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Simulation of radiant cooling performance with evaporative cooling sources

  • Author(s): Moore, Timothy
  • et al.
Abstract

This project addresses three closely related and similarly complex questions: First, using currently available simulation software, what methods might be appropriate for comparing slabintegrated radiant cooling to more conventional alternatives, such that the results are sufficiently fair and comprehensive to support system selection and design? Second, what is the relative performance of representative system configurations across a set of climates that test presumed strengths and limitations? Third, what useful conclusions can be drawn from such comparisons to inform the selection, application, design, and control of hydronic radiant cooling?

The particular approach taken to answering these questions is rooted in the contention that useful results must effectively capture five essential aspects of slab-integrated hydronic radiant cooling: a) radiant heat transfer between surfaces; b) the effects of thermal capacity, lag, and decrement in the chilled slab; c) the limitations of evaporative cooling water sources; d) the potential of various control strategies for maintaining thermal comfort while minimizing energy consumption and peak loads; and e) the challenges and benefits of integrating the operation and control of hydronic and airside space conditioning systems.

This report describes whole-building simulations of slab-integrated hydronic radiant cooling with mechanical ventilation, plus a more conventional all-air cooling system as a point of reference. Simulations are performed using Virtual Environment (VE)—an interconnected set of building performance-modeling tools from Integrated Environmental Solutions (IES). Methods are described for the modeling of hydronic radiant cooling slabs. Among these, THERM, a simple two-dimensional finite-element heat transfer tool from Lawrence Berkeley National Laboratory, is used for determining properties of the heat transfer path between the hydronic circuits and cooling surfaces. Attention is also given to modeling limitations of evaporative cooling as a supply water source for the radiant system and waterside economizer for the all-air baseline system. In preparing the models, emphasis was placed on achieving similar degrees of equipment and controls optimization for both systems using methods that could be replicated in the context of practical design processes.

Cooling-season performance is evaluated in terms of system dynamics, thermal comfort, peak loads, and energy consumption for a prototypical office building in Denver, Sacramento, Los Angeles, and San Francisco. The Denver climate was used to optimize system dynamics and performance for minimum energy consumption and peak power. Sacramento—the hottest of the four—was the focus for optimizing and evaluating thermal performance with aggressive hydronic slab nighttime precooling. For the San Francisco climate, added emphasis was placed on optimizing the economizer controls and performance for the all-air baseline system. In all cases, equipment, airflow, and other key parameters were evaluated and re-sized accordingly.

The slab-integrated hydronic radiant cooling is augmented by a dedicated outside air system (DOAS) for conditioning of ventilation air. The hydronic cooling and DOAS utilize only indirect evaporative cooling sources. The supply water source for the hydronic slabs and cooling coils is a closed-circuit cooling tower. The DOAS also incorporates a heat exchanger for sensible energy recovery and indirect-evaporative cooling of ventilation air via a spray chamber in the exhaust air stream. The reference baseline is a modern variable-air-volume system with an efficient watercooled chiller and fully integrated control resets for supply air temperature and airsideeconomizer operation. A waterside economizer or waterside “free cooling” (WSFC)—essentially the same cooling water source as is used for the hydronic radiant system—and nighttime precooling cycle were modeled as an additional scenario for the baseline system. The DOAS and VAV system use identical high-efficiency fans and motors (differing only in size).

Simulation results (Figures 1–4) suggest strong energy-saving potential for radiant cooling systems in both Colorado and California climates. In Denver (Figure 1), the simulated radiant cooling plus dedicated outside air system (Radiant+DOAS) with precooling uses an estimated 71% less energy than the standard VAV baseline system and 62% less than the same VAV system using waterside free cooling and a nighttime precooling control strategy. This comparison includes heating for cool mornings, which must be coordinated with the nighttime slab precooling strategy. In Sacramento (Figure 2), the Radiant+DOAS uses an estimated 59% less energy relative to the baseline VAV system and 56% less than the VAV with waterside free cooling, regardless of the inclusion of precooling controls. For this hot but relatively dry climate, the added fan energy for precooling with the all-air VAV system, given its capacity for WSFC is sized for chiller heat rejection, offsets the savings from reduced daytime chiller operation. In Los Angeles (Figure 3), where daytime temperature are more moderate and nighttime temperatures tend not to dip quite as low, precooling—in this case used only for the Radiant+DOAS—confers a lesser net benefit. For San Francisco (Figure 4), where cooling loads are reduced and airside “free cooling” is readily available through economizer operation (which still requires the use of fans), total energy for both systems is considerably lower. However, the effectiveness of waterside free cooling in this climate contributes to even greater reduction of energy consumption for the otherwise already very efficient hydronic radiant system.

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