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Open Access Publications from the University of California
Cover page of Application of Gagge’s Energy Balance Model to Determine Humidity-Dependent Temperature Thresholds for Healthy Adults Using Electric Fans During Heatwaves

Application of Gagge’s Energy Balance Model to Determine Humidity-Dependent Temperature Thresholds for Healthy Adults Using Electric Fans During Heatwaves

(2021)

Heatwaves are one of the most dangerous natural hazards causing more than 166,000 deaths from 1998–2017. Their frequency is increasing, and they are becoming more intense. Electric fans are an efficient, and sustainable solution to cool people. They are, for most applications, the cheapest cooling technology available. However, many national and international health guidelines actively advise people not to use them when indoor air temperatures exceed the skin temperature, approximately 35°C. We used a human energy balance model, to verify the validity of those recommendations and to determine under which environmental (air temperature, relative humidity, air speed and mean radiant temperature) and personal (metabolic rate, clothing) conditions the use of fans would be beneficial. We found that current guidelines are too restrictive. Electric fans can be used safely even if the indoor dry-bulb temperature exceeds 35°C since they significantly increase the amount of sweat that evaporates from the skin. The use of elevated air speeds (0.8m/s) increases the critical operative temperature at which heat strain is expected to occur by an average of 1.4°C for relative humidity values above 22%. We also analysed the most extreme weather events from 1990 to 2014 recorded in the 115 most populous cities worldwide, and we determined that in 103 of them the use of fans would have been beneficial. We developed a free, open-source, and easy-to-use online tool to help researchers, building practitioners, and policymakers better understand under which conditions electric fans can be safely used to cool people.

Cover page of Transformation Towards a Carbon-Neutral Residential Community with Hydrogen Economy and Advanced Energy Management Strategies

Transformation Towards a Carbon-Neutral Residential Community with Hydrogen Economy and Advanced Energy Management Strategies

(2021)

Cleaner power production, distributed renewable generation, building-vehicle integration, hydrogen storage and associated infrastructures are promising for transformation towards a carbon-neutral community, whereas the academia provides limited information through integrated solutions, like intermittent renewable integration, hydrogen sharing network, smart operation on electrolyzer and fuel cell, seasonal hydrogen storage and advanced heat recovery. This study proposes a hybrid electricity-hydrogen sharing system in California, United States, with synergistic electric, thermal and hydrogen interactions, including low-rise houses, rooftop photovoltaic panels, hydrogen vehicles, a hydrogen station, micro and utility power grid and hydrogen pipelines. Advanced energy management strategies were proposed to enhance energy flexibility and grid stability. Besides, simulation-based optimizations on smart power flows of vehicle-to-grid interaction and electrolyzer are conducted for further seasonal grid stability and annual cost saving. The obtained results indicate that, the green renewable-to-hydrogen can effectively reduce reliance on pipelines delivered hydrogen, and the hydrogen station is effective to address security concerns of high-pressure hydrogen and improve participators’ acceptance. Microgrid peer-to-peer sharing can improve hydrogen system efficiency under idling modes. Furthermore, the integrated system can reduce the annual net hydrogen consumption in transportation from 127.0 to 1.2 kg/vehicle. The smart operation (minimum input power of electrolyzer and fuel cell at 65 and 80 kW) can reduce the maximum mean hourly grid power to 78.2 kW by 24.2% and the annual energy cost to 1228.5 $/household by 38.9%. The proposed district hydrogen-based community framework can provide cutting-edge techno-economic guidelines for carbon-neutral transition with district peer-to-peer energy sharing, zero-energy buildings, hydrogen-based transportations together with smart strategies for high energy flexibility.

Cover page of Quantification on Fuel Cell Degradation and Techno-Economic Analysis of a Hydrogen-Based Grid-Interactive Residential Energy Sharing Network with Fuel-Cell-Powered Vehicles

Quantification on Fuel Cell Degradation and Techno-Economic Analysis of a Hydrogen-Based Grid-Interactive Residential Energy Sharing Network with Fuel-Cell-Powered Vehicles

(2021)

Hydrogen-based (H2-based) interactive energy networks for buildings and transportations provide novel solutions for carbon-neutrality transition, regional energy flexibility and independence on fossil fuel consumption, where vehicle fuel cells are key components for H2-electricity conversion and clean power supply. However, due to the complexity in thermodynamic working environments and frequent on/off operations, the proton exchange membrane fuel cells (PEMFCs) suffer from performance degradation, depending on cabin heat balance and power requirements, and the ignorance of the degradation may lead to the performance overestimation. In order to quantify fuel cell degradation in both daily cruise and vehicle-to-grid (V2G) interactions, this study firstly proposes a two-space cabin thermal model to quantify the ambient temperature of vehicle PEMFCs and the power supply from PEMFCs to vehicle HVAC systems. Afterwards, a stack voltage model is proposed to quantify the fuel cell degradation for multiple purposes, such as daily transportation and V2G interactions. Afterwards, the two models are coupled in a community-level based building-vehicle energy network, consisting of twenty single residential buildings, rooftop PV systems, four hydrogen vehicles (HVs), a H2 station, community-served micro power grid, local main power grid, and local H2 pipelines, located in California, U.S.A. Comparative analysis with and without fuel cell degradation is conducted to study the impact of dynamic fuel cell degradation on the energy flexibility and operating cost. Furthermore, a parametrical analysis is conducted on the integrated HV quantity and the grid feed-in tariff to reach trade-off strategies between associated fuel cell degradation costs and grid import cost savings. The results indicate that, in the proposed hydrogen-based building-vehicle energy network, the total fuel cell degradation is 3.16% per vehicle within one year, where 2.50% and 0.66% are caused by daily transportation and V2G interactions, respectively. Furthermore, in the H2-based residential community, the total fuel cell degradation cost is US$6945.2, accounting for 33.4% of the total operating cost at $20770.61. The sensitivity analysis results showed that, when the HV quantity increases to twenty, the fuel cell degradation of each HV decreases to 2.50%, whereas the total fuel cell degradation cost increases to 42.8% of the total operating cost. Last but not the least, the cost saving by V2G interactions can compensate the fuel cell degradation cost when the grid feed-in tariff is reduced by 40%. Research results can provide basic modelling tools on dynamic fuel cell degradation, in respect to vehicle power supply, vehicle HVAC and V2G interactions, together with techno-economic feasibility analysis, paving path for the development of hydrogen energy for the carbon-neutrality transition.

Cover page of Optimizing energy conservation measures in a grocery store using present and future weather files

Optimizing energy conservation measures in a grocery store using present and future weather files

(2019)

Grocery stores are one of the most energy intensive building types, which makes targets for zero net energy (ZNE) particularly challenging. This study builds on a prior computational optimization study to identify combinations of energy conservation measures (ECMs) for an existing grocery store in San Francisco. As the climate changes, also the retrofit recommendations based on simulation results from historical-based weather files may vary. In this paper, we looked at how the optimization results change when accounting for climatechanges over the building’s service life by using future weather files. We found that the expected changes in future weather are sufficient to alter retrofit recommendations. This type of analysis is thus important to ensure that buildings designed now can continue to meet performance objectives into the future.

Cover page of Sensitivity of passive design strategies to climate change

Sensitivity of passive design strategies to climate change

(2018)

Observed global warming trends undermine the conventional practice of using historic weather files, such as Typical Meteorological Year (TMY), to predict building performance during the design process. In order to limit adverse impacts such as improperly sized mechanical equipment or thermal discomfort, it is important to consider how the building will perform in the future. Like all passive design strategies, natural ventilation, relies on local climate to be effective in improving building performance. This paper combines future weather files with whole building energy simulations to assess the sensitivity and feasibility of natural ventilation in providing thermal comfort in three locations, representing different climate types. The results show how building performance, as measured by thermal comfort metrics, changes over time. Natural ventilation can provide a buffer against warming climate, but only to a certain extent. Future weather files are useful for identifying where and when there is a risk that an exclusively passive design is no longer possible.

Cover page of Designing for the future: Are today’s building codes locking in the wrong strategies by using past climate data?

Designing for the future: Are today’s building codes locking in the wrong strategies by using past climate data?

(2018)

California has set goals for zero net energy buildings and greenhouse gas emissions reductions that will be achieved in part through the state’s building energy codes. Decisions about what measures to include in code are informed by building energy models that rely on historical climate data. However, even under moderate emissions scenarios, by 2050 mean temperatures in California are projected to increase by almost 4 degrees Fahrenheit compared to pre-1990 levels and there is evidence that current day temperatures are already shifted from the historical record. Not only do these energy models underlie cost-effectiveness analyses which influence the prescriptive code, they inform building system selection and sizing, and they are the basis for program incentive awards. While the general trends are predictable – as temperatures increase, average cooling energy increases and heating decreases – the effects of future climate on the state’s building policies have not been thoroughly analyzed. To what extent will lower winter heating loads increase the business case for buildings to electrify? Under future climate, are increased cooling efficiency measures cost-effective that aren’t today? How will future climate affect the energy and emissions performance of California’s buildings and what policies can be adopted today to future-proof them? This paper starts to address these questions by examining the performance of prototype buildings within a subset of California’s climate zones under past and future climate scenarios. It models energy efficiency measure variants to these prototypes and compares the energy, emission, cost, and thermal load outcomes under future climate scenarios compared to historical design weather and makes policy recommendations based on the results.

Cover page of PMV-based event-triggered mechanism for building energy management under uncertainties

PMV-based event-triggered mechanism for building energy management under uncertainties

(2017)

This paper provides a study of the optimal scheduling of building operation to minimize its energy cost under building operation uncertainties. Opposed to the usual way that describes thermal comfort using a static range of air temperature, the optimization of a tradeoff between energy cost and thermal comfort predicted mean vote (PMV) index is addressed in this paper. In order to integrate the calculation of the PMV index with the optimization procedure, we develop a sufficiently accurate approximation of the original PMV model which is computationally efficient. We develop a model-based periodic event-triggered mechanism (ETM) to handle the uncertainties in the building operation. Upon the triggering of predefined events, the ETM determines whether the optimal strategy should be recalculated. In this way, the communication and computational resources required can be significantly reduced. Numerical results show that the ETM method is robust with respect to the uncertainties in prediction errors and results in a reduction of more than 60% in computation without perceivable degradation in system performance as compared to a typical closed-loop model predictive control.

Cover page of Measuring the effectiveness of San Francisco's planning standard for pedestrian wind comfort

Measuring the effectiveness of San Francisco's planning standard for pedestrian wind comfort

(2016)

In 1985, San Francisco adopted a wind comfort standard in its Downtown Area Plan in response to increasing concerns about the city’s downtown public open spaces becoming excessively windy. After 30 years of implementation, this study revisits the standard and examines its effectiveness in promoting pedestrian comfort. 701 valid samples were collected from 6 months of field study, which combined surveying pedestrians and on-site collection of microclimate data. Statistical analysis and an assessment using the physiological equivalent temperature (PET) show that 11 mph (4.92 m/s), the comfort criterion in places for walking, performs as an effective determinant of outdoor comfort in San Francisco. This study sheds light on climate-resilience of cities as they have become key urban challenges today.