Condensate Harvesting from Large Dedicated Outside Air-Handling Units with Heat Recovery
|Publication Date:||1 January 2009|
With the adoption of building service systems requiring designs to minimize their environmental impact, innovative as well as obvious natural resource conservation measures are being explored.
One element of conservation considered in design is to minimize water usage in buildings. Modern designs often use green design practices suggested by organizations like the US Green Building Council and their LEED new construction guidance, where points toward receiving an overall rating are assigned to reducing annual water consumption by 20% and 30% from a baseline fixture flow rates determined by the Energy Policy Act of 1992 (USGBC, 2007). Hydronic systems makeup for equipment such as cooling towers is not generally included in building baseline water consumption rates for LEED, however should be considered in whole building water consumption estimates. Typical water conservation measures are utilizing low and zero flow fixtures, utilizing grey water for non-potable uses, and minimizing high water demand landscaping. Other, less common, water conservation measures utilizes the harvesting of water producing sources to offset the annual water consumption to achieve a net annual water reduction. These water producing sources include storm water recovery and air conditioning condensate harvesting.
Condensate from air conditioners, dehumidifiers, and refrigeration units can provide facilities with a steady supply of relatively pure water for many processes. Laboratories are excellent sites for this technology because they typically require dehumidification of large amounts of 100% outside air (DOE, 2005).
Another element considered in building design is indoor air quality and building envelope pressurization. Ventilation for Acceptable Indoor Air Quality, ASHRAE Standard 62.1-2004 recommends appropriate ventilation levels for various building and occupancy types. If buildings have excessive exhaust requirements due to fume hoods, user required air change rates, or other process exhaust, theminimum ventilation air requirements are potentially higher. Outdoor ventilation air commonly contains a higher moisture concentration and temperature than what is desired in the space. Conditioning this air by both reducing the temperature as well as reducing the moisture content is required.
Because some designs require increased levels of outside ventilation air for a variety of reasons, the potential to recover the energy being expelled by the exhaust system and transfer to the incoming ventilation air is high. Due to this potential, ASHRAE has incorporated policy regarding energy recovery, outlined in the Energy Standard for Buildings except Low- Rise Residential Buildings, ASHRAE Standard 90.1. To satisfy ASHRAE 90.1, fan systems that have a design air flow rate of 5000 cfm (2358 L / sec) or greater and have a minimum outside ventilation air flow that is equal to 70% or more of the supply air shall have an energy recovery system with at least 50% recovery effectiveness (ASHRAE, 2004). There are exceptions to this requirement, however for the purpose of this paper, no exceptions are taken.
To centralize the process of pre-conditioning ventilation air and the recovery of exhaust air energy, large energy recovery units or DOAHUs with energy recovery means are commonly used. These units are designed to provide preconditioned ventilation air either directly to the occupied space or ducted to an additional AHU. All building exhaust is also ducted to these units, which exchanges sensible and latent energy with the incoming ventilation air.
There are several types and configurations of DOAHU available for use. They have heat recovery means, either by an enthalpy wheel, glycol run around loop, air-to-air heat exchanger, or others. In addition, they often have cooling and heating coils, as well as humidifiers depending on the application. The type of DOAHU studied in this paper is one with an energy recovery device, pre-heat and cooling coil downstream of the recovery device, and a heating coil in the reheat position, shown in Figure 1.
These units pre-condition large quantities of moisture laden air down to a more moisture neutral state, where the humidity ratio1 is close to the delivered supply air humidity ratio. This process produces large quantities of condensate, which is commonly discharged to the sanitary sewer systems. Some water treatment facilities operate at near capacity and do not allow or discourage condensate disposal into the sanitary sewer system (ICC, 2006).
The focus of this paper is to determine the feasibility of coupling both the water conservation and indoor air quality and building envelope pressurization elements of design by harvesting air conditioning condensate for non-potable water supplementation in large commercial, institutional, and medical buildings, where large volumes of outside air are required.
1. Humidity ratio is defined as for given moisture sample it is the ratio of the mass of water vapor to the mass of dry air in the sample (ASHRAE, 1993).