Distributed polygeneration: desiccant-based air handling units interacting with microcogeneration systems
G. Angrisani*,1, C. Roselli1, M. Sasso1 1 DING, Università degli Studi del Sannio, Piazza Roma 21, 82100 Benevento, Italy
Distributed Polygeneration (DP) systems are small size energy conversion devices able to efficiently supply two or more energy outputs (electric, cooling and heating) to the end-user, rather than the simple single-output equipments. In many applications in tertiary and residential sectors, they allow to achieve energy, economic and environmental benefits. In a barely analyzed layout of a DP system, thermal energy from a microcogenerator is used to regenerate a desiccant wheel (DW) in a hybrid air handling unit (AHU), in which a chiller is used to meet the room sensible load. The main advantages of this unit, with respect to conventional systems, are: sensible and latent loads can be separately controlled; the cooling machine can operate with a higher chilled water temperature, achieving a higher COP and consistent energy savings; a better IAQ can be obtained, due to sanitizing effects of desiccants.
The aim of this chapter is to review the researches carried out to evaluate the performance of systems in which a microcogenerator (MCHP) interacts with a desiccant-based HVAC (Heating, Ventilation and Air Conditioning) unit. Furthermore, experimental and numerical analysis are described to evaluate the performance of the MCHP/HVAC-DW installed at Department of Engineering, University of Sannio (Italy), as a function of various operating conditions and parameters, in order to establish its effectiveness, compared to a conventional HVAC system.
Keywords: distributed polygeneration; desiccant-based air handling units; microcogeneration; experimental and numerical analysis
1. Introduction
The bulk of electric power used in the world is delivered by centralized power plants, most of them utilizing large, fossil-fuel combustion or nuclear power plant. Distributed Generation (DG) is the application of small scale generators, located in the customers’ premises and connected to the electric grid, or at an isolated site, to provide electrical power to final users. By avoiding or reducing transmission and distribution costs and related energy losses, DG can provide lower operating costs and energy/environmental benefits in many cases. Furthermore, small, modern generators can be more efficient and less costly to operate than large and old generators. Since the “size” effect does not always lead to energy savings and pollutant emissions reduction, there is the need to support the diffusion of on-site small complex energy conversion devices, Distributed Polygeneration (DP), which are able to efficiently supply two or more energy outputs (electric, cooling and heating) to the end-user, rather than the simple single-output equipments. Microcogenerators are the most common systems for DP. Cogeneration, or Combined Heat and Power (CHP), represents the combined “production” of electric (and/or mechanical) and thermal energy (heating), starting from a single primary energy source, [1]. It is a well-established technology, which has important benefits and has been noted by the European Community as one of the first strategy to save primary energy and reduce greenhouse gas emissions – with respect to the reference separate “production” by large thermal power stations – and avoid network losses. Furthermore, in many applications in tertiary (hotels, hospitals, commercial buildings) and residential sectors, distributed microtrigeneration systems (Micro Combined Cooling, Heating and Power, MCCHP) allow to achieve energy, economic and environmental benefits. The “heart” of these energy conversion systems is a prime mover (PM), based on different technologies (Stirling, Reciprocating Internal Combustion – RIC, Fuel Cell, Gas Turbine, and so on), especially designed to operate in stationary conditions for a long time with high efficiency and very low pollutant emissions. At the moment, the most mature technology available on the market, which is gas-fired RIC engines, achieves small installation space, high thermal efficiency, low noise, vibrations and maintenance requirement as well as long life service, [2]. In CCHP (Combined Cooling, Heating and Power) systems, the prime mover drives (mechanically, electrically or thermally) electric generators (G) and/or electric heat pumps, Thermally-activated Cooling Systems (TCS: absorption heat pumps, desiccant wheels and so on), allowing a wide range of operating conditions, to match thermal (heating and cooling) and electric end-users' requirements, and achieving significant primary energy and emissions savings, with respect to the separate "production" of the same amounts of energy. In particular, the use of thermal energy from cogeneration for space cooling purposes during summer is highly favorable (Fig.1), as it allows to increase the number of operating hours per year, reducing the economic payback time. MCCHP systems have another outstanding advantage as they are typically fuelled with natural gas. In fact, especially in Mediterranean areas, there is an increasing demand of summer cooling energy in tertiary and residential sectors, usually satisfied by electrically-driven units, that largely dominate the HVAC market; this involves peaks of electricity
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demand and black-outs. Gas Cooling Technologies can shift energy demand in summer from electricity to gas, at the same time allowing the utilization of the natural gas surplus during the warm season.
Fig. 1 A CCHP system. In a barely analyzed configuration of a MCCHP system, thermal energy recovered from a prime mover is used to regenerate a desiccant wheel (DW) contained in an hybrid air handling unit (MCCHP=MCHP/HVAC-DW system). The desiccant wheel is a rotor, filled with a desiccant material (i.e. silica gel), in which humid air is dehumidified by the desiccant material, to balance latent loads of the ambient. To guarantee continuous operation, the wheel has to be regenerated by a hot air stream. The waste heat of a small cogeneration plant can be effectively used to regenerate the desiccant material, while the cogenerated electricity can drive a chiller to meet the room sensible load. The main advantages of this technology, with respect to conventional systems based on cooling dehumidification, are:
• sensible and latent loads can be separately controlled; • very low dew point temperatures of process air, lower than -6.0 °C, can be achieved; • the cooling machine can operate with a higher chilled water temperature, with a higher COP; • due to the higher value of the COP, electric energy requirement of the cooling machine is reduced; • as the cooling machine has to handle only the sensible load of the process air, a reduction of its size and the
refrigerant fluid mass is obtained; this consequently determines a lower environmental impact, both in terms of direct impact (ozone layer reduction and greenhouse effect due to refrigerant fluids) and indirect one (the reduced electric energy use determines lower equivalent CO2 emissions of the power plants);
• consistent energy savings can be obtained, thanks to the increase in the overall energy efficiency; • a better IAQ can be obtained, due to sanitizing effects of desiccants. Indeed, desiccant systems avoid the
formation of condensed water; this strongly reduces the presence of microorganisms as bacteria, viruses and fungi.
Desiccant dehumidification is not a new technology (the first patent by the American engineer Pennington goes back to ‘30s), but the recent developments in desiccant materials and cycles make it a viable alternative or integration to conventional air conditioning systems. In fact, the traditional niche markets (special areas like electronics, food and arms storage, pharmaceutical industry, hospitals, etc.) are greatly expanding towards application characterized by high
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latent loads, such as supermarkets, ice arenas, restaurants, cinemas, school and museums. Moreover, during last years, thanks to its benefits, this technology is spreading in residential and tertiary sectors and office buildings too. The aim of this chapter is to review the investigations related to MCHP/HVAC-DW systems. Furthermore, experimental tests and numerical analysis are described to evaluate the performance of the MCHP/HVAC-DW installed at the Engineering Department, University of Sannio (Italy), [3, 4, 5]. The performance of the system, especially suitable for residential and small commercial users, has been evaluated as a function of various operating conditions (outdoor and supply air thermal-hygrometric conditions, partial load operation of the cogenerator), in order to establish its effectiveness, compared to a conventional HVAC system. Finally, the influence of the electric grid efficiency is considered to analyse the polygeneration system in different electricity mix scenarios.
2. Review of researches about desiccant-based air handling units interacting with microcogeneration systems
In the most common configuration of a MCCHP system, the MCHP provides thermal energy to an absorption heat pump, which provides cooling energy to the final user. On the other hand, few investigations were carried out to evaluate the performance of desiccant-based air handling units interacting with microcogeneration systems. In [6], the results of a simulation model, carried out to design an experimental hybrid HVAC system, were reported. The test facility is placed in the South of Italy, in a humid town. A microcogenerator supplies electric energy to an electric heat pump and other electric devices. Waste heat recovered from the MCHP is utilized in summer to regenerate the DW. Possible excess of thermal energy can be used to produce domestic hot water. During winter waste heat is directly used for heating purposes, using the fan-coils as well as the air handling unit. Regarding energy performance, results indicate an electricity saving >30% in comparison to state-of-the-art solutions based on conventional technology. In [7, 8] a hybrid HVAC system coupled with a MCHP was analyzed. The test facility is placed in Hamburg, Germany. The HVAC system consists of a small CHP plant, a desiccant assisted ventilation system and a system interacting with the ground, consisting of borehole heat exchangers, for sensible cooling instead of an electric driven compression chiller. The radiant floor system of the building is used for cooling. Thermal energy recovered from the MCHP, at a temperature between 55 °C and 65 °C, is used to heat the regeneration air, while electric energy supplied by the MCHP powers the electric devices of the office. This system is compared, by means of an energy analysis, with other systems, such as a hybrid HVAC system without the microcogenerator and a conventional HVAC system. It was found that considerable primary energy savings can be achieved (70%) using desiccant air conditioning with borehole heat exchangers. But even if an electric chiller is used, savings of 30% in primary energy can be accomplished. Starting costs for the demonstration plant were not higher than for a conventional system, but running costs could be reduced drastically. In [9], the performance of a desiccant cooling system, in which thermal energy for regeneration is provided by heat recovery from a gas-fired reciprocating internal combustion engine, was evaluated. The system offers sufficient sensible and latent cooling capacities for a wide range of climatic conditions. Energy efficiency and water consumption of the desiccant cooling system were also evaluated and compared with those of a conventional system. In particular, at 35 °C ambient temperature, the desiccant cooling system operates with an electrical COP of about 5.3, that is more efficient than typical conventional systems. As regards water consumption of the desiccant cooling system, related to direct and indirect evaporative coolers, it is compared with the water use associated with the centralized electricity generation in conventional fossil fuels-based power plants. Unlike the common perceptions, the water use of the desiccant-based AHU is not substantially different from that one in conventional power “production” systems. In [10], a natural gas fired cogeneration system is employed in the Crowtree leisure complex (Sunderland). Waste heat from the cogenerator is used to regenerate a desiccant wheel, to provide dehumidification for the indoor swimming pools. The total wetted area of the swimming pools and the floor area of the associated changing rooms is about 2,400 m2, requiring a ventilation volumetric airflow rate of 155,520 Nm3/h. To satisfy the thermal and electric load profiles, the installation of two small-scale gas fired reciprocating cogenerators is considered, with electric power output of 185 kW and 95 kW. A payback period of about 4 years has been predicted for the system. In [11], a system consisting of a hybrid photovoltaic-thermal generator, a cogeneration unit, a reversible heat pump and a desiccant wheel dehumidification plant, connected to the electric grid and the air conditioning plant of a building, is described. During all the year, the photovoltaic cells and the cogeneration plants feed the heat pump and the internal electrical loads. During summer months, the heat recovered from the hybrid façade and the cogenerator is used for dehumidifying the renewal air, while the heat pump is employed for cooling purposes. In winter, however, the heat generated from both the façade and cogenerator is used for heating purposes. In [12], several HVAC configurations for product drying based on desiccant wheels are investigated, in order to find systems which reach high primary energy savings through the appropriate integration of refrigerating machines, adsorption wheels and cogenerative engines. Simulations are carried out for different values of sensible to latent ambient load ratio and the effect of ambient and outside air conditions is evaluated for each configuration. In this way,
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the results can be adapted to different fields and they are not limited to a specific application. It is shown that primary energy savings can reach 70-80% compared to the reference technology based on cooling and heating coils. In some cases, mechanical power output of gas engines is not further converted in electricity by means of electric generators, but it is directly used to drive the compressor of vapour compression cooling equipments. In [13, 14], a hybrid air conditioning system, incorporating an engine-driven chiller and a desiccant dehumidification system, was experimentally tested to measure the performance of the engine-driven chiller and the dehumidification capacity and to provide reliable data for energy consumption and operation cost. A comparison between theoretically predicted and measured values of dehumidification capacity was presented. The waste heat recovered from engine-cooling system and exhaust gases was experimentally determined. Also the performance improvement of chiller, due to higher temperatures of chilled water (only sensible cooling is needed), was measured. Economic benefits of the hybrid air conditioning system over the conventional electric chiller were calculated for a reference building; the results revealed that, under present electricity/gas price ratio, about 30-40% savings on operation costs can be achieved by the hybrid system. In [15], a GEDH (Gas Engine Desiccant Hybrid) system is analyzed. It consists of a regenerative dehumidifier, heat exchanger, evaporative cooler, heating and cooling coils and fans and it is used in summer to balance latent and part of the sensible load of a building. The remaining sensible heat load is taken by a gas engine driven lead chiller, supplying waste heat to fire the desiccant unit, and by an electric motor driven peak chiller, that switches on for peak sensible cooling load only. In winter, the lead gas engine driven chiller supplies heat through the condenser as well as its waste heat to heating coils, while the desiccant unit recovers heat from the exhaust air. The GEDH cycle is compared with a traditional system using electrically driven chillers and a gas-fired boiler, considering climatic and operating data of 4 cities (Brisbane, Melbourne, Sydney and Memphis) for an office application. The calculated payback time ranges from 0.59 to 5.73 years, suggesting that desiccant hybrid air-conditioning systems can be competitive in many climates with conventional systems, if desiccant units are commercially available. Furthermore, other commercial buildings may have higher latent load ratios and peak-load equivalent operating hours, which make hybrid desiccant cooling more attractive.
3. The polygeneration system installed at the Engineering Department of University of Sannio: experimental and numerical analysis.
At the Engineering Department of University of Sannio, in Benevento (Southern Italy), a desiccant air handling unit coupled to a natural gas-fired reciprocating internal combustion engine cogenerator, an electric chiller and a natural gas-fired boiler, has been experimentally analyzed. The test facility can be used for the evaluation of the performances, in Mediterranean climate, of both the main components of the HVAC system and the complete plant. The hybrid HVAC system is based on the dehumidification of outdoor air by a desiccant wheel and its subsequent cooling by an electric chiller. In Fig. 2, the layout of the test facility is shown. Nominal characteristics of the devices are the following:
• cogenerator: electric power Pel = 6.00 kW (about 0.200-0.400 kW is used for self-consumption); thermal power Pth = 11.7 kW, gross electric efficiency ηel = 28.8%, thermal efficiency ηth = 56.2%. The MCHP supplies thermal power for the regeneration of the desiccant wheel by recovering heat from the exhaust gas and from the engine jacket;
• air-cooled water chiller: cooling capacity Pco = 8.50 kW, COP = 3.00; • boiler: thermal power Pth = 24.1 kW, efficiency ηb = 90.2%. The boiler can be used to supply thermal energy
when the hybrid HVAC system is powered by separate “production” systems, or to integrate the thermal power available from the MCHP. In fact, the maximum regeneration air temperature that can be reached with the thermal recovery of the MCHP (65 °C) could be insufficient to reach the desired supply air humidity ratio in hot and humid climates; hence thermal power is supplied also by the boiler, allowing to reach a higher regeneration temperature.
As regards the AHU, there are three outdoor air streams, with nominal volumetric flow rate of 800 m3/h: • process air, dehumidified by the desiccant wheel (1-2) to the supply humidity ratio, pre-cooled by the cooling
air stream in an air-to-air cross flow heat exchanger (2-3), finally cooled to the supply temperature by a cooling coil interacting with the chiller (3-4); it is used to maintain thermal and humidity comfort values in the conditioned space;
• regeneration air, heated by the heating coil interacting with the MCHP (1-5) and/or by the heating coil interacting with the boiler (5-6); it is used to regenerate the desiccant wheel (6-7);
• cooling air, cooled by a direct evaporative cooler (1-8) and then used to pre-cool the process air exiting the desiccant wheel (8-9).
The volumetric flow rates of the three air streams can be controlled by means of manual shutters or varying the velocity of the fans.
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The three air streams are entirely drawn from the outdoor (state 1, common to the three airflows), therefore no recirculation is carried out. Desiccant-based AHUs normally use two air flows (process and regeneration) and a recuperative heat exchanger between them to pre-cool the process air flow and pre-heat the regeneration one; to this aim, regeneration air, drawn from outside or indoor ambient, is usually cooled in a direct evaporative cooler, in order to reduce its temperature and enhance the heat exchange in the recuperator. But the evaporative cooling process increases regeneration air humidity, hence reducing its desorption capacity; this could determine an insufficient capability of the desiccant cooling system in balancing ventilation and internal latent loads, in particular in Mediterranean climates. Therefore, in this work a new layout of the desiccant-based AHU is investigated: it uses an air-to-air heat exchanger between the process air flow and a cooling air flow. In Fig. 3, the energy flows of the MCCHP ≡ MCHP/HVAC-DW system in summer operation are reported, [16]. Net electric power produced by the MCHP ( , ), taking into account cogenerator and AHU auxiliary consumption ( , ), can be split between the chiller ( , ) and the direct use ( , – lights, appliances, etc.): by means of re parameter (0÷1), which represents the electric energy share provided to the chiller, different operating modes can be considered. Thermal power recovered is used to regenerate the desiccant wheel. MCHP,pP is the primary power input to
the MCHP, and ∗ are its thermal and net electric efficiency, respectively. COP is the Coefficient Of Performance of the chiller.
Fig. 2 The layout of the test facility. The polygeneration system can also operate in winter mode: the cogenerator directly supplies electricity and thermal energy to the user, while the desiccant-based AHU is inactive, [17]. However, in this section, only the summer operating mode will be investigated. As the main benefit of a polygeneration system is the possibility to save primary energy with respect to the separate “production” of equal energy outputs, the natural approach is to quantify the primary energy saving by means of the Fuel Energy Saving Ratio (FESR), [18], defined as:
= (1)
where and are the primary energy input of a proposed alternative system (AS, experimentally measured) and a conventional one for comparison purposes (CS, numerically calculated), respectively. A similar index is also defined by European directive, [19, 20], for cogeneration systems. Furthermore, the environmental impact is a key factor in selecting the proper energy system. A simplified approach is based on the evaluation of equivalent carbon dioxide emissions of the analyzed energy systems. The comparison is then based on the equivalent CO2 avoided emissions, ∆CO2, [21], defined as:
∆ = (2)
where and are the equivalent carbon dioxide emissions of AS and CS, respectively.
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Energy and environmental comparisons are tipycally performed on an annual basis. However, the FESR and ΔCO2 indicators can be evenly used when a shorter time period is assumed. In the following, the evaluation of the performance is based on three hours long tests in stationary conditions, therefore they will be based on the average measured primary power and hourly equivalent CO2 emissions. Both FESR and ΔCO2 are strongly influenced by energy performance parameters and fuel used by both alternative and conventional systems. For instance, the power plant efficiency and the emission factor can be characterized with respect to a specific country mix (e.g. Italy), the Best Available Technology (Gas Turbine Combined Cycle) or through other different approaches, [22].
Fig. 3 Energy flows of the MCHP/HVAC-DW system.
3.1 Experimental analysis
In [3, 16, 23–27], experimental tests were carried out to analyze a MCHP/HVAC-DW system. A thermo-economic analysis was carried out, comparing the hybrid polygeneration system with a conventional HVAC system. Different polygeneration systems (Alternative System, AS) were considered, with different energy conversion devices. In Fig. 4, the energy performances in terms of overall efficiency (PER, Primary Energy Ratio) of AS I (Desiccant based AHU powered by the MCHP), AS II (Desiccant based AHU powered by the electric grid and a natural gas-fired boiler) and CS (reference Conventional System, based on “cooling dehumidification and separate “production”) are compared to establish if the matching of the hybrid HVAC system with a microcogeneration system is effective. The results of 39 tests are reported: AS I has always a higher PER than AS II (about 23% higher, on average), but in some cases the CS provides the best energy performance. Therefore, the energy convenience of the hybrid polygeneration system should be carefully investigated, as a function of boundary operating conditions (i.e. outdoor thermal-hygrometric conditions). In Fig. 5 the average results of the energy and environmental comparison between the alternative systems and the conventional one are reported. The energy efficiency of both electric grid ( , = 45.2%) and boiler ( , = 90%) has been evaluated, with respect to Italy, in accordance with the European Directive 2004/8/EC [19] and its associated Commission Decision [20].
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Equivalent CO2 emissions of electric grid and boiler were set to 0.531 kgCO2/kWhel ([21]) and 0.200 kgCO2/kWhp, respectively. AS I can assure energy savings of about 18% and emissions savings of about 25% if compared with AS II; contrarily, these savings reduce to about 7% and 20% if AS I is compared with CS. AS II is not energetically and environmentally convenient with respect to CS. These results allow to state that a desiccant-based AHU has to be matched with a MCHP in order to achieve benefits with respect to the conventional system, therefore in the following only the AS I (simply named Alternative System, AS) will be considered. Obviously, the energy and environmental performances of the MCHP/HVAC-DW system vary not only with the type of system, but also with other operating variables, such as, for example, outdoor air and supply air thermal-hygrometric conditions, MCHP partial load conditions and electric grid efficiency.
Fig. 4 Primary Energy Rate of Alternative and Conventional Systems for 39 experimental tests.
3.1.1 Effect of supply air thermal-hygrometric conditions
In order to highlight the influence of supply air thermal-hygrometric conditions, the authors experimentally evaluated the energy and environmental performances of a MCHP/HVAC-DW with respect to a reference conventional system,
Fig. 5 Average results of energy and environmental comparison between alternative and conventional systems.
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Fig. 6 ([23]). Each curve is composed of several experimental tests. However, to improve clearness of charts, only the trend curve are shown, but to highlight the dispersion of the experimental data, the R2 value (determination coefficient) for each curve was calculated. It is defined as:
= ∑ ,∑ , (3)
where , are the values estimated by the trend curves, , are the experimental values, is the average of experimental values and N denotes the number of measurements. R2 varies between 0 and 1 and expresses how well the trend line fits the data (0 means no fit, 1 means perfect fit). Both FESR and ΔCO2 increase when supply air humidity ratio decreases. In fact, the reduction in supply air humidity ratio determines a decrease in the supply air dew point temperature; this involves a decrease in the chilled water temperature, produced by the electric chiller to dehumidify the air in the conventional system; therefore its COP strongly reduces. As a consequence, energy consumptions and emissions increase with respect to the system based on adsorption dehumidification: the desiccant dehumidification technology is therefore particularly indicated when a very low supply humidity ratio is needed. The R2 values are satisfactorily high.
3.1.2 Effect of outdoor air thermal-hygrometric conditions
In order to highlight the influence of outdoor air thermal-hygrometric conditions, the authors, [3], experimentally evaluated the energy and environmental performances of a MCHP/HVAC-DW with respect to a reference conventional system. To investigate the effect of outdoor air thermal-hygrometric conditions on the performance of the chiller interacting with both the alternative and the conventional system, both full and part load operating conditions have been considered, in agreement with literature, [28].
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Fig. 6 Average results of energy and environmental comparison between alternative and conventional systems – outdoor temperature = 29.6 °C, outdoor humidity ratio = 11.3 g/kg, MCHP electric power = 5.15 kW (average values).
Fig. 7 Psychrometric chart showing the area with FESR <0 and that with FESR >0 (MCHP at nominal electric power).
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In Fig. 7 outdoor air thermal-hygrometric conditions that get a positive FESR are shown. The hybrid HVAC system interacting with the MCHP requires less primary energy than the conventional system for an outdoor humidity lower than about 11.5 g/kg and an outdoor temperature in the range 25-36 °C. For an outdoor humidity higher than 13.0 g/kg, the MCHP/HVAC-DW system is no more energetically suitable. The authors also showed that FESR increases when outdoor humidity decreases, reaching a maximum value for 8.00 g/kg (24 %, 31 % and 35 % for outdoor temperature equal to 25.0 °C, 29.0 °C and 33.5 °C, respectively). Therefore FESR increases with outdoor temperature as the COP of the chiller in the CS decreases more than the COP of the chiller interacting with the desiccant-based AHU. The equivalent CO2 avoided emissions show the same trend as FESR, achieving a maximum value of 43%.
3.1.3 Effect of MCHP partial load conditions
To point out the influence of the cogenerator partial load ratio, the authors carried out some tests to evaluate the influence of this operating variable on the global energy performance of the MCHP/HVAC-DW in comparison with the conventional system, [3]. The net electric power for computers, lights, etc.., was gradually increased up to 1.5 kW to allow the full load of the cogenerator (the electric power supplied by the MCHP, that has a nominal electric power of 6 kW, to the chiller and auxiliaries is about 4.5 kW). Figure 8 shows that FESR increases with the net electric power supplied to the final user, so it is convenient to operate the MCHP at full load for the maximum number of hours (FESR of about 24%): this, in fact, causes an increase in the electric efficiency of the microcogenerator. Also in this case, the reduction in equivalent CO2 emissions shows the same trend as FESR; it achieves the maximum value (35%) at full load operation of the MCHP.
3.1.4 Effect of electric grid efficiency
The authors evaluated the influence of electric grid efficiency on the performance of the MCHP/HVAC-DW system with respect to the reference one, at constant operating conditions, Fig. 9 ([3]). In the base case (electric grid efficiency = 45.2%), FESR is about 22%; then it increases with the reduction in electric grid efficiency. Even if the electric energy “production” is based on the Best Available Technology (Gas Turbine Combined Cycle power plants, , = 0.58), FESR remains positive (about 6%). These values are in good agreement with quite similar polygeneration systems and operating conditions analyzed in literature [7, 8]. The environmental analysis is not influenced by the value of , , as equivalent CO2 emissions of reference conventional system are calculated on the basis of electricity drawn from the grid, not on the basis of the related primary power.
3.1.5 Effect of cooling air flow and chiller COP
In [28], the effect of the cooling air flow on the system performance is investigated. To this aim, three systems are considered: System 1, in which cooling air fan is off; System 2, in which cooling air fan is on, but the humidifier pump is not active; System 3, in which both cooling air fan and humidifier are on. In all systems, the MCHP is electric-lead and provides electric energy to chiller and auxiliaries as well as thermal energy to regenerate the desiccant wheel.
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Fig. 8 FESR and ΔCO2 as a function of net electric power supplied to the final user – outdoor temperature = 30.5 °C, outdoor humidity ratio = 10.2 g/kg, supply air humidity ratio = 6.17 g/kg (average values).
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Passing from system 1 → 2 → 3, Pel,MCHP decreases due to the reduction of Pel,chil, as the activation of the cooling air flow and the humidifier pump reduces the cooling load on the chiller. The energy analysis shows that System 2 and System 3 obtain a FESR of about 9% and 11%, respectively, with respect to System 1.
The effect of the chiller COP on the overall system performances (with both the cooling air flow fan and the humidifier pump switched on) is analyzed considering two different chillers, the latter (case B) having a COP about 23% higher than the former (case A). In this analysis, the MCHP operates at full electric power in both cases, therefore the reduction of Pel,chil for the case B (due to the increase of COP) determines an increase of the electric power available for the final user (Pel,us equal to 0.27 kW and 0.76 kW, for case A and B respectively). Thus, the difference in Pel,us between the two cases (0.49 kW) has to be taken from the grid in the case A. The final result is that case B can achieve a FESR of about 6% and a ΔCO2 of about 12% with respect to case A.
4. Conclusions
The transition from conventional centralized energy systems, based on separate “production”, to decentralized ones is currently in progress. This is due to the market availability of a wide variety of small scale energy conversion systems, allowing for the satisfaction of different energy requirements (electricity, cooling and heating) with a great potential of primary energy saving, greenhouse gas emission and operating costs reduction. A particularly interesting technology is represented by desiccant-based dehumidification systems, eventually integrated with conventional cooling equipments. As regards the available energy source for regeneration, attention has been focused on the coupling of desiccant based AHU with microcogenerators. The system is able to guarantee significant benefits in terms of fuel energy saving and equivalent CO2 emissions reductions, with respect to conventional air conditioning systems, and it is especially indicated in hot and humid climates, such as in Mediterranean countries. The energy and environmental advantages depend on several operating conditions (partial load ratio of the cogenerator, outdoor and supply air thermal-hygrometric conditions and electric grid efficiency). The best energy and environmental results are obtained when the MCHP supplies its maximum electric and thermal power, and when a very low humidity ratio of the supply air is required. Finally, the positive effect of the cooling air flow and of an increase of the chiller COP has been highlighted. Energy savings often give rise to economic savings. To that way, before installing such a system, a careful economic analysis has to performed, in order to establish if a reasonable value of the pay-back period (usually between five and ten years for this type of investment) can be obtained. Presently, the first cost of both MCHP and desiccant wheel does not allow to obtain an acceptable pay-back period. Anyway, there are several subjects involved in the definition of the economic variables concerning this type of energy conversion system, including the institutional sector and the private one (gas utilities, manufacturers, etc.). For example, government grants along with attractive rates for electricity export to the grid may significantly encourage MCHP and DW market penetration [29]. To conclude, the key factors that can sustain the diffusion of microtrigeneration and desiccant-based air conditioning systems are: • primary energy savings, • reduction of greenhouse gas emissions, • shift from centralized to distributed energy “production” systems to avoid distribution losses, thus assuring high quality power supply and finally increasing the network availability.
0
10
20
30
40
0.36 0.40 0.44 0.48 0.52 0.56 0.60
FES
R [
%]
Electric grid efficiency [-]
BAT
Base case
Fig. 9 FESR as a function of electric grid efficiency with MCHP at nominal electric power – outdoor temperature = 35.2 °C, outdoor humidity ratio = 10.2 g/kg, supply air humidity ratio = 6.27 g/kg (average values).
Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)____________________________________________________________________________________________________
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Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)____________________________________________________________________________________________________
CO2 equivalent carbon dioxide emissions, kg COP Coefficient Of Performance, - E Energy, kJ FESR Fuel Energy Saving Ratio, - P Power, kW PER Primary Energy Ratio, - R2 Determination coefficient, dimensionless re electric energy share provided to the chiller, dimensionless Greek symbols ΔCO2 equivalent CO2 avoided emissions, - η efficiency or effectiveness, - Acronyms AHU Air Handling Unit AS Alternative System CCHP Combined Cool, Heat and Power CHP Combined Heat and Power CS Conventional System DG Distributed Generation DP Distributed Polygeneration DW Desiccant Wheel G Electric generator GEDH Gas Engine Desiccant Hybrid HVAC Heating, Ventilation and Air Conditioning IAQ Indoor Air Quality MCCHP Micro Combined Cooling, Heating and Power MCHP Micro Combined Heat and Power PM Prime Mover RIC Reciprocating Internal Combustion TCS Thermally-activated Cooling Systems Subscripts aux auxiliaries b boiler chil chiller co cooling el electric MCHP Micro Combined Heat and Power p primary ref reference system th thermal us user Superscripts AS Alternative System CS Conventional System * related to net electric efficiency
Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)____________________________________________________________________________________________________
