Michael J. Brandemuehl

Improved Energy Capture in Series String Photovoltaics via Smart Distributed Power Electronics

This paper proposes an improved module integrated converter to increase energy capture in the photovoltaic (PV) series string. Prototypes for self-powered, high efficiency dc-dc converters that operate with autonomous control for tracking the maximum power of solar panels locally and on a fine scale are simulated, built and tested. The resulting module is a low-cost, reliable smart PV panel that operates independently of the geometry and complexity of the surrounding system. The controller maximizes energy capture by selection of one of three possible modes: buck, boost and pass-through. Autonomous controllers achieve noninteracting maximum power point tracking and a constant string voltage. The proposed module-integrated converters are verified in simulation. Experimental results show that the converter prototype achieves efficiencies of over 95% for most of its operating range. A 3-module PV series string was tested under mismatched solar irradiation conditions and increases of up to 38% power capture were measured.

Characterization of Power Optimizer Potential to Increase Energy Capture in Photovoltaic Systems Operating Under Nonuniform Conditions

Power optimizers, which perform power conversion and distributed maximum power point tracking (DMPPT) at the subarray level, are available to mitigate losses associated with nonuniform operating conditions in grid-tied photovoltaic (PV) arrays, yet there is not a good understanding of their potential to increase energy capture. This paper develops and demonstrates a methodology for the use of a detailed software tool that can accurately model both partial shading and electrical mismatch at the subpanel level in a PV array. Annual simulations are run to examine the device-independent opportunity for power recovery in arrays with light, moderate, and heavy shading, and subpanel electrical mismatch variations based on measurements from a monocrystalline silicon array. It is found that in unshaded arrays, the potential energy gain is <; 1% using power optimizers, but in shaded arrays it increases to 3-16% for panel-level DMPPT and 7-30% for cell-level DMPPT. In the set of simulated cases, panel-level power optimization recovers 34-42% of the energy that is lost to partial shading.

Guidelines for improved performance of ice storage systems

Abstract This paper describes simulation-based results of an investigation of a commercial cooling plant with an ice storage system. Various ice storage systems, chiller compressors, and building types were analyzed under four different control strategies. Optimal control as the strategy that minimizes the total operating cost (demand and energy charges) served as a benchmark to assess the relative performance of three conventional controls (chiller-priority, constant-proportion, and storage-priority control) and to determine aspects in need of improvement in order to apply these conventional controls better and to enhance the cost saving potential of ice storage systems. Independent of the non-cooling electrical load profile, it was found that good efficiency of the cooling plant in the icemaking mode and rate structures with strong load-shifting incentives are prerequisites for making cool storage successful. Chillers with poor performance at subfreezing evaporator temperatures require significant on- to off-peak differentials in the energy and demand rates to yield substantial savings. The relative performance benefit of optimal control over conventional controls increases when rate-based load-shifting incentives are weak. With cooling-related electrical loads being large compared to non-cooling loads, all conventional controls improve their performance when slowly recharging during off-peak periods to contain off-peak demand. On-peak demand reduction of storage-priority is near-optimal for many cases. Guidelines are presented to improve the load-shifting performance of chiller-priority and constant-proportion control.

Advances in Near-Optimal Control of Passive Building Thermal Storage

Using a simulation and optimization environment, this paper presents advances toward near-optimal building thermal mass control derived from full factorial analyses of the important parameters influencing the passive thermal storage process for a range of buildings and climate/utility rate structure combinations. Guidelines for the application of, and expected savings from, building thermal mass control strategies that can be easily implemented and result in a significant reduction in building operating costs and peak electrical demand are sought. In response to the actual utility rates imposed in the investigated cities, fundamental insights and control simplifications are derived from those buildings deemed suitable candidates. The near-optimal strategies are derived from the optimal control trajectory, consisting of four variables, and then tested for effectiveness and validated with respect to uncertainty regarding building parameters and climate variations. Due to the overriding impact of the utility rate structure on both savings and control strategy, combined with the overwhelming diversity of utility rates offered to commercial building customers, this study cannot offer universally valid control guidelines. Nevertheless, a significant number of cases, i.e., combinations of buildings, weather, and utility rate structure, have been investigated, which offer both insights and recommendations for simplified control strategies. These guidelines represent a good starting point for experimentation with building thermal mass control for a substantial range of building types, equipments, climates, and utility rates.

Evaluation of commercial building demand response potential using optimal short-term curtailment of heating, ventilation, and air-conditioning loads

Mismatches in power supply and demand due to infrastructure design and commercial development result in negative economic impacts and societal disruption. These impacts are exacerbated by unusually hot weather as well as energy infrastructure failures. Demand reductions by large commercial and industrial customers are increasingly sought by electrical utilities as a means to control severe supply–demand mismatches. Large electricity consumers have used thermal storage systems, on-site electricity generation, shifting of production processes, and short-term curtailment as means to manage and control their demand during peak demand times. Utility notification may be sent to request a reduction of load for a given duration when demand reaches a specified percent of available supply. This paper examines the interconnected nature of the building heating, ventilation, and air-conditioning systems as they apply to short-term demand response (DR) by conducting a reference case investigation into optimal control of building cooling systems for DR.

Expanded microchannel heat exchanger: design, fabrication, and preliminary experimental test

This article first reviews non-traditional heat exchanger geometry, laser welding, practical issues with microchannel heat exchangers, and high effectiveness heat exchangers. Existing microchannel heat exchangers have low material costs, but high manufacturing costs. This article presents a new expanded microchannel heat exchanger design and accompanying continuous manufacturing technique for potential low-cost production. Polymer heat exchangers have the potential for high effectiveness. This article discusses one possible joining method – a new type of laser welding named ‘forward conduction welding’, used to fabricate the prototype. The expanded heat exchanger has the potential to have counter-flow, cross-flow, or parallel-flow configurations, be used for all types of fluids, and be made of polymers, metals, or polymer–ceramic precursors. The cost and ineffectiveness reduction may be an order of magnitude or more, saving a large fraction of primary energy. The measured effectiveness of the prototype with 28 µm thick black low-density polyethylene walls and counterflow, water-to-water heat transfer in 2 mm channels was 72 per cent, but multiple low-cost stages could realize the potential of higher effectiveness.

Module mismatch loss and recoverable power in unshaded PV installations

Distributed electronics which optimize power in PV systems have the potential to improve energy production even under unshaded conditions. This work investigates the extent to which mismatch in the unshaded electrical characteristics of PV panels causes system-level power losses, which can be recovered in arrays employing power optimizers. Of particular interest is how this potential for power recovery is affected by factors such as available light, cell temperature, panel technology, and field degradation. A system for simultaneous collection of panel-level I-V curves over an entire array is designed. This system is used to acquire high and low light module performance data for a variety of arrays at the National Renewable Energy Laboratory (NREL) test facility. The measured data show moderately low variation in module maximum power and maximum power producing current in all of the arrays. As a group, the tested arrays do not show any strong correlations between this variation and array age, technology type, or operating conditions. The measured data are used to create individual panel performance models for high and low light conditions. These models are then incorporated in annual hourly energy simulations for each array. Annual mismatch loss (and thus potential for increased energy capture using power optimizers) is found to be minimal, <;1 % for all of the sampled arrays. Due to the nature of the tested arrays, these results may or may not be indicative of typical PV array behavior; further investigation is planned over a larger group of PV installations to determine the general applicability of this studys results.

Heat Conduction of Inert Gas-Hydrogen Mixtures in Parabolic Trough Receivers

The annulus of a parabolic trough receiver is normally evacuated to prevent heat conduction between the internal absorber pipe and the external glass envelope. In the past, this vacuum has sometimes been compromised by hydrogen permeation from the heat transfer fluid through the absorber pipe. Heat conduction, and consequently receiver thermal loss, can be significantly increased by the presence of hydrogen in the annulus. Supplying receivers with inert gases in the annulus, or injecting receivers with inert gases after the vacuum has been compromised, could mitigate these heat losses. This study measures parabolic trough receiver heat conduction in the transition, temperature jump, and continuum regimes for argon-hydrogen and xenon-hydrogen mixtures at an absorber temperature of 350°C. Test results show that small heat loss increases over evacuated values are associated with the 95% inert gas/5% hydrogen mixtures and that from a performance perspective gas-filled HCEs would likely induce a 1–3% plant revenue decrease relative to evacuated receivers, but would protect against hydrogen-induced heat loss as long as there was sufficient quantity of inert gas in the annulus. Sherman’s interpolation formula predicted the inert gas and 95% inert gas/5% hydrogen mixture test results within experimental and model uncertainty, but did not accurately capture the larger hydrogen molar fraction test results. The source of this discrepancy will be further investigated.Copyright

Analysis of Potential for Mitigation of Building-Integrated PV Array Shading Losses Through Use of Distributed Power Converters

Partial shading of building-integrated photovoltaic (BIPV) arrays is very common, as they are limited by building geometry and most often installed in crowded urban or suburban environments. Power losses in shaded BIPV systems tend to be disproportionately large, due in large part to mismatches in operating conditions between panels. Maximum power point tracking at a modular level, which can be achieved through the use of module integrated dc-dc converters (MICs), may be used to mitigate some of these losses. This paper investigates the potential power gain provided by MICs for several representative partially shaded BIPV array scenarios. A flexible, comprehensive simulation model for BIPV systems is developed, which allows for variations in insolation and temperature at the PV cell level, while accurately modeling MICs and their effect on array performance. Shadows from nearby objects are mapped onto the modeled BIPV arrays and simulated on an annual, hourly basis, with varying array configuration as well as object size and placement. Results of these simulations show that the impact of MICs on system power output varies depending on factors such as radiation availability, time shaded throughout the year, shadow size and distribution on the array, and inverter design. Annual power gains of 3–30% are realized for a moderately shaded system with MICs when compared to conventional approaches. Further opportunities for increased energy capture in a BIPV system with MICs are identified and discussed.Copyright

Parabolic Trough Receiver Thermal Testing

NREL has fabricated a parabolic trough receiver thermal loss test stand to quantify parabolic receiver off-sun steadystate heat loss. At an operating temperature of 400°C, measurements on Solel UVAC2 and Schott PTR70 receivers suggest off-sun thermal losses of approximately 370 W/m receiver length. For comparison, a receiver from the field with hydrogen in its annulus loses approximately 1000 W/m receiver length. The UVAC2 heat loss results agree within measurement uncertainty to previously published data, while the PTR70 results are somewhat higher than previously published data. The sensitivity of several receiver performance parameters is considered and it is concluded that differences in indoor and outdoor testing cannot account for the difference in PTR70 thermal loss results.