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Informing Design Through Analysis

Building Envelope and Fenestration

The building envelope and fenestration system was designed with the help of the whole house energy model built using EnergyPlus. The main design principle for the envelope and fenestration was to minimize the space loads. This allows for smaller sized equipment, which decreases capital costs, and cuts the amount of energy consumed by the HVAC system. Two ways to reduce space loads are building a tight envelope and minimizing solar gains. Parametric runs, which varied the insulation R-value, infiltration, window solar heat gain coefficient (SHGC), and window U-value, were completed to determine what design would provide the most energy efficient envelope and fenestration system.

High Efficiency Water to Water Heat Pump

Using the EnergyPlus whole house energy model, the water to water heat pump was sized based on the peak heating and cooling loads. A water to water heat pump was chosen because it is compatible with the system with two thermal storage tanks and is inherently more efficient than air to air or air to water systems. In addition, it is possible to use refrigerant R410A instead of R22 with this system, which has zero ozone depleting potential and will lessen the house’s environmental impact.

Thermal Storage Tanks

TRNSYS was used to model the thermal storage tanks because EnergyPlus does not have the capability of modeling stratified tanks. Using the outputs from the EnergyPlus energy simulation, the optimum tank water temperatures in terms of minimized energy consumption were determined. TRNSYS was also used to size the tanks to be capable of providing storing two days of heating and cooling capacity based on expected building loads.

Indoor Heat Exchangers

Microsoft Excel and PHOENICS were used in the design of the indoor heat exchangers. Standard convection and conduction heat transfer relations were used to construct the heat exchanger model in Excel. The PHOENICS model helped determine the optimum air flow rates to meet the space loads while maintaining proper thermal comfort. The heat exchanger pipe diameter (1/2"), spacing (1 1/2" O.C.) and arrangement (staggered) was chosen so that the output of the exchangers was as high as possible at the given water and air temperatures. The water temperatures in both modes were determined from the thermal storage tank design. Air flow rates across the exchangers were estimated at 50 feet per second and the thermal comfort levels in the space were verified with the PHOENICS model. Radiation heat transfer was not modeled in Excel due to the complicated equation set but was assumed to provide an additional 25% of space conditioning in both heating and cooling modes.

Building Integrated Photovoltaic Thermal Collection System (BIPV/T)

The thermal component of the BIPV/T system will remove heat from the backside of the photovoltaic panels and supply the heat as hot water to the hot storage tank to supplement the heat pump. A CFD model of the system was built to estimate the amount of heat the system could remove and approximate the exiting water temperature from the system. Further studies conducted with a heat transfer network modeled in MATLAB confirmed the results of the PHOENICS model.

Building Integrated Photovoltaic System (BIPV) and Battery Bank

Electric loads for three locations were derived from the energy model developed for this project. Power production estimates for several PV modules were made using TMY2 and manufacturer data and then confirmed with PV Watts. A PV module and array capacity was selected to be large enough for bad weather in D.C. A building-integrated frame was found capable of use with our PV modules and others. Inverters were recommended by the manufacturer. Inverter efficiency data was used to determine optimal size relative to PV production in Colorado. A tool from the inverter manufacturer was used to specify the number of panels in the array’s strings. The electric load predicted for the house was used to design a battery bank that will offer normal operation for four days of zero PV power. Wiring followed NEC 2005.

Airflow System

The airflow system was designed to provide adequate air circulation and outdoor air to the spaces while consuming very little energy and increasing the convection heat transfer of the indoor heat exchangers. The system is completely housed within the plenum above the kitchen. A separate duct is included to provide air to the heating and cooling coil serving the bedroom. The bathroom includes a dedicated exhaust fan.