy. In a different way, a control system has to decrease ower

2 Proceedings of the 5th IASM/WSAS Int. Conference on eat Transfer, Thermal ngineering and nvironment, Athens, Greece, August 25-27, outut of the oly-generation unit or the suerfluous energy must be rejected or accumulated. 2 Nowadays utilizing of oly-generation The Czech Reublic is country with numerous alications of district heating systems. Nearly all cities utilize any kind of a combined heat and ower roduction. Cogeneration systems roduce 16 % of electricity roduced in the Czech Reublic. The cogeneration systems with ower outut greater than 3 MW e frequently use the coal and biomass as a rimary fuel. These systems engage either thermal vaor cycles with stem turbines or combination of gas turbine cycles and steam turbine cycles. Also waste incineration lants utilize the steam turbine technology for cogeneration. Solo gas turbines are occasionally used in systems with ower outut range from 1.5 to 3 MW. Smaller cogeneration systems are nearly all fed by natural gas and based on the stroke engine technology. The fig. 1 shows ercentage of different cogeneration systems oerating in the Czech Reublic. Tri-generation systems are used only in limited number of alications with significant requirements on amount of heat and cool suly. Tyical objects for alications are university buildings, hositals, office buildings and some technologies. sufficient temerature level (above 90 C) for common heating systems 59 % of the fuel energy. ngine tye fficiency % lectricity eat Total Petrol Diesel Table 1 The efficiency of cogeneration units based on a stroke engine technology 9 % of energy is taken out from the system by thermal losses. The fig. 2 shows the real energy fluxes in the considered cogeneration unit. Fig. 2 nergy fluxes in the stroke engine cogeneration unit Fig. 1 Percentage of different cogeneration systems oerating in the Czech Reublic, Poly-generation of energy 3.1 Cogeneration A cogeneration reresents the most common tye of oly-generation frequently based on a stroke engine basis. Different tyes of stroke engines are used at cogeneration units and their efficiency vary in range mentioned in the Table 1. The articular cogeneration unit considered in this study for a detail analysis has technical arameters: energy of fuel is transformed to electricity with efficiency 32%, heat from flue gasses and from an engine cooling jacket is used as a source of heat with a 3.2 Tri-generation Cool for air-conditioning units is commonly roduced in cooling cycles driven by electricity. Cool roduction requires significant consumtion of energy. Majority of cooling systems utilize a vaor-comressor cycle. The second ossible way of the cool roduction is to utilize an absortion cycle technology driven by heat energy from a cogeneration unit. This system consists from a cogeneration unit connected with a convenient absortion system for roduction of cool water. eat from a cogeneration unit is transorted by hot water to a heat exchanger of the absortion cycle, see fig. 3. This system is called the tri-generation technology for combined roduction of electricity, heat and cool. The tri-generation can be used with advantage in energy central suly systems of buildings and its oeration can be otimize to minimal consumtion of the rimary energy sources with conservation of full comfort in an energy utilizing.

3 Proceedings of the 5th IASM/WSAS Int. Conference on eat Transfer, Thermal ngineering and nvironment, Athens, Greece, August 25-27, Utilizing of the tri-generation technology significantly rotracts a year oerating eriod of a olygeneration system causing extended savings of rimary energy sources. The fig. 4 shows relation between the comarative saving of rimary energy Q CR and the ower to heat ratio of a oly-generation unit PR. Qsave QCR = (4) η + PP ηp The curve labeled 1 was obtained for cogeneration systems. The curve 2 was obtained for tri-generation systems. Fig. 3 The trigeneration technology with an absortion cycle 3.3 nvironmental benefit of oly-generation The oly-generation main hilosohy is decreasing of consumtion of rimary energy sources. Poly-generation reresents more effective utilizing of a rimary energy in comarison with searate roduction of different kinds of energy. The ower to heat ratio PR is arameter for exressing of the quality of energy transformation rocesses and the entire technological efficiency of a oly-generation assembly PR =, (1) where is a roduced electricity outut and is heat sulied to a heating system. The saving of a rimary energy in the cogeneration systems can be exressed as + CP Q = +, (2) save η η η PP P CP where η PP is the efficiency of a ower lant, η P is the efficiency of a heating lant and η CP is the efficiency of a cogeneration lant. The saving of a rimary energy in tri-generation systems can be exressed as C CCP + + C Q = + +, (3) save η η η η PP P CP CCP Fig. 4 The relationshi between the comarative saving of rimary energy Q CR and the ower to heat ratio of a oly-generation unit PR. We must strictly distinguish an area scale for the environmental assessment of a oly-generation technology. Utilizing of a oly-generation decreases a roduced emission rate in large areas (involving distant ower lants). If we asses an environmental burden in local scale areas, a oly-generation system can resent a significant local source of emissions located directly in urban areas. xosure of oulation to these emissions can be more danger than exosure to emissions from distant ower lants. From this reason, emission threshold values of stationary stroke engines must be minimized. where C means amount of cool sulied to air-condition system, η CP is the efficiency of a vaor cooling unit and η CCP is the efficiency of a tri-generation lant.

4 Proceedings of the 5th IASM/WSAS Int. Conference on eat Transfer, Thermal ngineering and nvironment, Athens, Greece, August 25-27, eating and cooling requirements The articular mid-size building located at the central art of the Czech Reublic was used for assessment of the alicability of tri-generation energy roduction for air-conditioning systems. The building arameters and energy requirements can be considered as reresentative arameters of mid-size buildings in the Czech Reublic. 4.1 eating requirements The heating season is 245 days/year for the region of the studied building. A temerature at this region varies from -15 C in winter to +35 C in summer. The fig. 5 shows the air temerature distribution curve for the average heating eriod. Fig. 5 The air temerature distribution curve for the average heating eriod The building altitude is 420 m. eat suly requirements were calculated in accordance with an actual legislation and the valid technical standards. The calculation took in to account the building heat loss, the heat required for necessary air exchange and hot water roduction. Results of the calculation are resent in the fig. 6 in form of the heat requirement distribution curve. A decreasing trend of the heat requirement distribution curve is characteristic for the heating season commonly taking 580 hours/year. The rest of a year, only a constant heat suly for rearing of hot water is required. In the fig. 6, the area labeled KJ reresents heat sulied by cogeneration unit. The area labeled PK1-3 reresents heat sulied by sulementary heat sources. In this study, we considered gas boilers oerating in the term of very low external air temeratures. Gas boilers are convenient as a sulementary source of heat for a cogeneration unit because they use the same fuel. Fig. 6 The heat requirement distribution curve for a cogeneration unit 4.2 Cooling requirements The cooling season in the studied locations takes about 90 days er year. An air-conditioning system must take away a heat load of a building and kee temerature of an indoor air on a required temerature level. Three main contributions of a heat load was considered, namely a heat load released from building equiment, a solar heat load by windows and a heat load enetrating by walls. The solar heat load by windows is a function of an actual solar heat flux and building arameters a building orientation, an area of windows, a tye of windows. The fig. 7 shows the results of a calculation for one day of the to cooling season. The maximal solar heat load was determined nearly exactly in the noon. A art of the solar load is absorbed by a building structure during day and released during night in due to decreasing of the indoor air temerature. From this reason, we can decrease the solar heat load by an accumulation caacity of the building, see fig. 7. The difference between the solar heat load minus the accumulation and the total heat load of the building is caused by a heat roduction from building equiment (lightning, comuters and technology) and a heat load enetrating by walls. Fig. 7 The building heat load for one articular day of the to cooling season

5 Proceedings of the 5th IASM/WSAS Int. Conference on eat Transfer, Thermal ngineering and nvironment, Athens, Greece, August 25-27, A tri-generation technology is used for roduction of cool water sulied to air-conditional units in this study. This system consists from a cogeneration unit connected with a convenient absortion system for roduction of cool water. eat from the cogeneration unit is transorted by hot water to a heat exchanger of the absortion cycle. Water cooled by an absortion cycle circulates between the air-condition units and the absortion cycle. 4.3 Cooling absortion cycle Utilizing of an absortion cycle is the only difference between cogeneration and tri-generation. From this reason, it is imortant to focus on detail erformance of the absortion cycle for correct evaluation of tri-generation. The absortion cycle consists from a generator, an absorber, a condenser and an evaorator, see fig. 8. These arts are connected by ies for transort of liquid mixtures and vaors. Throttle valves are used for deressurizing of fluid mixtures. A um enables to ressurize liquid mixtures. A liquid refrigerant evaorates in the evaorator. The heat flux Q 2 is taken away from cooled medium. The refrigerant vaors are led to the absorber, where the oor liquid mixture absorbs the refrigerant vaors. The absortion heat Q 4 is released during the absortion rocess. The saturated liquid mixture is transorted to the generator oerating on a higher-ressure level. The coefficient of erformance (COP) evaluates absortion cycle effectiveness. COP = Q 2 / Q 1. (5) The mixture of water and LiBr is considered as the most frequently used working medium in absortion systems. There is a hysical limitation for convenient working temeratures of an absortion cycles [1]. The COP is aroximately constant for all variations of convenient temeratures. The temerature of a hot water leaving the cogeneration unit is required 95 C for correct oeration of the absortion cycle driven by a hot water. The theoretical COP value is The cooling caacity is calculated as Q 2 = Q 1 COP. (6) Utilizing of a cool roducing absortion cycle driven by hot water from a cogeneration unit increases the heat requirement during summer eriod. The corresonding heat requirement distribution curve is shown in fig. 9. Fig. 9 The heat requirement distribution curve for tri-generation unit Fig. 8 The simle one-stage absortion cycle The driving heat Q 1 is sulied to the generator by hot water from a cogeneration unit. The refrigerant vaors release the mixture during increasing of the mixture temerature. The oor hot mixture returns to the absorber via a throttle valve. The released refrigerant vaors continue to the condenser. The vaors are cooled to lower temerature and condensate to the liquid form. The condensation heat Q 3 is released during this rocess. Then the liquid refrigerant asses through another throttle valve and enters the evaorator. 5 Results and discussion The Czech Reublic legislation encourages utilizing of all kinds of oly-generation systems. Local distributors of electricity must buy at a secial rice all electricity roduced by oly-generation systems. Year oeration hours of oly-generation systems are limited only by actual heat requirements. The cogeneration unit used in the studied building runs 6350 hour er year without utilizing of an absortion cycle. This eriod is extended to 7250 hour er year in a case of utilizing of the tri-generation technology driven by heat from a cogeneration unit. This extension enables to oerate the cogeneration unit effectively even during summer eriod, commonly without heat requirements.

6 Proceedings of the 5th IASM/WSAS Int. Conference on eat Transfer, Thermal ngineering and nvironment, Athens, Greece, August 25-27, Poly-generation of energy is connected with saving of a rimary energy sources. The cogeneration technology decreases consumtion of the rimary energy by 29 %. The tri-generation technology decreases consumtion of the rimary energy by 20 %. We calculated the ayback eriod as a basic economical criterion. Inut arameters corresond to situation in the Czech Reublic in the year The ayback of the cogeneration and the trigeneration technology was evaluated close to 4.5 years in comarison with the searate roduction of electricity, heat and cool. An investment of a tri-generation technology is more exensive in comarison with a cogeneration technology. But a tri-generation technology offers higher annual saving and higher rofit during the life-time eriod. In this contribution we assumed not utilizing of waste heat from the tri-generation technology. This low temerature heat can be utilized for reheating of water or heating of swimming ools. In these cases the saving of the rimary energy is more significant. A tri-generation technology connected with an aircondition unit is rofitable assembly convenient for midsize building in conditions of the Czech Reublic. Other advantages of this system are: uniform annual consumtion of electricity, air-conditioning without burden of an electricity distribution net, back u source of energy. 6 Conclusion This aer introduces connection of oly-generation systems and an air-condition unit for mid-size buildings. A cogeneration unit based on a stroke engine is considered as the most frequently used ly-generation system. The aer focused on connection of a cogeneration unit with an absortion cycle. This assembly forms a tri-generation unit simultaneously generating electricity, heat and cool. The articular mid-size building was used for detail analysis of the cogeneration and the tri-generation technology. Utilizing of a cogeneration technology causes 29 % decreasing of rimary energy sources consumtion. Tri-generation decreases consumtion of rimary energy sources by 20 %. A tri-generation system offers another advantage namely more uniform oeration of a oly-generation system during the entire year eriod. A cogeneration unit used in the studied building runs 6350 hour er year without utilizing of an absortion cycle. This eriod is extended to 7250 hour er year in a case of utilizing of a tri-generation technology driven by heat from the cogeneration unit. The ayback eriod of the studied oly-generation system was calculated 4.5 years. A trigeneration technology connected with an air-condition unit was evaluated as rofitable assembly convenient for mid-size building in conditions of the Czech Reublic. Acknowledgements: This work is art of research suorted by the Ministry of ducation of the Czech Reublic under the grant MSM References: [1] Ab-Sortion Machines for eating and cooling in Future nergy Systems, Final reort from Annex 24 of the IA eat Pum Programme, October [2] Petchers, Neil, Combined heating, cooling & ower handbook, Lilburn: The Fairmont Press, Inc [3] Grossman G.: Advanced modular simulation of oen absortion systems, Keynote Lecture, Proceedings, the 7th International Sortion eat Pum, Conference, Shanghai, China, 2002, [4] Posisil J., Skala Z., Additional Auxiliary Lines for -T-x Diagram of Ammonia-Water System, Conference roceeding of International Sortion eat Pum Conference 2005, Denver, USA, 2005 [5] Camanari S., Boncomagni L., Macchi., Microturbines and Trigeneration: Otimalization Strategies and Multile ngine Configuration ffects, ASM Transaction, Vol. 126, 2004 [6] minciuc., Corre O., Athanasovici V., Tazerout M., Fuel saving and CO 2 emissions for trigeneration systems, Alied Thermal ngineering 23, , 2003