Prediction of thermal buffer zone effectiveness in real‐size buildings—An experimental and analytical study

Global warming is caused by greenhouse gas (GHG) emissions produced from the use of fossil fuel–based energy sources. Buildings consume about 30% to 35% of the global energy use, which makes buildings a major contributor to the global warming problem. A long‐term plan has been established at the Thermal Processing Laboratory (TPL) at McMaster University to investigate the use of various renewable energy–based technologies to achieve net‐zero energy buildings (NZEB) in Canada. This paper presents results of an investigation of the effectiveness of using a thermal buffer zone (TBZ) in real‐size buildings. A TBZ is a closed passage built around the building that allows air to passively redistribute heat energy from solar radiation received on the south side throughout the building. A TBZ offers an effective solution of the overheating problem usually experienced on the south side of the building, and at the same time, it helps in reducing the heating load of the north side of the building. An experimental setup employing TBZ in a lab‐scale model of a typical building floor has been built. An analytical model of the TBZ has been developed. The experimental data has been used to validate the developed analytical model, which then was used to predict the performance of the TBZ implemented in a real‐size building floor, considering four cases. Results of the first three case studies considering the use of TBZ in cold and hot climates, with and without thermal insulation, show that the predicted effectiveness of TBZ could reach 117% and 72.5% in the winter and summer, respectively. Results of the fourth case study considering the effect of integrating a fan with the TBZ show that a fan is beneficial up to a certain fan power, beyond which the use of the fan would not be feasible. Results presented herein confirm that the TBZ is an effective means of integrating solar energy into buildings, thereby reducing buildings' fossil fuel–based energy consumption.     

The un-utilizability design method for collector-storage walls

The monthly-average auxiliary energy requirement of a building with a collector-storage (Trombe) wall is estimated using upper and lower theoretical limits to system performance. These two limits on the building auxiliary energy requirements result from considering the building and collector-storage wall to have either zero thermal capacity or infinite thermal capacity. With zero thermal capacity, all solar gain in excess of the load, on an instantaneous basis, is not useful and must be dumped. With infinite thermal capacity, the house is able to store any gain in excess of the instantaneous load and use it at some later time. Auxiliary energy use by real systems will fall between these two theoretical bounds. An empirical correlation is presented for the fraction of the load met by the collector-storage wall, F, for systems with finite capacity. The correlation is based on the solar radiation statistic, utilizability. The correlation is compared to yearly TRNSYS simulation results for a wide variety of system types. The root-mean-square difference between F found from the un-utilizability method and from TRNSYS simulations is less than 4 per cent. One advantage of this method over other simplified design methods is that this method covers a much larger range of design parameters.     

Optimal mixed convection for maximal energy recovery with vertical porous channel (solar wall)

A vertical open-ended channel filled with a porous medium, with an imposed heat flux and a heat loss coefficient on one of its walls, is studied numerically. A fan can enhance the self-driven flow, and therefore a mixed convection regime is considered. The objective is to maximize the overall energy recovery (heat transfer to the fluid minus fan power). Correlations are developed for optimal pressure drop to be imposed by the fan and maximal energy recovery, as a function of the Rayleigh number, the channel aspect ratio, and the heat loss coefficient. The optimal allocation of the total energy losses (i.e., sum of the heat loss and fan power) is shown. Potential applications include solar wall and solar chimney used for ventilation and preheating of makeup air in buildings.     

Analytical model for predicting the performance of photovoltaic array coupled with a wind turbine in a stand-alone renewable energy system based on hydrogen

We present the results of an analysis of the performance of a photovoltaic array that complement the power output of a wind turbine generator in a stand-alone renewable energy system based on hydrogen production for long-term energy storage. The procedure for estimating hourly solar radiation, for a clear sunny day, from the daily average solar insolation is also given. The photovoltaic array power output and its effective contribution to the load as well as to the energy storage have been determined by using the solar radiation usability concept. The excess and deficit of electrical energy produced from the renewable energy sources, with respect to the load, govern the effective energy management of the system and dictate the operation of an electrolyser and a fuel cell generator. This performance analysis is necessary to determine the effective contribution from the photovoltaic array and the wind turbine generator and their contribution to the load as well as for energy storage.     

Numerical simulation of underground Seasonal Solar Thermal Energy Storage (SSTES) for a single family dwelling using TRNSYS

A system for capturing and storing solar energy during the summer for use during the following winter has been simulated. Specifically, flat plate solar thermal collectors attached to the roof of a single family dwelling were used to collect solar thermal energy year round. The thermal energy was then stored in an underground fabricated Seasonal Solar Thermal Energy Storage (SSTES) bed. The SSTES bed allowed for the collected energy to supplement or replace fossil fuel supplied space heat in typical single family homes in Richmond, Virginia, USA. TRNSYS was used to model and simulate the winter thermal load of a typical Richmond home. The simulated heating load was found to be comparable to reported loads for various home designs. TRNSYS was then used to simulate the energy gain from solar thermal collectors and stored in an underground, insulated, vapor proof SSTES bed filled with sand. Combining the simulation of the winter heat demand of typical homes and the SSTES system showed reductions in fossil fuel supplied space heating in excess of 64%. The optimization of the SSTES scheme showed that a 15 m³ bed volume, 90% of the south facing roof, and a flow rate of 11.356 lpm through the solar collectors were optimal parameters. The overall efficiency of the system ranged from 50% to 70% when compared to the total useful energy gain of the solar collectors. The overall efficiency was between 6.1% and 7.6% when compared to the total amount of solar radiation incident upon the solar collectors.     

Analysing the benefits of hybridisation and storage in a hybrid solar gas turbine plant

ABSTRACT

A dynamic model of a tower-driven, hybrid solar gas turbine power plant is presented to highlight the benefits of hybrid operation as well as the development a novel plant configuration to improve solar fraction by leveraging a packed-bed thermal energy storage (TES). Relative to solar-only plant, hybridisation increases solar-to-electric efficiency (STE) by 30%. Introduction of a passive packed-bed TES only leads to slight improvement in solar energy utilisation and displacement of solar load. A novel plant configuration, which utilises a recycle stream to charge TES, is presented to improve solar energy utilisation. The recycle stream gives freedom to manipulate the thermal capacity of flow in the tower collector to control collector exit temperature and direct excess solar energy to TES. Employment of the proposed plant configuration leads to a 10.8% and 11.3% improvement in yearly STE and solar fraction, respectively, relative to a plant not utilising such a control scheme.     

Experimental performance assessment of an online energy management strategy for varying renewable power production suppression

Lately, interest in renewable sources, especially wind and solar energy, has shown a significant increase in all over the world that mostly depends on climate-threatening conventional fossil fuels. Besides, hybrid use of these power sources with suitable back-up units provides many advantages compared to sole use of these sources. In this regard, a hybrid system consisting of a wind turbine for utilizing the wind energy, photovoltaic panels for solar energy, fuel cell for providing back-up power and a battery unit for storing the possible excess energy production and supplying the transient load is considered in this study. Experimental assessment of this system in different case studies including the real time measured dynamic power demand of an office block is realized. The collaborative actions of the proposed hybrid system with a fuzzy logic based energy management strategy during fluctuations of renewable-based power production are investigated. Thus, results of this study may be valuable for evaluating the feasibility of stand-alone hybrid renewable energy units for future power systems.     

Reduced storage and balancing needs in a fully renewable European power system with excess wind and solar power generation

The storage and balancing needs of a simplified European power system, which is based on wind and solar power generation only, are derived from an extensive weather-driven modeling of hourly power mismatches between generation and load. The storage energy capacity, the annual balancing energy and the balancing power are found to depend significantly on the mixing ratio between wind and solar power generation. They decrease strongly with the overall excess generation. At 50% excess generation the required long-term storage energy capacity and annual balancing energy amount to 1% of the annual consumption. The required balancing power turns out to be 25% of the average hourly load. These numbers are in agreement with current hydro storage lakes in Scandinavia and the Alps, as well as with potential hydrogen storage in mostly North-German salt caverns.     

Energy management in horticultural applications through the closed greenhouse concept,state of the art

The commercial greenhouse has the highest demand for energy as compared to all other agricultural industry sectors. Here, energy management is important from a broad sustainability perspective. This paper presents the state-of-the-art regarding one energy management concept; the closed greenhouse integrated with thermal energy storage (TES) technology. This concept is an innovation for sustainable energy management since it is designed to maximize the utilization of solar energy through seasonal storage. In a fully closed greenhouse, there is no ventilation which means that excess sensible and latent heat must be removed. Then, this heat can be stored using seasonal and/or daily TES technology, and used later in order to satisfy the heating demand of the greenhouse. This assessment shows that closed greenhouse can, in addition to satisfying its own heating demand, also supply the demand for neighboring buildings. Several energy potential studies show that summer excess heat of almost three times the annual heating demand of the greenhouse. However, many studies propose the use of some auxiliary system for peak load. Also, the assessment clearly point out that a combination of seasonal and short-term TES must be further explored to make use of the full potential. Although higher amount of solar energy can be harvested in a fully closed greenhouse, in reality a semi-closed greenhouse concept may be more applicable. There, a large part of the available excess heat will be stored, but the benefits of an integrated forced-ventilation system are introduced in order to use fresh air as a rapid response for primarily humidity control. The main conclusion from this review is that aspects like energy efficiency, environmental benefits and economics must be further examined since this is seldom presented in the literature. Also, a variety of energy management scenarios may be employed depending on the most prioritized aspect.     

太阳辐射对辐射地板供冷性能的影响

以长沙地区某南向设窗的实验房间为研究对象,采用EnergyPlus能耗模拟软件建立辐射地板结合风机盘管供冷空调系统的数值模型,并通过实验验证该模型的可靠性。通过规范计算及模拟分析量化不同窗墙比下太阳辐射得热对该系统中辐射地板供冷能力及风机盘管系统设计容量的影响,并提出一种在太阳负载下辐射系统容量预测的简化方法。     

Evaluating the limits of solar photovoltaics (PV) in traditional electric power systems

In this work, we examine some of the limits to large-scale deployment of solar photovoltaics (PV) in traditional electric power systems. Specifically, we evaluate the ability of PV to provide a large fraction (up to 50%) of a utility system's energy by comparing hourly output of a simulated large PV system to the amount of electricity actually usable. The simulations use hourly recorded solar insolation and load data for Texas in the year 2000 and consider the constraints of traditional electricity generation plants to reduce output and accommodate intermittent PV generation. We find that under high penetration levels and existing grid-operation procedures and rules, the system will have excess PV generation during certain periods of the year. Several metrics are developed to examine this excess PV generation and resulting costs as a function of PV penetration at different levels of system flexibility. The limited flexibility of base load generators produces increasingly large amounts of unusable PV generation when PV provides perhaps 10–20% of a system's energy. Measures to increase PV penetration beyond this range will be discussed and quantified in a follow-up analysis.     

Modeling and design of a solar thermal system for hybrid cooking application

This paper proposes a hybrid solar cooking system where the solar energy is brought to the kitchen. The energy source is a combination of the solar thermal energy and the Liquefied Petroleum Gas (LPG) that is in common use in kitchens. The solar thermal energy is transferred to the kitchen by means of a circulating fluid. The transfer of solar heat is a twofold process wherein the energy from the collector is transferred first to an intermediate energy storage buffer and the energy is subsequently transferred from the buffer to the cooking load. There are three parameters that are controlled in order to maximize the energy transfer from the collector to the load viz. the fluid flow rate from collector to buffer, fluid flow rate from buffer to load and the diameter of the pipes. This is a complex multi energy domain system comprising energy flow across several domains such as thermal, electrical and hydraulic. The entire system is modeled using the bond graph approach with seamless integration of the power flow in these domains. A method to estimate different parameters of the practical cooking system is also explained. Design and life cycle costing of the system is also discussed. The modeled system is simulated and the results are validated experimentally.     

Performance analysis of a conventional PV/T mixed mode dryer under no load condition

A solar dryer integrated with photovoltaic powered DC fan has been designed and installed at Solar Energy Park, Indian Institute of Technology Delhi. The dryer has been coupled to a solar air heater having blackened surface of absorber for improved energy collection efficiency and a drying chamber with chimney. An analytical expression for characteristic equation for photovoltaic/thermal mixed mode dryer has been derived as a function of design and climatic parameters. The experiment was carried out for forced mode under no load conditions during April 2008 and validated with theoretical results for New Delhi climatic condition. This paper also shows the detailed analysis of thermal energy, exergy, and electrical energy gain by considering four weather conditions (a, b, c, and d type) for five different cities (New Delhi, Bangalore, Mumbai, Srinagar, and Jodhpur) of India. Copyright © 2009 John Wiley & Sons, Ltd.     

Demand side management for remote area power supply systems incorporating solar irradiance model

This paper presents a technique for generating the daily electricity load profile for remote areas in the Middle East from first principles, using diversified demand. The generated load profile includes the energy required to run a small desalination unit to provide the necessary freshwater. Demand side management (DSM) is used in this study to smooth out the daily peaks and fill valleys in the load curve to make the most efficient use of energy resources. Finally, the load profile is compared with real data for six houses collected from Safri area in the Sultanate of Oman. These data may be used as the basis to obtain load profiles of other remote areas of the Middle East since the weather and social factors are similar. The modified hourly variation factor based on weather and economic and social factors of the Middle East is obtained.A solar irradiance model is incorporated in the system to utilise the solar energy available in the Middle East region.     

Thermoeconomic analysis of a standalone solar hydrogen system with hybrid energy storage

A comprehensive thermoeconomic analysis is presented for a novel integrated solar hydrogen energy system for standalone operation. The proposed system includes a solar PVT module (photovoltaic thermal), a FC (Fuel cell) and a battery to meet the electrical load demand and domestic hot water over a year. The PVT component works as a primary energy source converting solar energy into electricity and heat. The excess electrical energy and hot water produced by PVT are consumed for producing hydrogen, which can be stored. The generated hydrogen is fed to the fuel cell to produce electricity and water to satisfy the demand. The proposed system is convenient for different seasons of the year because in all time, produced power satisfy the demand. The first and second laws of thermodynamics are used to evaluate the performance of each component and the overall system. Economic assessment of this system is also conducted considering the net present cost, and the system performance is optimized based on this parameter. The overall electrical efficiency of the system is obtained as 9% and the levelized cost of electricity is determined as $ 0.286/kWh. For a steady operation of system, integrating a battery system is convenient when solar energy is not available for a short term. When there is a longer-term shortage of solar radiation, up to 8 days, the electricity can be supplied by utilizing the hydrogen storage system.     

The effects of solar chimneys on thermal load mitigation of office buildings under the Japanese climate

The study investigated the performance of a solar chimney, which is integrated into a south facade of a one-story building, as well as the effect on the heating and cooling loads of the building by using a CFD simulation and an analytical model. A C programming code was developed for the calculation of the heating and cooling loads by the heat balance method. The analytical equations of a solar chimney were incorporated into the heat balance calculation. The results showed that the fan shaft power requirement was reduced by about 50% in annual total due to the natural ventilation. It was also found that the solar chimney was beneficial to reduce the heating load by about 20% during the heating season. The annual thermal load mitigation was estimated as 12% by taking the increase of the cooling load into account.     

Development of a photovoltaic powered forced circulation grain dryer for use in the tropics

In tropical Africa, a high percentage of crop loss, of the order of 10–50%, occurs during the drying period due to either permanently high relative humidity and sudden rainfall periods, or such periods coinciding with the harvesting season. The traditional sun-drying technique is labour-intensive and requires a lot of land per unit throughput. Due to lack of any control, sun-dried grain is usually prone to overheating, cracking, vermin and dust contamination, rewetting during sudden rains, predation by animals and vandalism by humans. All this leads to significant product loss and impaired quality. A solar maize dryer incorporating a directly coupled photovoltaic (PV) powered d.c. fan was developed and field-tested for small scale use in Malawi, central Africa. The dryer has a capacity of 90 kg and it has been designed to utilise forced air circulation without the use of external power supplies like grid electricity, fossil fuel and batteries, which are either very expensive or non-existent in the rural areas of central Africa. A main design constraint was that the drying air temperature should not exceed 60°C, which is the international drying standard for maize grain used for human consumption. Temperatures in excess of 60°C lead to grain overheating, cracking and subsequent microbial attack. Results showed that the incorporation of a PV-driven d.c. fan provided some form of passive control over the air flow and hence the drying air temperature. The dryer was coupled to a solar air heater having a sun-tracking facility and optimised blackened sisal rope grids for improved energy collection efficiency of the order of 80%. Grain drying with this solar dryer technology, compared with sun drying, reduced the drying time by over 70%. Grain quality, texture, and flour quality and flavour improved significantly with the dryer, as grain was permanently protected from sudden rains, vermin and dust contamination. Although the capital cost of the solar dryer was found to be high (about US$900), the dryer was found to be cost-effective with a payback period of less than one year if it is used to dry grain for purchasing by the Cereal Boards.     

Technico-economic analysis of off grid solar PV/Fuel cell energy system for residential community in desert region

A technico-economic analysis based on integrated modeling, simulation, and optimization approach is used in this study to design an off grid hybrid solar PV/Fuel Cell power system. The main objective is to optimize the design and develop dispatch control strategies of the standalone hybrid renewable power system to meet the desired electric load of a residential community located in a desert region. The effects of temperature and dust accumulation on the solar PV panels on the design and performance of the hybrid power system in a desert region is investigated. The goal of the proposed off-grid hybrid renewable energy system is to increase the penetration of renewable energy in the energy mix, reduce the greenhouse gas emissions from fossil fuel combustion, and lower the cost of energy from the power systems. Simulation, modeling, optimization and dispatch control strategies were used in this study to determine the performance and the cost of the proposed hybrid renewable power system. The simulation results show that the distributed power generation using solar PV and Fuel Cell energy systems integrated with an electrolyzer for hydrogen production and using cycle charging dispatch control strategy (the fuel cell will operate to meet the AC primary load and the surplus of electrical power is used to run the electrolyzer) offers the best performance. The hybrid power system was designed to meet the energy demand of 4500 kWh/day of the residential community (150 houses). The total power production from the distributed hybrid energy system was 52% from the solar PV, and 48% from the fuel cell. From the total electricity generated from the photovoltaic hydrogen fuel cell hybrid system, 80.70% is used to meet all the AC load of the residential community with negligible unmet AC primary load (0.08%), 14.08% is the input DC power for the electrolyzer for hydrogen production, 3.30% are the losses in the DC/AC inverter, and 1.84% is the excess power (dumped energy). The proposed off-grid hybrid renewable power system has 40.2% renewable fraction, is economically viable with a levelized cost of energy of 145 $/MWh and is environmentally friendly (zero carbon dioxide emissions during the electricity generation from the solar PV and Fuel Cell hybrid power system).     

A simplified method for estimating the monthly-average performance of photovoltaic systems

A method is presented for estimating the monthly-average conventional energy displaced by photovoltaic systems. Monthly-average array efficiency is estimated in terms of array parameters and monthly-average meteorological data. Monthly-average excess capacity is estimated for systems having a constant load during daylight hours. If the system does not have battery storage, excess capacity must be dissipated or fed back to the utility. With battery storage, a portion of the excess capacity can be stored for later use. A method is presented for estimating the monthly-average system performance for a constant 24 hr-per-day load with a battery of specified capacity.