A TRNSYS Model of a Hybrid Lighting System

Abstract

During the past two years, considerable effort has been expended in developing the TRNSYS Hybrid Lighting Model. The work has resulted in a software tool that can simulate impacts associated with utilizing the hybrid lighting technology. All of the physical parameters of a hybrid lighting system are included as variables within the software. By utilizing this work, the hybrid lighting team can make design decisions based upon computer predictions of the performance of a hybrid lighting system. The hybrid lighting system is modeled using the transient system simulation program TRNSYS. The TRNSYS model is implemented using interconnected components, which include a weather generator, radiation processors, a hybrid lighting model, a building model, building schedules, utility rate schedules, and output components. The results from the simulation include the annual energy and monetary savings gained from the hybrid lighting system. An economic model has been incorporated into the hybrid lighting model to calculate the break-even capital cost of a hybrid lighting system based on the annual savings. A narrow-band and wide-band hybrid lighting model has been developed. The wide- band model uses direct normal solar radiation from either a TMY2 data file or the TRNSYS weather generator. The incoming direct normal radiation is weighted by the average spectral properties of the hybrid lighting components which include concentrator reflectance, secondary element transmittance and reflectance, thermal photovoltaic quantum efficiency, light fiber attenuation, and luminaire efficiency. The narrow-band model uses TMY2 data or the TRNSYS weather generator to obtain the magnitude of the direct normal radiation, but the direct normal spectral distribution is predicted based on the atmospheric transmission model SMARTS. The narrow-band model predicts the direct normal spectral radiation at five nanometer bandwidths. Next it reads the available spectral component data, applies the component data to the solar spectral distribution, and calculates the amount of light and electricity that is generated by the system. The outputs from both models include the light produced by the hybrid lighting system as well as electricity generated by the TPV. Within TRNSYS, the light output from the hybrid lighting system model is sent to the building model. The building is modeled using the TRNSYS type 56 multi-zone building model. Type 56 is a FORTRAN subroutine which is designed to provide detailed thermal models of buildings. The model consists of two windowless 2500 m2 zones. One zone uses efficient fluorescent lighting and the other zone uses hybrid lighting with dimmable fluorescent auxiliary lighting. Identical schedules in the two zones simulate the heating, cooling, and ventilation of a typical mixed-use environment. Gains in the model account for the people, computers, and lights in the building. Cooling in the building is supplied using a chiller with a constant COP of 3 and heating loads are met using an 80 % efficient natural gas furnace. Using local utility rate schedules, energy costs can be calculated for the two zones of the building model with the difference representing the energy savings due to the hybrid lighting system. The hybrid lighting model calculates the break-even capital cost of a hybrid lighting system based on the system energy savings. The break-even capital cost is defined as the initial cost of the hybrid lighting system that will provide a life cycle savings (LCS) of zero over the economic lifetime. At this point in the design stage, realistic component prices are not available for determining economic parameters such as years to payback, LCS, or return on investment. Instead the break-even capital cost was calculated to be used as a price target where the energy savings predicted by the TRNSYS model will economically compensate for the system components. Simulations were performed to determine effectiveness of the hybrid lighting technology across the United States. Hybrid lighting systems located in Tucson, AZ and Honolulu, HI performed best with break-even capital costs of $2050 and $2800 based on a 10 year analysis period. Other daylighting strategies were evaluated to determine their cost competitiveness with hybrid lighting. Photovoltaics and toplighting were both evaluated using TRNSYS models. The break-even capital cost of the hybrid lighting system was approximately five times that of a toplighting or photovoltaic system. Photovoltaics are not an economic alternative, but the low cost and simple nature of toplighting makes it a very competitive alternative to hybrid lighting.