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Extract from the Proceedings of Building Simulation and Optimization 2012

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Extract from the Proceedings of Building Simulation and Optimization 2012 First Building Simulation and Optimization Conference Loughborough, UK BSO12 10-11 September 2012 MODELING AND SIMULATION OF BUILDING ECONOMIZER AND ENERGY RECOVERY SYSTEMS FOR OPTIMUM PERFORMANCE Stephen Treado, Xing Liu Pennsylvania State University University Park, PA appropriate to provide “free cooling”, the rough ABSTRACT equivalent of opening the windows on a nice day This paper describes how building economizer and rather than using mechanical cooling. The former energy/enthalpy recovery system control and study has demonstrated that the energy savings operation can be modelled and simulated to evaluate associated with economizer could be very significant and optimize their performance. A improved method (Brandemuehl et al., 1999). The energy saving of the to determine optimum temperature setpoints and robust control strategy on cooling coil energy outdoor air fractions is discussed and demonstrated. consumption which was evaluated by over one year’s Through the simulation, the new strategy based on comparison tests on two air-handling units in a total system energy use is shown to be able to save building was 88.47% in contrast to the energy up to 11% of energy under certain conditions and to consumption using the conventional control (damper be most useful for dry climates. at fixed position) in Hong Kong (Wang et al., 2004). Energy recovery systems exploit the “free heating or INTRODUCTION cooling” provided by exchanging energy between the exhaust and outdoor airstreams. In both of these Buildings are constructed to serve a purpose, namely systems, some control logic must be implemented to to provide a comfortable, safe, healthy and determine how and when the systems should be productive environment for human endeavours. operated to obtain the most favourable outcome. They require the investment of scarce resources and substantial time and effort to design, build and This paper examines the current methods used for operate, so efficiency and cost effectiveness are controlling building economizer and energy recovery always considerations. In particular, energy efficient systems, and related methods for modeling and performance, and the means to achieve it, is of simulating their performance. It identifies some critical interest to building designers and owners. A inconsistencies in approaches and suggests some tension exists, however, between building energy improved models and simulation methods to more efficiency, thermal performance and occupant health accurately evaluate energy performance and increase and comfort, whereby the means employed to the energy savings potential from the use of these minimize energy usage may negatively impact technologies. thermal and air quality within the building’s conditioned space. This is a constant challenge faced SIMULATION by building designers and operators, and the focus of There are several important aspects related to the many design codes, standards and tools. In addition control, modeling and simulation of building HVAC to requiring building envelope components to reduce economizer and energy recovery systems. The or control unwanted heat transfer, building codes discussion will commence with economizer control, impose minimum requirements for ventilation air, and then extend to the analysis of energy recovery sometimes known as fresh air, to control the buildup systems. The actual implementation of the of contaminants in the indoor air. Ventilation economizer control, for example, can be done in standards are based on a combination of factors, different ways depending on the installed sensors and usually including building use, size and occupancy, actuators. The basic strategies (ASHRAE Standard and inadequate ventilation has been linked to 90.1-2010) employed include the following problems with moisture, allergens and generally conditions, under which the outdoor air intake unhealthy conditions. damper would be opened beyond the minimum It is within this context that building economizer required for ventilation: cycles and energy recovery systems have evolved. • fixed dry bulb-outdoor air dry bulb The introduction of ventilation air from the outside temperature less than a set value may impact heating or cooling loads since the • differential dry bulb- outdoor air dry bulb outdoor air needs to be conditioned to maintain the temperature less than return air temperature indoor comfort conditions. This means that in • fixed enthalpy- outdoor air enthalpy less extreme climate conditions (winter, summer) than a set value additional energy may be required to heat or cool the • differential enthalpy- outdoor air enthalpy ventilation air as compared to a sealed building. An less than return air enthalpy economizer cycle utilizes outdoor air when © Copyright IBPSA-England, 2012 - 433 - • combination- some combination of the effect is described more fully below. Second, above outdoor air conditions in region C, being warmer but • optimal- a combined strategy that dryer than the return air, may provide reduced maximizes energy savings cooling loads with partial economizer operation. The differential modes require at least two sensors, This can be a substantial benefit in certain climate and the enthalpy-based strategies require the regions. A method for optimal control in this region measurement of relative humidity or some other will be outlined below. Third, the optimal control moisture content indicator, while the combination logic in Figure 1 is based on minimizing cooling coil strategies require multiple temperature and enthalpy loads, but the energy is actually expended to operate sensors. Lacking these sensors limits the control a chiller and pump to provide chilled water to the strategies that can be implemented. Even when the coil, and a fan to provide conditioned air to the space. sensors are installed, sensor errors and other The energy inputs to these components depend on a measurement problems can seriously compromise number of factors, including chilled water supply performance, particularly for enthalpy measurements, temperature, part load ratio and condenser entering which are notoriously unreliable (Zhou et al., 2008). temperature. Optimizing economizer operation to Several studies have illustrated the potential minimize energy input by adjusting setpoints without degradation of economizer energy performance due diminishing comfort will be demonstrated. to sensor uncertainty (Seem et al., 2010). That said, A typical air handling unit (AHU) has hydronic this paper will not explore that issue any further, but heating and cooling coils, a supply fan, and ducts, will focus on the theoretical basis for economizer dampers and control valves. For the sake of control assuming properly functioning and simplicity, the system is assumed to control the air reasonably accurate sensors. temperature within a single zone, although that assumption can readily be extended to a multi-zone A schematic representation of optimum economizer case without loss of applicability. A source of hot control logic based on (Taylor, 2010) is shown in and chilled water is further assumed to provide the Figure 1 in the form of a psychrometric chart, which heating and cooling functions to the room air by way is formatted to show a wide range of air temperature of the respective heating and cooling coils. Outdoor and moisture conditions. Note that lines of constant air can be drawn in to the system through the outdoor dry bulb temperature are nearly vertical on a air damper, and similarly exhausted through the psychrometric chart, increasing from left to right on exhaust air damper. These two dampers work in the horizontal axis, while moisture content increases conjunction with the return air damper to admit the on the vertical axis, and lines of constant enthalpy desired quantity of outdoor air to meet ventilation, slant diagonally downward from left to right. Two pressurization and economizer requirements for the air condition points define the boundaries for building zone. The two basic modes of fan operation optimum economizer control, one being the return air are constant air volume (CAV) or variable air volume temperature Tr (which is assumed to be equal to the (VAV), with the latter being preferred for its energy zone air temperature) and the supply air temperature saving potential. Ts (which is assumed to be saturated). If the condition of the air entering the cooling coil is above In either CAV or VAV operation, the zone heating the moisture content defined by the supply air and cooling loads are met by supplying conditioned temperature intersection with the saturation curve, air to the zone such that the product of the mass flow condensation will occur on the coil (wet coil) while if rate of the supply air, the specific heat of air and the the moisture content is below the line, the temperature change of the air from supply (Ts) to condensation will not occur (dry coil). Thus, it will return (Tr) are equal to the zone thermal load: be advantageous to use 100% outdoor air if the outdoor air condition lies in region A, based on lower ̇ ̇ (1) enthalpy for the wet coil, or region B, based on lower dry bulb temperature for the dry coil. In regions D The difference between the CAV and VAV modes is and E, the use of outdoor air is not advantageous, that as loads vary, supply air temperature is varied by while in region F, some fraction of outdoor air may the control system for CAV, and airflow rate is be beneficial. varied for VAV. Thus, supply air temperature is held at a setpoint (although
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