Paul Reeves

Tracking and stationary flat plate solar collectors : yearly collectible energy correlations for photovoltaic applications

Abstract For tracking and stationary flat plate (nonconcentrating) solar collectors, we develop easy-to-use, closed form, analytic formulae for yearly collectible energy as a function of radiation threshold. Primary applications include central-station photovoltaic systems. These correlations include the explicit dependence on: yearly average clearness index, latitude, and ground cover ratio (shading effects), and are in excellent agreement with data-based results for 26 U.S. SOLMET stations. They also incorporate appropriate functional forms that ensure accurate results for photovoltaic system design and, in particular, for systems with buy-back thresholds. Both beam and diffuse shading are treated properly and diffuse shading is found to represent a 2%–6% loss that has systematically been ignored in past studies. Sample sensitivity studies illustrate evaluation of the energetic advantage of tracking vs. stationary deployment and its significant dependence on ground cover ratio. The impact of isotropic versus anisotropic modeling of diffuse radiation is quantified and shown to give rise to non-negligible differences (up to 10%) in yearly collectible energy. We also determine an optimal tracking strategy for two-axis trackers which, however, is found to yield a 0%–2% energetic advantage relative to conventional normal-incidence tracking.

The Hathaway "Solar Patriot" House: A Case Study in Efficiency and Renewable Energy

This report details the monitored and modeled performance of a solar home outside of Washington, D.C. We modeled the home energy performance using DOE2.2, performed numerous short-terms tests on the home and monitored its occupied performance for 29 months. The home uses modular construction, solar water heating, a ground-coupled heat pump, efficient appliances and compact fluorescent lighting to reduce its energy consumption by 35% compared to the Building America research benchmark home. The addition of 6kW of photovoltaics increases the savings to 67% compared to the Building America research benchmark. A more efficient shell to reduce space conditioning loads would have brought the home closer to its zero energy goals. However, even with efficient lighting and appliances, the lights, appliance and plug loads are a significant energy consumer. About 4 kW of PV are required to meet the needs of these loads alone. To achieve the zero energy goal with no further efficiency increases, the Hathaway house would need about 2.6 kW of PV in addition to the 6.0 kW it now has.

Thermal Performance of Unvented Attics in Hot-Dry Climates: Results from Building America

Unvented attics have become a more common design feature implemented by Building America partners in hot-dry climates of the United States. More attention is being focused on how this approach affects heating and cooling energy consumption. By eliminating the ridge and eave vents that circulate outside air through the attic and by moving the insulation from the attic floor to the underside of the roof, an unvented attic become a semi-conditioned space, creating a more benign environment for space conditioning ducts.

Evaluation of a High-Performance Solar Home in Loveland, Colorado

Building America (BA) partner McStain Neighborhoods built the Discovery House in Loveland, Colorado, with an extensive package of energy-efficient features, including a high-performance envelope, efficient mechanical systems, a solar water heater integrated with the space-heating system, a heat-recovery ventilator (HRV), and ENERGY STAR™ appliances. The National Renewable Energy Laboratory (NREL) and Building Science Consortium (BSC) conducted short-term field-testing and building energy simulations to evaluate the performance of the house. These evaluations are utilized by BA to improve future prototype designs and to identify critical research needs. The Discovery House building envelope and ducts were very tight under normal operating conditions. The HRV provided fresh air at a rate of about 75 cfm (35 l/s), consistent with the recommendations of ASHRAE Standard 62.2. The solar hot water system is expected to meet the bulk of the domestic hot water (DHW) load (>83%), but only about 12% of the space-heating load. DOE-2.2 simulations predict wholehouse source energy savings of 54% compared to the BA Benchmark [1]. The largest contributors to energy savings beyond McStain’s standard practice are the solar water heater, HRV, improved air distribution, high-efficiency boiler, and compact fluorescent lighting package.

Thermal Performance of Unvented Attics in Hot-Dry Climates: Results From Building America

As unvented attics have become a more common design feature implemented by Building America partners in hot-dry climates of the United States, more attention has been focused on how this approach affects heating and cooling energy consumption. By eliminating the ridge and eave vents that circulate outside air through the attic in most new houses and by moving the insulation from the attic floor to the underside of the roof, an unvented attic becomes a semiconditioned space, creating a more benign environment for space conditioning ducts. An energy trade-off is made, however, because the additional surface area (and perhaps reduced insulation thickness) increases the building loss coefficient. Other advantages and disadvantages, unrelated to energy, must also be considered. This paper addresses the energy-related effects of unvented attics in hot-dry climates based on field testing and analysis conducted by the National Renewable Energy Laboratory.Copyright

Field Evaluation of a Near Zero Energy Home in Oklahoma

The authors evaluated a Zero Energy Home (ZEH) built by Ideal Homes in Edmond, Oklahoma, that included an extensive package of energy-efficient technologies and a photovoltaic (PV) array for site electricity generation. The ZEH was part of a Building America (BA) research project in partnership with the Building Science Consortium to exhibit high efficiency technologies while keeping costs within the reach of average home buyers, and was a modified version of a production 1584-ft2 , three-bedroom, single-story, slab-on-grade design with attached garage. The home included a tight, well-insulated envelope, an energy recovery ventilator, high-performance windows, tankless gas water heater, efficient lights and appliances, and a ground source heat pump (GSHP). We conducted a series of short-term tests beginning in August 2005, and have collected long-term data under occupied conditions since February 2006. The GSHP performance was disappointing until the outdoor unit was replaced, after which time the efficiency began to meet expectations. However, the electricity use of the replacement unit was higher than expected because of an unusually low cooling setpoint. Based on the measured test results, the predicted whole-house energy savings compared to the BA Benchmark was 96%, with savings of 55% for efficiency measures alone.Copyright