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A new heat pump desiccant dehumidifier for supermarket application
Energy and Buildings 39 (2007) 59–65 www.elsevier.com/locate/enbuild
A new heat pump desiccant dehumidifier for supermarket application Renato M. Lazzarin, Francesco Castellotti * Department of Management and Engineering, University of Padova, Stradella S. Nicola 3, 36100 Vicenza, Italy Received 9 February 2006; received in revised form 25 March 2006; accepted 3 May 2006
Abstract Recently a new equipment for dehumidification was put onto the market. It is a self-regenerating liquid desiccant cooling system able to dehumidify, heating or cooling the ambient air by an electric heat pump that is a part of the equipment. Its operation is here studied in a supermarket application where air temperature and relative humidity play a very important role and the air-conditioning becomes necessary not only to assure a suitable thermal comfort, but also to make the refrigerated display cabinets operate properly. In this paper possible energy savings, compared to a traditional mechanical dehumidification, are evaluated by means of a numerical model that simulates a typical Italian supermarket. # 2006 Elsevier B.V. All rights reserved. Keywords: Chemical dehumidification; Supermarkets; Energy savings; Modelling
1. Introduction The relative importance of the latent load is growing in airconditioning. The latent load is due to people or particular activities such as cooking or drying. This increasing importance can be attributed to a better care in insulating and reducing solar gains by special glazing and to the use of higher air changes to reach good comfort standards. In fact, the outside humidity is normally well higher in summer than inside set values in many climates so that the contribution to the latent load can be absolutely prevalent [1]. This latent load is usually satisfied by cooling the air below dew point. The low temperature requested reduces the possible COP of cooling equipment and often obliges to provide the inefficient process of post-heating. An interesting alternative is chemical dehumidification which is often produced in hygroscopic wheels via solid adsorption. Less widespread is the process of liquid absorption [2–5] that is utilised to our knowledge by only one manufacturer with the well known brand Kathabar. 2. The innovative machine Recently a new equipment for dehumidification was put onto the market [6]. It is a self-regenerating liquid desiccant cooling * Corresponding author. Tel.: +39 0444 998778; fax: +39 0444 998888. E-mail address: [email protected] (F. Castellotti). 0378-7788/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2006.05.001
system able to dehumidify, heating or cooling the ambient air by an electric heat pump that is a part of the equipment. It appears in the shape similar to a traditional air-handling unit (Fig. 1), but it puts together inside in a new way a chemical dehumidification system and an electric vapour compression heat pump: in such a way it can be defined as a hybrid machine. Its operation can be followed by the numbers reported in the figure. Outside air or re-circulated is sucked in 1, through 2 it is filtered, dehumidified and cooled, finally in 3 the dry and cool air can be introduced into the ambients. Outside air for regeneration is taken in 4, in 5 one can find the control system and the heat pump compressor. Through 6 regeneration air is heated by the heat pump condenser. This hot air contacts the desiccant in 7 in a structured cellulose packing and regenerates it. Finally, reactivation air is exhausted in 8 at a higher humidity. How the equipment works, it is better illustrated in Fig. 2, according to which the equipment can be subdivided in three parts: regeneration and dehumidification sections and the heat pump. Starting from the right side, the air to be treated flows thorough a honeycomb cellulose media where enters in contact with the concentrated solution (LiCl–H2O), previously cooled in the heat pump evaporator: the air flows out dehumidified and the solution gets diluted. The provided cooling does not only balance the exothermic effect of the absorption process that allows the dehumidification, but, if necessary, it can give an additional effect that produces cool dry air. Now thermal energy is requested to regenerate the desiccant solution: this energy is supplied to the solution in a primary heat
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Nomenclature AHU COP HVAC t
air-handling unit coefficient of performance heating ventilation air-conditioning temperature (8C)
Greeks letter w air relative humidity (%) Subscripts a store air o outside air
pump condenser and to the regeneration air in a secondary heat pump condenser (left side). Hot air enters in contact with hot diluted solution and removes water from it: so hot and wet air is exhausted and concentrated solution moves towards dehumidification section. Just in the middle of the scheme the heat pump scroll compressor is represented: the refrigerant can be R22, R407c or R417a. How the concentrated solution returns to the absorber and the diluted solution goes to the regenerator is not clear. This is operated in the information supplied by the manufacturer by a fluid equaliser, described as a patented device. Adding a third condenser at the outlet of the process air, the machine can provide a slight post-heating: such an operation can satisfy a request of dehumidification and heating, typical in the Italian fall season. In the provided data sheets (to = 30 8C, wo = 70%), COP varies from 3.2 to 4.2, depending on the capacity. The smallest model treats 250 m3/h, removing 3 l/h of water from the processed air
and cooling down it of 7 8C; the electric demand is about 1 kW. The highest capacity model (rooftop) can handle 4,760 m3/h, removing 42 l/h of water from the air and providing at the meantime a cooling of 6 8C; the electric demand is about 9 kW. 3. Store and HVAC systems modelling A model of a typical Italian supermarket is used to investigate on the energy requirements of a traditional airconditioning management, that is a control of sensible and latent loads by means of mechanical dehumidification followed by post-heating process. All the assumptions about the supermarket modelling are detailed in [7]; here they are briefly reported. We consider a conditioned space of 2,600 m2 and a volume of about 10,000 m3. As regards the outside conditions, we consider the weather data reported in the Test Reference Year [8] for the city of Rome, in the centre of Italy. The thermal gains of the supermarket are due to the occupants with a maximum presence of 0.1 people/m2, scaled in a typical daily profile. The contribution of the interior lights and other devices is assumed equal to 30 W/m2. The opening hours are from 8 to 20 with Sunday opening. The installed refrigerated case capacity is 100 kW of which 75 kW at normal temperature case ( 5 8C) and 25 kW at low temperature case ( 25 8C), with an equal distribution of vertical and horizontal cases. The coupling and the interactions between conditioned air and refrigerated display cabinets are discussed in detail in [9]. 3.1. Traditional air-handling unit The required fresh air is 9 l/s per person, hence a flow rate of 8,500 m3/h considering the maximum occupancy; the total air
Fig. 1. The desiccant heat pump system studied [6]. (1) Inlet of outside or re-circulated air flow; (2) filtration, dehumidification and cooling section; (3) outlet of dry and cool processed air flow; (4) inlet of outside air flow; (5) heat pump section; (6) heating section by condenser; (7) regeneration section; (8) outlet of hot and wet regeneration air flow.
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Fig. 2. Heat pump chemical dehumidifier schematic view [6].
Fig. 3. Schematic view of the air-handling unit.
supply rate is 42,500 m3/h (about 4.2 volumes/h), provided by an air-handling unit (AHU) working at constant rate with centrifugal fans absorbing about 32 kW (Fig. 3). The airhandling unit can vary the outdoor air flow rate (and the recirculated one) according to an enthalpic economiser operating mode [10]. Finally, a crossed flow air heat exchanger works with the outdoor and the exhausted air with an efficiency of 50% in the sensible thermal exchange. In the primary energy evaluations we consider a heat production and distribution with an efficiency of 80%; to produce chilled water we assume a COP of 4 if there is a simple demand of air cooling and a COP of 3 if there is a demand both of cooling and dehumidification. The overall efficiency in the electricity production to evaluate primary energy needs is set to 39%.
into account both the sensible and latent effect) at ta = 30 8C and wo = 70%. The electric needs result in about 18 kW. As depicted in Fig. 5, the two machines are placed upwards the traditional AHU and their operation covers only the latent load of the supermarket with a step-by-step function. Downwards the two machines, a mixing section with a by-pass
3.2. Innovative air-handling unit The innovative AHU differs from a traditional only for the utilization of two desiccant dehumidifiers. Each machine treats an outdoor air flow of 4,250 m3/h providing dehumidification and cooling as illustrated in Fig. 4. The rated COP is 4.2 (taking
Fig. 4. Air-conditions at the outlet section of the desiccant dehumidifier varying the inlet air-conditions [6].
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Fig. 5. Schematic view of the air-handling unit with two desiccant dehumidifiers upwards (innovative air-handling unit).
outdoor air provides the minimum fresh air flow rate. Of course, the sensible load is only partially satisfied by the two machines and mainly by the cold battery that is supplied by an electric chiller with an average COP of 4. In the present analysis the desiccant dehumidifiers are switched-on only in summer time, because of too low outdoor air temperature in winter time to regenerate the desiccant solution. The thermal behaviour of the building is modelled in dynamic state by means a of software [11] that runs with a 15 min time resolution; air-handling unit, cabinets and desiccant dehumidifier operations are simulated with three dedicated types, developed by the Authors. 4. The air-handling unit loads Considering the primary energy requirement of the AHU for the average day in July, in Fig. 6 the contributions of cooling, heating and fans are reported varying the percentage of remote
condensers and the cooling mode for the built-in condensers. We distinguish the results for the traditional and innovative AHU, where the contribution of the desiccant machines is pointed out. It is possible to appreciate how the post-heating requirement is made almost null and the cooling is strongly lowered, just because of the dehumidification produced by the desiccant machines that cover the latent load and permit an efficient cooling in the cold battery (COP = 4) that controls only the sensible load. Therefore, the primary energy consumption for the airconditioning can be lowered from 26% to 63%, depending on the cabinets’ condensers configuration. Of course, thanks to the efficient dehumidification of the desiccant machines, the highest savings are reached in the scenarios characterized by high dehumidification loads compared to cooling ones: in fact, in water-cooled scenarios (or in 100% remote air-cooled scenario) the cabinets condensation heat is not rejected into the store air and so their cooling gain increases, reducing the corresponding AHU sensible load but not the latent one.
Fig. 6. Primary energy consumption of the traditional or innovative air-handling unit for the average day in July, varying the percentage of remote condensers (and so the complementary of built-in) and the cooling mode for the condensers.
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Fig. 7. Primary energy consumption of the supermarket with the traditional or innovative air-handling unit for the average day in July, varying the percentage of remote condensers (and so the complementary of built-in) and the cooling mode for the condensers.
5. The overall energy consumption Here the analysis regards the overall primary energy consumption of the supermarket for the average day in July; the results are reported in Fig. 7. Regarding the cabinets contribution, as discussed in [7], its variation depends on their layout (built-in or remote condensers) and on the different condensation temperature (air- or water-cooled). The contributions of the AHU are those shown in Fig. 6, save for the water-cooled scenarios where the heat recovery of the cabinets makes almost null the heating net consumption of the supermarket. What can be observed is that the previous energy savings in the air-conditioning management determine substantial energy savings also in the global energy requirement of the supermarket: the primary energy can be lowered from 5% to 22%, varying the cabinets’ configuration.
6. The store air-conditions and the overall energy consumption Here the discussion regards the variations of the overall supermarket energy consumption due to a different set point for air temperature and relative humidity, again comparing the traditional and the innovative scenarios. We refer the discussion to the scenario with 75% of remote water-cooled condensers and 25% built-in air-cooled condensers. Of course, in the next graphs the cabinets contribution, as well as the lights and other devices, does not vary in the comparison. 6.1. July average day Both in the traditional and innovative scenario, the total energy consumption decreases slightly if the store air temperature increases (Fig. 8). In such a case the desiccant
Fig. 8. Supermarket primary energy consumption with the traditional (left graph) or innovative (right graph) air-handling unit, varying the store air temperature with constant air relative humidity for the average July day.
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Fig. 9. Supermarket primary energy consumption with the traditional (left graph) or innovative (right graph) air-handling unit, varying the store air relative humidity with constant air temperature for the average July day.
machines contribution increases with the lowering of the store air temperature that means a lowering in the air humidity ratio at constant air relative humidity, thus a higher dehumidification demand. On the contrary the air relative humidity sensitivity in summer operation is much higher with the traditional AHU (Fig. 9): a strong reduction occurs in the air-handling unit load with the increasing of the air relative humidity, here limited to a maximum acceptable for comfort of 60%. Such a tendency is partially counterbalanced by an increasing cabinets contribution; however, the first effect is more important so that the minimum of overall consumption moves towards humidity levels well higher than usual. The result is quite different with the innovative AHU scenario, where the total consumption is not dependent on air relative humidity. In fact, the contribution of air-conditioning downwards the desiccant machines (labelled as AHU in the graph) does not vary with the air relative humidity because such processes control only the sensible load, whereas the latent load is satisfied by the desiccant dehumidifiers with a contribution that gets higher with lower level of humidity maintained in the store. Such a slope is equal but with opposite sign to the cabinets slope, the effects counterbalance each other and the final result is a flat curve: maintaining a relative humidity of
40% rather than 60% has no consequence in supermarket global energy consumption. 6.2. Annual simulation Obviously the above discussion should be extended over a whole year. Air relative humidity is the most influent factor on the performances of the cabinets and on the loads of the airhandling unit. An annual simulation was carried out at a winter store air temperature of 19 8C and at summer store air temperature of 26 8C. The results are reported in Fig. 10 where the minimum in the overall energy requirements with the traditional AHU is at about 62% of relative humidity. With the desiccant machines operating only in summer time, the point of minimum moves to about 50% of relative humidity, but its value is slightly higher than the minimum reached with the traditional AHU. The desiccant machines operation determines substantial energy savings only when there is a consistent dehumidification requirement: in Italian climate such a condition is limited to a period not longer than 2 months a year. As already discussed in [7], though in winter time in supermarkets air-conditioning dehumidification request is not negligible because of high levels of indoor vapour production due to high levels of
Fig. 10. Annual supermarket primary energy consumption with the traditional (left graph) or innovative (right graph) air-handling unit, varying the store air relative humidity with constant air temperature (19 8C in winter and 26 8C in summer).
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Fig. 11. Annual supermarket primary energy consumption with the traditional or innovative air-handling unit, varying the store air relative humidity with constant air temperature (19 8C in winter and 26 8C in summer), for two different Italian climates.
occupancy, operation of the desiccant machines is not allowed when outdoor temperature is lower than 15 8C, that is the operation limit to regenerate the solution, provided by the manufacturer. A development of the considered equipment could overcome this constraint due to the solution regeneration. 7. The climate influence Of course the climate is the other important variable in the evaluations: all the previous considerations referred to the climate of Rome. A final diagram (Fig. 11) is proposed for a quite different climate: Milan in the north of Italy with a colder and wetter climate. One can note the shifting of the point of minimum towards 50% of air relative humidity, both for Rome and Milan. In the graph it is possible to appreciate that for the climate of Rome the innovative AHU determine a minimum slightly higher than the traditional plant, whereas for the climate of Milan the minimum of the two curves is the same. Such a conclusion confirms the best attitude of the desiccant dehumidifiers in a wetter climate. 8. Conclusions This paper studies the application of an innovative desiccant dehumidifier in the air-conditioning of an Italian supermarket placed in Rome. The modelling of the supermarket regards the building, the air-handling unit, as well as the refrigerated display cabinets operation, whose interactions with the store air have to be computed properly. Two desiccant machines are considered for satisfying the AUH dehumidification load, providing even a slight cooling at the meantime. The primary energy consumption for air-conditioning for the average day of July can be lowered from 26% to 63%, depending on the cabinets’ condensers configuration, thanks to the more efficient dehumidification produced by the desiccant machines with respect to a mechanical dehumidification followed by post-heating, typical of traditional AHU systems.
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Such savings in the air-conditioning management determine substantial energy savings also in the global energy requirement of the supermarket: the primary energy can be lowered from 5% to 22%, varying the cabinets’ configuration. The dependency of the supermarket total energy consumption on store air relative humidity is completely different for a traditional air treatment and that here considered. Whereas the traditional AHU requirement is strongly reduced increasing the relative humidity till an optimum value around 60%, no variation occurs with the innovative AHU: maintaining a relative humidity of 40% rather than 60% has no consequence in supermarket global energy consumption. As far as the annual performances are concerned, results do not differ very much since in this study the innovative equipment is used only in summer months. The here presented innovative machines find their best application with a dehumidification load resulting from a more humid climate or a higher density of cabinets remotely condensed that emphasize the latent load on the sensible for the AHU. In the modelled supermarket no substantial energy savings can be predicted in comparison to a traditional mechanical dehumidification, but the optimum air relative humidity set-point moves from 62 to 50%. An air relative humidity of 62% is close to the upper limit of hygro-thermal comfort range for the occupants. Moreover, reducing it to 50% can be a benefit in the food exposition in the cabinets to prevent frost formation on them.
References [1] R.M. Lazzarin, La deumidificazione chimica fra teoria e pratica (Chemical dehumidification between theory and practice), in: Proceedings of the 43th International Conference on AICARR, Ventilation e IAQ, Milan, 2002. [2] R.M. Lazzarin, A. Gasparella, G.A. Longo, Chemical dehumidification by liquid desiccants: theory and experiment, International Journal of Refrigeration 22 (1999) 334–347. [3] R.M. Lazzarin, G.A. Longo, A. Gasparella, Theoretical analysis of an open-cycle absorption heating and cooling system, International Journal of Refrigeration 19 (3) (1996) 160–167. [4] ASHRAE, Sorbents and desiccants, in: Fundamentals Handbook, ASHRAE, 1993 (Chapter 19). [5] ASHRAE, Desiccant dehumidification and pressure drying equipment, in: System and Equipment Handbook, ASHRAE, 1992 (Chapter 22). [6] DryKor Ltd., Beyond air conditioning—our dry air is cool, in: Illustrative Brochure, DryKor Ltd., 2001. [7] R.M. Lazzarin, F. Castellotti, Influence of refrigerated display cabinets on supermarket thermal loads, submitted to Building & Environment Journal. [8] CEE, Test Reference Year Weather Data Sets for Computer Simulations of Solar Energy Systems and Energy Consumption in Buildings, Commission of the European Communities, Directorate General XII for Science, Research and Development, 1985. [9] R.M. Lazzarin, F. Castellotti, Modelling of interactions between refrigerated display cabinets and store environment in supermarket, submitted to Building & Environment Journal. [10] P. Mazzei, F. Minichiello, D. Palma, Desiccant HVAC systems for commercial buildings, Applied Thermal Engineering 22 (2002) 545–560. [11] AA, VV, TRNSYS, A Transient Systems Simulation Program, Solar Energy Laboratory, University of Wisconsis, Madison, 2005.
A new heat pump desiccant dehumidifier for supermarket application Renato M. Lazzarin, Francesco Castellotti * Department of Management and Engineering, University of Padova, Stradella S. Nicola 3, 36100 Vicenza, Italy Received 9 February 2006; received in revised form 25 March 2006; accepted 3 May 2006
Abstract Recently a new equipment for dehumidification was put onto the market. It is a self-regenerating liquid desiccant cooling system able to dehumidify, heating or cooling the ambient air by an electric heat pump that is a part of the equipment. Its operation is here studied in a supermarket application where air temperature and relative humidity play a very important role and the air-conditioning becomes necessary not only to assure a suitable thermal comfort, but also to make the refrigerated display cabinets operate properly. In this paper possible energy savings, compared to a traditional mechanical dehumidification, are evaluated by means of a numerical model that simulates a typical Italian supermarket. # 2006 Elsevier B.V. All rights reserved. Keywords: Chemical dehumidification; Supermarkets; Energy savings; Modelling
1. Introduction The relative importance of the latent load is growing in airconditioning. The latent load is due to people or particular activities such as cooking or drying. This increasing importance can be attributed to a better care in insulating and reducing solar gains by special glazing and to the use of higher air changes to reach good comfort standards. In fact, the outside humidity is normally well higher in summer than inside set values in many climates so that the contribution to the latent load can be absolutely prevalent [1]. This latent load is usually satisfied by cooling the air below dew point. The low temperature requested reduces the possible COP of cooling equipment and often obliges to provide the inefficient process of post-heating. An interesting alternative is chemical dehumidification which is often produced in hygroscopic wheels via solid adsorption. Less widespread is the process of liquid absorption [2–5] that is utilised to our knowledge by only one manufacturer with the well known brand Kathabar. 2. The innovative machine Recently a new equipment for dehumidification was put onto the market [6]. It is a self-regenerating liquid desiccant cooling * Corresponding author. Tel.: +39 0444 998778; fax: +39 0444 998888. E-mail address: [email protected] (F. Castellotti). 0378-7788/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2006.05.001
system able to dehumidify, heating or cooling the ambient air by an electric heat pump that is a part of the equipment. It appears in the shape similar to a traditional air-handling unit (Fig. 1), but it puts together inside in a new way a chemical dehumidification system and an electric vapour compression heat pump: in such a way it can be defined as a hybrid machine. Its operation can be followed by the numbers reported in the figure. Outside air or re-circulated is sucked in 1, through 2 it is filtered, dehumidified and cooled, finally in 3 the dry and cool air can be introduced into the ambients. Outside air for regeneration is taken in 4, in 5 one can find the control system and the heat pump compressor. Through 6 regeneration air is heated by the heat pump condenser. This hot air contacts the desiccant in 7 in a structured cellulose packing and regenerates it. Finally, reactivation air is exhausted in 8 at a higher humidity. How the equipment works, it is better illustrated in Fig. 2, according to which the equipment can be subdivided in three parts: regeneration and dehumidification sections and the heat pump. Starting from the right side, the air to be treated flows thorough a honeycomb cellulose media where enters in contact with the concentrated solution (LiCl–H2O), previously cooled in the heat pump evaporator: the air flows out dehumidified and the solution gets diluted. The provided cooling does not only balance the exothermic effect of the absorption process that allows the dehumidification, but, if necessary, it can give an additional effect that produces cool dry air. Now thermal energy is requested to regenerate the desiccant solution: this energy is supplied to the solution in a primary heat
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Nomenclature AHU COP HVAC t
air-handling unit coefficient of performance heating ventilation air-conditioning temperature (8C)
Greeks letter w air relative humidity (%) Subscripts a store air o outside air
pump condenser and to the regeneration air in a secondary heat pump condenser (left side). Hot air enters in contact with hot diluted solution and removes water from it: so hot and wet air is exhausted and concentrated solution moves towards dehumidification section. Just in the middle of the scheme the heat pump scroll compressor is represented: the refrigerant can be R22, R407c or R417a. How the concentrated solution returns to the absorber and the diluted solution goes to the regenerator is not clear. This is operated in the information supplied by the manufacturer by a fluid equaliser, described as a patented device. Adding a third condenser at the outlet of the process air, the machine can provide a slight post-heating: such an operation can satisfy a request of dehumidification and heating, typical in the Italian fall season. In the provided data sheets (to = 30 8C, wo = 70%), COP varies from 3.2 to 4.2, depending on the capacity. The smallest model treats 250 m3/h, removing 3 l/h of water from the processed air
and cooling down it of 7 8C; the electric demand is about 1 kW. The highest capacity model (rooftop) can handle 4,760 m3/h, removing 42 l/h of water from the air and providing at the meantime a cooling of 6 8C; the electric demand is about 9 kW. 3. Store and HVAC systems modelling A model of a typical Italian supermarket is used to investigate on the energy requirements of a traditional airconditioning management, that is a control of sensible and latent loads by means of mechanical dehumidification followed by post-heating process. All the assumptions about the supermarket modelling are detailed in [7]; here they are briefly reported. We consider a conditioned space of 2,600 m2 and a volume of about 10,000 m3. As regards the outside conditions, we consider the weather data reported in the Test Reference Year [8] for the city of Rome, in the centre of Italy. The thermal gains of the supermarket are due to the occupants with a maximum presence of 0.1 people/m2, scaled in a typical daily profile. The contribution of the interior lights and other devices is assumed equal to 30 W/m2. The opening hours are from 8 to 20 with Sunday opening. The installed refrigerated case capacity is 100 kW of which 75 kW at normal temperature case ( 5 8C) and 25 kW at low temperature case ( 25 8C), with an equal distribution of vertical and horizontal cases. The coupling and the interactions between conditioned air and refrigerated display cabinets are discussed in detail in [9]. 3.1. Traditional air-handling unit The required fresh air is 9 l/s per person, hence a flow rate of 8,500 m3/h considering the maximum occupancy; the total air
Fig. 1. The desiccant heat pump system studied [6]. (1) Inlet of outside or re-circulated air flow; (2) filtration, dehumidification and cooling section; (3) outlet of dry and cool processed air flow; (4) inlet of outside air flow; (5) heat pump section; (6) heating section by condenser; (7) regeneration section; (8) outlet of hot and wet regeneration air flow.
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Fig. 2. Heat pump chemical dehumidifier schematic view [6].
Fig. 3. Schematic view of the air-handling unit.
supply rate is 42,500 m3/h (about 4.2 volumes/h), provided by an air-handling unit (AHU) working at constant rate with centrifugal fans absorbing about 32 kW (Fig. 3). The airhandling unit can vary the outdoor air flow rate (and the recirculated one) according to an enthalpic economiser operating mode [10]. Finally, a crossed flow air heat exchanger works with the outdoor and the exhausted air with an efficiency of 50% in the sensible thermal exchange. In the primary energy evaluations we consider a heat production and distribution with an efficiency of 80%; to produce chilled water we assume a COP of 4 if there is a simple demand of air cooling and a COP of 3 if there is a demand both of cooling and dehumidification. The overall efficiency in the electricity production to evaluate primary energy needs is set to 39%.
into account both the sensible and latent effect) at ta = 30 8C and wo = 70%. The electric needs result in about 18 kW. As depicted in Fig. 5, the two machines are placed upwards the traditional AHU and their operation covers only the latent load of the supermarket with a step-by-step function. Downwards the two machines, a mixing section with a by-pass
3.2. Innovative air-handling unit The innovative AHU differs from a traditional only for the utilization of two desiccant dehumidifiers. Each machine treats an outdoor air flow of 4,250 m3/h providing dehumidification and cooling as illustrated in Fig. 4. The rated COP is 4.2 (taking
Fig. 4. Air-conditions at the outlet section of the desiccant dehumidifier varying the inlet air-conditions [6].
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Fig. 5. Schematic view of the air-handling unit with two desiccant dehumidifiers upwards (innovative air-handling unit).
outdoor air provides the minimum fresh air flow rate. Of course, the sensible load is only partially satisfied by the two machines and mainly by the cold battery that is supplied by an electric chiller with an average COP of 4. In the present analysis the desiccant dehumidifiers are switched-on only in summer time, because of too low outdoor air temperature in winter time to regenerate the desiccant solution. The thermal behaviour of the building is modelled in dynamic state by means a of software [11] that runs with a 15 min time resolution; air-handling unit, cabinets and desiccant dehumidifier operations are simulated with three dedicated types, developed by the Authors. 4. The air-handling unit loads Considering the primary energy requirement of the AHU for the average day in July, in Fig. 6 the contributions of cooling, heating and fans are reported varying the percentage of remote
condensers and the cooling mode for the built-in condensers. We distinguish the results for the traditional and innovative AHU, where the contribution of the desiccant machines is pointed out. It is possible to appreciate how the post-heating requirement is made almost null and the cooling is strongly lowered, just because of the dehumidification produced by the desiccant machines that cover the latent load and permit an efficient cooling in the cold battery (COP = 4) that controls only the sensible load. Therefore, the primary energy consumption for the airconditioning can be lowered from 26% to 63%, depending on the cabinets’ condensers configuration. Of course, thanks to the efficient dehumidification of the desiccant machines, the highest savings are reached in the scenarios characterized by high dehumidification loads compared to cooling ones: in fact, in water-cooled scenarios (or in 100% remote air-cooled scenario) the cabinets condensation heat is not rejected into the store air and so their cooling gain increases, reducing the corresponding AHU sensible load but not the latent one.
Fig. 6. Primary energy consumption of the traditional or innovative air-handling unit for the average day in July, varying the percentage of remote condensers (and so the complementary of built-in) and the cooling mode for the condensers.
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Fig. 7. Primary energy consumption of the supermarket with the traditional or innovative air-handling unit for the average day in July, varying the percentage of remote condensers (and so the complementary of built-in) and the cooling mode for the condensers.
5. The overall energy consumption Here the analysis regards the overall primary energy consumption of the supermarket for the average day in July; the results are reported in Fig. 7. Regarding the cabinets contribution, as discussed in [7], its variation depends on their layout (built-in or remote condensers) and on the different condensation temperature (air- or water-cooled). The contributions of the AHU are those shown in Fig. 6, save for the water-cooled scenarios where the heat recovery of the cabinets makes almost null the heating net consumption of the supermarket. What can be observed is that the previous energy savings in the air-conditioning management determine substantial energy savings also in the global energy requirement of the supermarket: the primary energy can be lowered from 5% to 22%, varying the cabinets’ configuration.
6. The store air-conditions and the overall energy consumption Here the discussion regards the variations of the overall supermarket energy consumption due to a different set point for air temperature and relative humidity, again comparing the traditional and the innovative scenarios. We refer the discussion to the scenario with 75% of remote water-cooled condensers and 25% built-in air-cooled condensers. Of course, in the next graphs the cabinets contribution, as well as the lights and other devices, does not vary in the comparison. 6.1. July average day Both in the traditional and innovative scenario, the total energy consumption decreases slightly if the store air temperature increases (Fig. 8). In such a case the desiccant
Fig. 8. Supermarket primary energy consumption with the traditional (left graph) or innovative (right graph) air-handling unit, varying the store air temperature with constant air relative humidity for the average July day.
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Fig. 9. Supermarket primary energy consumption with the traditional (left graph) or innovative (right graph) air-handling unit, varying the store air relative humidity with constant air temperature for the average July day.
machines contribution increases with the lowering of the store air temperature that means a lowering in the air humidity ratio at constant air relative humidity, thus a higher dehumidification demand. On the contrary the air relative humidity sensitivity in summer operation is much higher with the traditional AHU (Fig. 9): a strong reduction occurs in the air-handling unit load with the increasing of the air relative humidity, here limited to a maximum acceptable for comfort of 60%. Such a tendency is partially counterbalanced by an increasing cabinets contribution; however, the first effect is more important so that the minimum of overall consumption moves towards humidity levels well higher than usual. The result is quite different with the innovative AHU scenario, where the total consumption is not dependent on air relative humidity. In fact, the contribution of air-conditioning downwards the desiccant machines (labelled as AHU in the graph) does not vary with the air relative humidity because such processes control only the sensible load, whereas the latent load is satisfied by the desiccant dehumidifiers with a contribution that gets higher with lower level of humidity maintained in the store. Such a slope is equal but with opposite sign to the cabinets slope, the effects counterbalance each other and the final result is a flat curve: maintaining a relative humidity of
40% rather than 60% has no consequence in supermarket global energy consumption. 6.2. Annual simulation Obviously the above discussion should be extended over a whole year. Air relative humidity is the most influent factor on the performances of the cabinets and on the loads of the airhandling unit. An annual simulation was carried out at a winter store air temperature of 19 8C and at summer store air temperature of 26 8C. The results are reported in Fig. 10 where the minimum in the overall energy requirements with the traditional AHU is at about 62% of relative humidity. With the desiccant machines operating only in summer time, the point of minimum moves to about 50% of relative humidity, but its value is slightly higher than the minimum reached with the traditional AHU. The desiccant machines operation determines substantial energy savings only when there is a consistent dehumidification requirement: in Italian climate such a condition is limited to a period not longer than 2 months a year. As already discussed in [7], though in winter time in supermarkets air-conditioning dehumidification request is not negligible because of high levels of indoor vapour production due to high levels of
Fig. 10. Annual supermarket primary energy consumption with the traditional (left graph) or innovative (right graph) air-handling unit, varying the store air relative humidity with constant air temperature (19 8C in winter and 26 8C in summer).
R.M. Lazzarin, F. Castellotti / Energy and Buildings 39 (2007) 59–65
Fig. 11. Annual supermarket primary energy consumption with the traditional or innovative air-handling unit, varying the store air relative humidity with constant air temperature (19 8C in winter and 26 8C in summer), for two different Italian climates.
occupancy, operation of the desiccant machines is not allowed when outdoor temperature is lower than 15 8C, that is the operation limit to regenerate the solution, provided by the manufacturer. A development of the considered equipment could overcome this constraint due to the solution regeneration. 7. The climate influence Of course the climate is the other important variable in the evaluations: all the previous considerations referred to the climate of Rome. A final diagram (Fig. 11) is proposed for a quite different climate: Milan in the north of Italy with a colder and wetter climate. One can note the shifting of the point of minimum towards 50% of air relative humidity, both for Rome and Milan. In the graph it is possible to appreciate that for the climate of Rome the innovative AHU determine a minimum slightly higher than the traditional plant, whereas for the climate of Milan the minimum of the two curves is the same. Such a conclusion confirms the best attitude of the desiccant dehumidifiers in a wetter climate. 8. Conclusions This paper studies the application of an innovative desiccant dehumidifier in the air-conditioning of an Italian supermarket placed in Rome. The modelling of the supermarket regards the building, the air-handling unit, as well as the refrigerated display cabinets operation, whose interactions with the store air have to be computed properly. Two desiccant machines are considered for satisfying the AUH dehumidification load, providing even a slight cooling at the meantime. The primary energy consumption for air-conditioning for the average day of July can be lowered from 26% to 63%, depending on the cabinets’ condensers configuration, thanks to the more efficient dehumidification produced by the desiccant machines with respect to a mechanical dehumidification followed by post-heating, typical of traditional AHU systems.
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Such savings in the air-conditioning management determine substantial energy savings also in the global energy requirement of the supermarket: the primary energy can be lowered from 5% to 22%, varying the cabinets’ configuration. The dependency of the supermarket total energy consumption on store air relative humidity is completely different for a traditional air treatment and that here considered. Whereas the traditional AHU requirement is strongly reduced increasing the relative humidity till an optimum value around 60%, no variation occurs with the innovative AHU: maintaining a relative humidity of 40% rather than 60% has no consequence in supermarket global energy consumption. As far as the annual performances are concerned, results do not differ very much since in this study the innovative equipment is used only in summer months. The here presented innovative machines find their best application with a dehumidification load resulting from a more humid climate or a higher density of cabinets remotely condensed that emphasize the latent load on the sensible for the AHU. In the modelled supermarket no substantial energy savings can be predicted in comparison to a traditional mechanical dehumidification, but the optimum air relative humidity set-point moves from 62 to 50%. An air relative humidity of 62% is close to the upper limit of hygro-thermal comfort range for the occupants. Moreover, reducing it to 50% can be a benefit in the food exposition in the cabinets to prevent frost formation on them.
References [1] R.M. Lazzarin, La deumidificazione chimica fra teoria e pratica (Chemical dehumidification between theory and practice), in: Proceedings of the 43th International Conference on AICARR, Ventilation e IAQ, Milan, 2002. [2] R.M. Lazzarin, A. Gasparella, G.A. Longo, Chemical dehumidification by liquid desiccants: theory and experiment, International Journal of Refrigeration 22 (1999) 334–347. [3] R.M. Lazzarin, G.A. Longo, A. Gasparella, Theoretical analysis of an open-cycle absorption heating and cooling system, International Journal of Refrigeration 19 (3) (1996) 160–167. [4] ASHRAE, Sorbents and desiccants, in: Fundamentals Handbook, ASHRAE, 1993 (Chapter 19). [5] ASHRAE, Desiccant dehumidification and pressure drying equipment, in: System and Equipment Handbook, ASHRAE, 1992 (Chapter 22). [6] DryKor Ltd., Beyond air conditioning—our dry air is cool, in: Illustrative Brochure, DryKor Ltd., 2001. [7] R.M. Lazzarin, F. Castellotti, Influence of refrigerated display cabinets on supermarket thermal loads, submitted to Building & Environment Journal. [8] CEE, Test Reference Year Weather Data Sets for Computer Simulations of Solar Energy Systems and Energy Consumption in Buildings, Commission of the European Communities, Directorate General XII for Science, Research and Development, 1985. [9] R.M. Lazzarin, F. Castellotti, Modelling of interactions between refrigerated display cabinets and store environment in supermarket, submitted to Building & Environment Journal. [10] P. Mazzei, F. Minichiello, D. Palma, Desiccant HVAC systems for commercial buildings, Applied Thermal Engineering 22 (2002) 545–560. [11] AA, VV, TRNSYS, A Transient Systems Simulation Program, Solar Energy Laboratory, University of Wisconsis, Madison, 2005.