Conference Year

2016

Keywords

DOAS, Air to Air Heat Recovery Unit, Indirect Evaporative Cooling, Sensible Heat Exchangers, Desiccant Dehumidification

Abstract

In hot and humid countries like India, the HVAC systems in the commercial and building sector are responsible for major part of energy consumption. About 30 to 70% of electrical power consumed in various facilities is for HVAC&R. Moreover air-conditioning systems need to be supplied with increased fresh air to improve Indoor Air Quality, IAQ, and ventilation to comply with latest norms. Treating fresh air typically consumes 20 to 40% of energy required for air-conditioning.  Dedicated Outdoor Air Systems are increasingly being considered in air-conditioning systems to improve Indoor Air Quality, IAQ and reduce energy consumption. Fresh air is treated separately before being introduced in the air conditioned space while limiting the increase in energy cost. This improves Indoor Air Quality, as more outdoor air can be introduced in the conditioned space while limiting the load and energy required. Energy is saved by decoupling sensible and latent loads.  DOAS usually operate along with some other sensible cooling equipment, mainly VRF system, radiant panels, RP or Fan Coil Units, FCU. Integration of non-compressor based DOAS needs to be investigated further to optimally integrate these systems with various existing conventional air-conditioning systems. VRF systems have emerged as energy efficient HVAC equipment in recent years, therefore judiciously combining DOAS with VRF system needs to be investigated.  This paper consolidates the understanding about various ways of integrating different DOAS and identifying best practices for energy efficient and cost effective integration with balance air-conditioning load handling system. As DOAS is site specific, supply air conditions like DBT, DPT and supply airflow rate affect the performance of DOAS. Assuming indoor air conditions to be 27oC DBT and 50% rh, data for different Indian climatic zones is used to evaluate performance of different non compressor based configurations like evaporative pre cooling, air to air heat recovery and indirect evaporative cooling.  A novel configuration, with less number of components is proposed and investigated for possible integration with existing air-conditioning systems. The configuration is cooling and desiccant dehumidification of fresh outdoor air by indirect evaporative cooling of exhaust air.  Comparative psychrometric analysis between the select DOAS configuration and previous configurations is performed for various climate conditions of different Indian Cities: Warm and Humid (Mumbai), Composite (New Delhi), Moderate (Bangalore) and Hot and Dry (Ahmedabad).  Theoretical analysis for heat and mass transfer was carried out based on patented nine enhanced passage aluminium extrusion for deployment in Mumbai. Overall heat transfer coefficient on outdoor air side is calculated as 13.89 W/m2K. Mass transfer coefficient on outdoor and indoor air side is obtained as 0.03 m/s and 0.04 m/s respectively. Assumed outdoor air conditions are 31.0 DBT and 80% rh. Water evaporated on indoor air side and water condensed in outdoor air side is obtained as 0.34 g/s and 0.37 g/s respectively. LMTD for the heat transfer process is obtained as 4.2oC for a counter flow heat and mass exchange. Energy and cost saving calculations suggest savings of 2171 INR/day from heat recovery and 38.6% increase in COP of parallel VRF system.

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