High Latent Load Research Articles

Annual Performance of Sensible and Total Heat Recovery in Ventilation Systems: Humidity Control Constraints for European Climates

Ensuring a comfortable indoor air quality requires a minimum fresh air supply by ventilation. Moreover, the improvement of the air tightness in new and refurbished high performance buildings enhances the role of mechanical ventilation and its importance in further increasing the energy efficiency. Indeed, a reduction of the ventilation load can be achieved by installing air-to-air heat recovery devices, whose potential energy savings can be easily assessed by means of their nominal effectiveness. However, this estimation does not consider the impact on the overall performance of the system, in particular when humidity control is needed. Proper control strategies can be defined on the basis of the indoor latent load to prevent preheating or avoid moisture recovery when dehumidification by cooling is then required. In this work, the energy saving potential of heat recovery systems has been analyzed, considering the impact of different control strategies on both energy and cost savings. The calculations have been generalized using the specific latent load, which allows for the analysis of strategies and savings based on typical utilization categories, without considering in detail all the building characteristics. Representative hourly weather data for 66 European cities have been used to evaluate sensible and total heat recovery devices. The energy and the cost saving results have been mapped per each European Köppen-Geiger climate class and each country, respectively. The proposed strategies based on humidity control can strongly reduce the attractiveness of total heat recovery with respect to sensible heat recovery in terms of energy and cost savings, especially when high specific latent loads are considered.

Low cost humidity controlled air-conditioning system for building energy savings in tropical climate

The viable solution to the high latent load that naturally occurs in tropical regions requires an alternative system that runs at relatively low energy consumption yet be able to provide indoor thermal comfort by effective handling of the excessive humidity. Although the existing outdoor air treatment system is a proven approach, it is unpopular in developing countries due to its high initial cost. In this paper, a new system termed Dual Air Handling Unit system is proposed to be the answer. The function of Humidity-control Air Handling Unit is to remove the moisture from the conditioned room up to the desired humidity level and in the process the room temperature is also fractionally reduced. The Temperature-control Air Handling Unit completes the task by removing the remaining sensible heat so that the room temperature is maintained at the required set-point. By reducing the relative humidity to 50%, a much lower value than that of the normal air-conditioning could offer, room temperature of the new system is shifted higher to 26°C in order to reduce the energy consumption. The simulation result shows that the proposed system offers energy savings up to 13.2% compared to the conventional air-conditioning system, without compromising the thermal comfort of the occupants.

Evaluation of thermal comfort in an historical Italian opera theatre by the calculation of the neutral comfort temperature

Theatres are a very complex environment where thermal comfort is difficult to control. They frequently operate at high occupancy level and tend to have high sensible and latent heat loads. An application of the neutral comfort temperature methodology is proposed to analyze this particular kind of building. Thermal-hygrometric parameters defined in ISO 7730, Ashrae Standard 55/2004 and ISO 10551, were measured at carefully selected locations in the occupied zone within the auditorium in the Fraschini theatre, in order to ensure a good representation of human exposure to thermal comfort. In addition, a subjective evaluation was achieved by the distribution of a dedicated multiple response questionnaire. More than 400 questionnaires were analyzed, comprehensive of information for the application of the traditional static and adaptive model. When heating system is on (1st and 2nd survey), thermal stratification is really relevant in the theatre (air temperature runs from 23 °C in the stalls area to 30 °C in the upper order), while when the theatre is in naturally ventilated, the thermal stratification is not relevant. An application of a new thermal comfort evaluation method is here proposed by means of the calculation of the neutral comfort temperature with both the analytical and adaptive methods. . The methodology was also applied to data acquired in 9 different open space offices, where a total amount of 846 people were interviewed. Results showed that the method is applicable to both theatres and open plan offices and could provide a simple tool to quickly evaluate thermal comfort conditions.

Performance analysis of a hybrid air-conditioning system dehumidified by liquid desiccant with low temperature and low concentration

Abstract For temperature and humidity independent control (THIC) air-conditioning systems that integrated liquid desiccant dehumidification technology, it is very good idea to use condensation heat of the vapor compression system for the liquid desiccant regeneration. However, the system is unable to work in situations of humid climate or high latent load, since the condensation heat may not satisfy the regeneration of the liquid desiccant. This paper proposed a novel THIC air-conditioning system based on liquid desiccant, in which liquid desiccant with low-concentration and low-temperature was used in air dehumidification, and the condensation heat could meet the need of the regeneration of desiccant solution. A validated mathematical model was used to investigate the performance of the novel system under different operation conditions. The results showed that under typical indoor condition in Nanjing summer, mass concentration range of 25.4–26.9% was suggested in the dehumidification process using LiCl–H 2 O, and the dehumidification temperature range of the desiccant was around 17–19 °C. The effects of the ratios of indoor sensible load to latent load, and the outdoor air temperature and humidity on the system performance were discussed. Performance comparisons showed that COP sys of the system was 13.6–116.1% more than that of the conventional system under different operation conditions.

Theoretical and Experimental Analysis of Desiccant Wheel Performance for Solar Hybrid Desiccant Air-Conditioning System

Dehumidification is considered as a key feature of HVAC systems for thermal comfort. Malaysia is located in the South East Asia region which has a sufficiently high temperature and humidity of approximately 80% RH. This requires a process which can reduce moisture in the air. Desiccant wheel is used to provide dehumidification. Desiccant wheel with model no. 770 WSG produced by NOVELAIRE having operating system 1:1 and maximum flow rate of 1500 cfm has been theoretically and experimentally analyzed for its performance. The results show good matching between theoretical and experimental results in term of sensible and latent effectiveness of dehumidification process as well as regeneration process. This desiccant wheel is capable of condensing water at 0.05kgH2O/s and providing 3.3639 kW load reduction. With this significant load reduction, the component integrated in solar hybrid desiccant air-conditioning system can perform comfort cooling to area with high latent load such as hall, hyper market and commercial building.

Programmed Death-Ligand 1 (PD-L1) expressed on Herpes Simplex Virus Type 1 (HSV-1) infected neurons limits survival of CD8 T cells specific for subdominant epitopes. (105.45)

Abstract CD8 T cells that reside in the trigeminal ganglia (TG) of HSV-1 latently infected mice maintain viral latency through persistent interaction with infected neurons. Interestingly, CD8 T cells in TG of C57BL/6 mice exhibit a 1:1 ratio of cells recognizing subdominant versus an immunodominant (gB498-505) HSV-1 epitope. We investigated the role of Programmed Death-1 (PD-1) and its ligand, PD-L1 (B7-H1) in determining this ratio. We found that subdominant CD8 T cells in the TG 1) preferentially express PD-1, 2) are functionally compromised, and 3) proliferate and undergo apoptosis at a higher rate than gB498-505-specific CD8 T cells. Moreover, PD-L1 is preferentially expressed on neurons with a high latent viral load, and its expression is regulated by interferon-γ. In B7-H1-/- mice and bone marrow chimeras lacking PD-L1 only on parenchymal cells, the ratio of subdominant cells to gB498-505-specific cells was dramatically increased (4:1). This was due to increased survival of PD-1+ subdominant CD8 T cells that remained non-functional, and did not augment CD8 T cell protection from HSV-1 reactivation in ex vivo TG cultures. We conclude that PD-1/PD-L1 interaction regulates the survival, but not the function of CD8 T cells specific for subdominant HSV-1 epitopes in latently infected TG. Blocking this interaction alone is not sufficient to reduce HSV-1 reactivation from latency, but might be effective if combined with blockade of other suppressor molecules.

Investigation on air-conditioning load profile and energy consumption of desiccant cooling system for commercial buildings in Hong Kong

In Hong Kong, conventional air-conditioning (AC) systems cause a high energy consumption and poor indoor thermal comfort. This paper investigated the AC load profiles of three typical local commercial buildings, i.e. office, hotel and retail, and examined the energy consumption of liquid desiccant cooling system (LDCS) for these buildings. Simulation results showed that the cooling load contributed to more than 98% of total annual load, with high latent load proportion from March to September. The high reheating ratio, i.e. from 60% to 90%, is found to be the main reason for the poor thermal comfort. Therefore, the LDCS which could handle sensible and latent loads separately was proposed. However, the electricity-driven LDCS was found not suitable for local commercial buildings. significant energy saving could be achieved by the solar-assisted LDCS system with thermal storage, when the installation area for solar collectors is larger than the minimum required one, i.e. 9.7m2/(kWm−2) for office and 12.7m2/(kWm−2) for hotel buildings. Energy saving of different system types with different installation areas was also studied. The solar-assisted LDCS shows more energy saving in winter but less in summer. Results and conclusions can be useful for researchers and local engineers.

Investigation on solar hybrid desiccant cooling system for commercial premises with high latent cooling load in subtropical Hong Kong

This study found that the solar hybrid desiccant cooling system (SHDCS) was more effective to handle the premises with high latent cooling load in the hot and humid climate compared to the conventional airconditioning system. The SHDCS was designed to use the solar-thermal desiccant cooling to tackle the latent load; and the electrical vapour compression refrigeration to cater the sensible load. In this study, the typical commercial premises with high latent load were the Chinese restaurant and the wet market. The effectiveness of SHDCS included both better indoor cooling performance and higher year-round energy-saving potential. The annual primary energy consumption of SHDCS could be lower than that of the conventional system by 49.5% in the Chinese restaurant and 13.3% in the wet market. For the premises with more stringent temperature and humidity requirements, like the Chinese restaurant, the contribution of SHDCS in energy saving would be significant. Utilization of solar energy could ensure the energy-saving potential of SHDCS for the premises with high latent load. As a whole, the study assures the wider application of solar air-conditioning in the subtropical Hong Kong.

Condensation Risk in a Room with a High Latent Load and Chilled Ceiling Panels and with Air Supplied from a Liquid Desiccant System

Radiant cooling has shown great advantages when it comes to improving thermal comfort and can also reduce energy demand. But the system may have a high condensation risk if used with a high latent load. This study proposes a new type of ventilation system that uses a radiant cooling panel and air supplied from a liquid desiccant dehumidification system, which provides very dry supply air and chilled water for radiant cooling. This study uses an office with a high latent load to demonstrate the application of such a system in a hot and humid climate. By using a validated computational fluid dynamics (CFD) program to calculate the distributions of air temperature, humidity, and velocity, as well as the predicted mean vote in the office, the results show that the new system could reduce the risk of condensation on the cooling panels and offer a comfortable indoor environment. The new system has great potential when it comes to use in hot and humid climate regions.

Energy savings potential of a hybrid desiccant dehumidification air conditioning system in Beirut

In this work, the transient performance of a hybrid desiccant vapor compression air conditioning system is numerically simulated for the ambient conditions of Beirut. The main feature of this hybrid system is that the regenerative heat needed by the desiccant wheel is partly supplied by the condenser dissipated heat while the rest is supplied by an auxiliary gas heater. The hybrid air conditioning system of the present study replaces a 23 kW vapor compression unit for a typical office in Beirut characterized by a high latent load. The vapor compression subsystem size in the hybrid air conditioning system is reduced to 15 kW at the peak load when the regeneration temperature was fixed at 75 °C. Also the sensible heat ratio of the combined hybrid system increased from 0.47 to 0.73. Based on hour by hour simulation studies for a wide range of recorded ambient conditions of Beirut city, this paper predicts the annual energy consumption of the hybrid system in comparison with the conventional vapor compression system for the entire cooling season. The annual running costs savings for the hybrid system is 418.39 USD for a gas cost price of 0.141 USD/kg. The pay back period of the hybrid system is less than five years when the initial cost of the hybrid air conditioning system priced an additional 1712.00 USD. Hence, for a 20-year life cycle, the life cycle savings of the hybrid air conditioning system are 4295.19 USD.

Performance analysis of liquid desiccant dehumidification systems

Desiccant systems find applications in a very large variety of industrial and daily usage products including the new HVAC installations. An overview of liquid desiccant technology has been presented in this paper along with a compilation of experimental performance data of liquid desiccant dehumidifiers, empirical dehumidification effectiveness and mass transfer correlations in a useful and easy to read tabular format. The latest trends in this area suggest that hybrid systems are of current interest to HVAC industry, not only for high latent load applications but also for improving indoor air quality. The paper presents a comprehensive comparative parametric analysis of packed bed dehumidifiers for three commonly used desiccant materials viz. triethylene glycol, lithium chloride and calcium chloride, using empirical correlations for dehumidification effectiveness from the literature. The analysis reveals significant variations and anomalies in trends between the predictions by various correlations for the same operating conditions, and highlights the need for benchmarking the performance of desiccant dehumidifiers.

Use of desiccant dehumidification to improve energy utilization in air-conditioning systems in Beirut

Humidity and indoor moist surrounding affect air cleanliness and protects harmful microorganisms when relative humidity is above 70%. In humid climates, the humidity issues are a major contributor to energy inefficiency in HVAC devices. The use of liquid desiccant dehumidification systems of supply air is a viable alternative to reduce the latent heat load on the HVAC system and improve efficiency. Thermal energy, at a temperature as low as 40–50°C, required for the operation of a liquid desiccant hybrid air conditioner can be efficiently obtained using a flat-plate solar collector. In this work a model of a solar-operated liquid desiccant system (using calcium Chloride) for air dehumidification is developed. The system utilizes packed beds of counter flow between an air stream and a solution of liquid desiccant for air dehumidification and solution regeneration. The desiccant system model is integrated with a solar heat source for performance evaluation at a wide range of recorded ambient conditions for Beirut city. Standard mass and energy balances are performed on the various components of the system and a computer simulation program is developed for the integrated system analysis. The desiccant system of the current study replaces a 3 TR (10.56 kW) vapour compression unit for a typical house as low latent load application, and is part of a hybrid desiccant–vapour compression system for a high latent load application, namely a small restaurant with an estimated cooling load of 11.39 TR (40 kW), including reheat. The relevant parameters of the desiccant system are optimized at peak load, and it is found out that there is an important energy saving if the ratio of the air flow rate in the regenerator to that in the dehumidifier is about 0.3 to 0.4. The COP of the desiccant unit is 0.41 for the house, and 0.45 for the restaurant. The size of the vapor compression unit of the restaurant is reduced to 8 TR when supplemented by a desiccant system. The performance is studied of the desiccant system integrated with a solar collector system and an auxiliary natural gas heater to heat the regenerator. The transient simulation of the solar desiccant system is performed for the entire cooling season. The solar fraction for the house is equal to 0.25, 0.47, and 0.68 for a collector area of 28.72, 57.44, and 86.16 m2, respectively. The solar fraction for the restaurant is 0.19, 0.38, and 0.54, for the same collector areas. The life cycle savings for the house run solely on desiccant system were positive only if natural gas is available at a cheap price. For the restaurant, the economic benefit of the desiccant system is positive, because the need for reheat in the vapor compression system is eliminated. For a gas price of 0.5638 $/kg, the payback period for the restaurant turned out to be immediate if the energy is supplied solely by natural gas, and 11 years if an 86.16 m2 solar collector is implemented to reduce the fuel consumption. Copyright © 2003 John Wiley & Sons, Ltd.

Studies on solid desiccant based hybrid air-conditioning systems

Desiccant based air-conditioning systems offer a promising alternative to conventional air-conditioning systems using vapour compression refrigeration especially under conditions involving high latent loads. The desiccant can be used either in a stand-alone system or coupled judiciously with a vapour compression system to achieve high performance over a wide range of operating conditions. In this paper, the results of a detailed study of solid desiccant-based hybrid air-conditioning systems are presented. The literature review revealed that various authors differ in their evaluation of the efficacy of these systems. This seems to be due to different methods of modelling of dehumidifier and differences in the operating conditions of the cycles employed. Accordingly, the performance of four hybrid cycles (which include a new proposed cycle) for typical hot-dry and hot-humid weather conditions has been evaluated using a detailed procedure for the analysis of rotary dehumidifier, the most commonly employed industrial dehumidifier, based on the analogy method of Maclaine-Cross and Banks [I.L. Maclaine-Cross, P.J. Banks, Coupled heat and mass transfer in regenerators — predictions using an analogy with heat transfer, Int. J. of Heat and Mass Transfer 15 (1972) 1225–1241]. Effect of room sensible heat factor, ventilation mixing ratio, and regeneration temperature has also been studied. The results show that solid desiccant-based hybrid air-conditioning systems can give substantial energy savings as compared to conventional vapour compression refrigeration based air-conditioning systems in most commonly encountered situations.

Energy conservation through hybrid air-conditioning cycles: computer modeling studies

SYNOPSIS In this paper the authors have presented a review, analysis and computer modelling of desiccant based hybrid air-conditioning (A/C) cycles for typical hot and humid conditions. Three hybrid cycles based on solid desiccant dehumidifier (viz. ventilation/condenser, recirculation/condenser and ventilation/heat exchanger cycles) have been evaluated and compared in performance with a conventional vapour compression A/C system. Computer modelling and analysis is based on dehumidifier performance data and the air psychrometric properties data correlations. Typical performance results in parametric variations for less and more humid conditions prevailing in Indian cities are presented. It is found that there is considerable scope for energy savings and improvement in the COP with hybrid A/C over the conventional vapour compresion cycle and desiccant cooling cycle alone for high latent heat load conditions.

Vapour-compression and liquid-desiccant hybrid solar space-conditioning system for energy conservation

This paper presents some energy conservation and techno-economic assessment studies of a hybrid solar space-conditioning system consisting of conventional vapour-compression (V-C) and liquid-desiccant cycles. The liquid-desiccant cycle is used to bring the humidity within the comfort range by partially converting the latent heat load (LHL) into a sensible heat load and then meeting this load with a conventional V-C cycle. In a hybrid V-C system, the heat rejected from the condenser is used to partially regenerate the desiccant to improve the coefficient of performance ( COP) of the system. Thermodynamic modelling of the hybrid V-C system with R-11 as the refrigerant and water-LiBr as the liquid desiccant has been carried out and the effect of operating parameters on the system COP and cooling capacity has been investigated. It is found that the hybrid V-C system is more promising under high latent heat load or higher ambient humidity conditions, and significant energy saving can be achieved over standard V-C cooling systems.

Psychometric technoeconomic assessment and parametric studies of vapor-compression and solid/liquid desiccant hybrid solar space conditioning systems

This communication presents some studies on hybrid vapor-compression (V-C) and solid/liquid desiccant solar air-conditioning systems. Desiccants are used to bring the humidity within the comfort range by partially converting the latent heat load (LHL) into sensible heat load and then meeting this load with conventional V-C cycle. In both the options of hybrid systems consisting of vapor-compression and solid/liquid desiccant cycles, heat rejected from the condenser of the V-C cycle is used to regenerate the desiccants to improve the coefficient of performance ( COP) of the systems. A thermodynamic assessment and technoeconomic feasibility study of hybrid V-C and solid/liquid desiccant solar air-conditioning systems have been presented. It is found that a hybrid system with desiccant cycle is more promising under high latent heat load and higher ambient humidity conditions and significant energy saving can be achieved over standard V-C cooling systems.