Overall thermal energy and exergy analysis of hybrid photovoltaic thermal array

In this paper, overall thermal energy and exergy analysis has been carried out for different configurations of hybrid photovoltaic thermal (PVT) array. The hybrid PVT array (10.08 m × 2.16 m) is a series and parallel combinations of 36 numbers of PV modules. A one-dimensional transient model for hybrid PVT array has been developed using basic heat transfer equations. On the basis of this transient model, an attempt has been made to select an appropriate hybrid PVT array for different climatic conditions (Bangalore, Jodhpur, New Delhi, and Srinagar) of India. On the basis of high grade energy (i.e. overall exergy gain), case-III has been selected as the most appropriate configuration because overall exergy for case-III is 12.9% higher than case-II. The overall thermal energy and exergy gain for Bangalore is 4.54 × 10⁴ kW h and 2.07 × 10⁴ kW h respectively which is highest in comparison to the other cities.     

Maximising the energy output of a PVT air system

Simultaneously generating both electricity and low grade heat, photovoltaic thermal (PVT) systems maximise the solar energy extracted per unit of collector area and have the added benefit of increasing the photovoltaic (PV) electrical output by reducing the PV operating temperature. A graphical representation of the temperature rise and rate of heat output as a function of the number of transfer units NTUs illustrates the influence of fundamental parameter values on the thermal performance of the PVT collector. With the aim of maximising the electrical and thermal energy outputs, a whole of system approach was used to design an experimental, unglazed, single pass, open loop PVT air system in Sydney. The PVT collector is oriented towards the north with a tilt angle of 34°, and used six 110 Wp frameless PV modules. A unique result was achieved whereby the additional electrical PV output was in excess of the fan energy requirement for air mass flow rates in the range of 0.03–0.05 kg/s m². This was made possible through energy efficient hydraulic design using large ducts to minimise the pressure loss and selection of a fan that produces high air mass flow rates (0.02–0.1 kg/s m²) at a low input power (4–85 W). The experimental PVT air system demonstrated increasing thermal and electrical PV efficiencies with increasing air mass flow rate, with thermal efficiencies in the range of 28–55% and electrical PV efficiencies between 10.6% and 12.2% at midday.     

System analysis of a multifunctional PV/T hybrid solar window

The work presented in this article aims to investigate a PV/T hybrid solar window on a system level. A PV/T hybrid is an absorber on which solar cells have been laminated. The solar window is a PV/T hybrid collector with tiltable insulated reflectors integrated into a window. It simultaneously replaces thermal collectors, PV-modules and sunshade. The building integration lowers the total price of the construction since the collector utilizes the frame and the glazing in the window. When it is placed in the window a complex interaction takes place. On the positive side is the reduction of the thermal losses due to the insulated reflectors. On the negative side is the blocking of solar radiation that would otherwise heat the building passively. This limits the performance of the solar window since a photon can only be used once. To investigate the sum of such complex interaction a system analysis has to be performed. In this paper results are presented from such a system analysis showing both benefits and problems with the product. The building system with individual solar energy components, i.e. solar collector and PV modules, of the same size as the solar window, uses 1100 kW h less auxiliary energy than the system with a solar window. However, the solar window system uses 600 kW h less auxiliary energy than a system with no solar collector.     

Technical and economic assessment of grid-independent hybrid photovoltaic–diesel–battery power systems for commercial loads in desert environments

Solar photovoltaic (PV) hybrid system technology is a hot topic for R&D since it promises lot of challenges and opportunities for developed and developing countries. The Kingdom of Saudi Arabia (KSA) being endowed with fairly high degree of solar radiation is a potential candidate for deployment of PV systems for power generation. Literature indicates that commercial/residential buildings in KSA consume an estimated 10–45% of the total electric energy generated. In the present study, solar radiation data of Dhahran (East-Coast, KSA) have been analyzed to assess the techno-economic viability of utilizing hybrid PV–diesel–battery power systems to meet the load requirements of a typical commercial building (with annual electrical energy demand of 620,000 kW h). The monthly average daily solar global radiation ranges from 3.61 to 7.96 kW h/m². NREL's HOMER software has been used to carry out the techno-economic viability. The simulation results indicate that for a hybrid system comprising of 80 kWp PV system together with 175 kW diesel system and a battery storage of 3 h of autonomy (equivalent to 3 h of average load), the PV penetration is 26%. The cost of generating energy (COE, US$/kW h) from the above hybrid system has been found to be 0.149 $/kW h (assuming diesel fuel price of 0.1 $/L). The study exhibits that for a given hybrid configuration, the operational hours of diesel generators decrease with increase in PV capacity. The investigation also examines the effect of PV/battery penetration on COE, operational hours of diesel gensets for a given hybrid system. Emphasis has also been placed on unmet load, excess electricity generation, percentage fuel savings and reduction in carbon emissions (for different scenarios such as PV–diesel without storage, PV–diesel with storage, as compared to diesel-only situation), cost of PV–diesel–battery systems, COE of different hybrid systems, etc.     

Energy matrices analyses of hybrid photovoltaic thermal (HPVT) water collector with different PV technology

In this paper, an attempt has been made to evaluate and compare the energy matrices of a hybrid photovoltaic thermal (HPVT) water collector under constant collection temperature mode with five different types of PV modules namely c-Si, p-Si, a-Si (thin film), CdTe and CIGS. The analysis is based on overall thermal energy and exergy outputs from HPVT water collector. The temperature dependent electrical efficiency has also been calculated under composite climate of New Delhi, India.It is observed that c-Si PV module is best alternative for production of electrical power. Maximum annual overall thermal energy and exergy is obtained for c-Si PV module. The maximum and minimum EPBT of 1.01 and 0.66 years on energy basis is obtained for c-Si and CIGS respectively, whereas on exergy basis maximum EPBT of 5.72 years is obtained for a-Si and minimum of 3.44 in obtained for CIGS PV module. EPF and LCCE increase with increasing the life time of the system.     

Evaluation of optimal power options for base transceiver stations of Mobile Telephone Networks Cameroon

Photovoltaic hybrid systems (PVHS) with 2 days of energy autonomy are shown to be optimal options for the supply of the daily energy demands of 33 base transceiver stations of MTN Cameroon. PVHS were computed for all sites using the technical data for a 150 Wp mono-crystalline module, the site specific hourly load data, the average monthly solar radiation and temperature. Hourly solar radiation data for all sites were downloaded using the solar resource module of HOMER and geographical coordinates of the selected sites. The 3-hourly temperature data available on a website maintained NASA was used to generate average monthly hourly temperatures needed in the calculation of the output of solar modules. The energy costs and breakeven grid distances for possible power options were computed using the Net Present Value Technique and financial data for selected power system components. The results with a PV module cost of 7.5 €/Wp, a remote diesel price of 1.12 €/l, a general inflation rate of 5% and a fuel escalation of 10% showed that the annual operational times of the diesel generator were in the range 3–356 h/year with renewable energy fractions in the range 0.89–1.00. However, only 22 PVHS had two parallel battery strings as stipulated in the request for proposal launched by MTN Cameroon in 2008. The PV array sizes evaluated for the 22 PVHS were found to be the range 2.4–10.8 kWp corresponding to daily energy demands in the range 7.31–31.79 kW h/d. The energy costs and breakeven grid distances determined were in the ranges 0.81–1.32 €/kW h and 10.75–32.00 km respectively.     

A model of residential energy end-use in Canada: Using conditional demand analysis to suggest policy options for community energy planners

We applied conditional demand analysis (CDA) to estimate the average annual energy use of various electrical and natural gas appliances, and derived energy reductions associated with certain appliance upgrades and behaviours. The raw data came from 9773 Canadian households, and comprised annual electricity and natural gas use, and responses to >600 questions on dwelling and occupant characteristics, appliances, heating and cooling equipment, and associated behaviours. Replacing an old (>10 years) refrigerator with a new one was estimated to save 100 kW h/year; replacing an incandescent lamp with a CFL/LED lamp was estimated to save 20 kW h/year; and upgrading an old central heating system with a new one was estimated to save 2000 kW h/year. This latter effect was similar to that of reducing the number of walls exposed to the outside. Reducing the winter thermostat setpoint during occupied, waking hours was estimated to lower annual energy use by 200 kW h/°C-reduction, and lowering the thermostat setting overnight in winter relative to the setting during waking hours (night-time setback) was estimated to have a similar effect. This information may be used by policy-makers to optimize incentive programs, information campaigns, or other energy use change instruments.     

Performance comparison of a double-axis sun tracking versus fixed PV system

In the present study, performance results of two double axis sun tracking photovoltaic (PV) systems are analyzed after one year of operation. Two identical 7.9 kWp PV systems with the same modules and inverters were installed at Mugla University campus in October 2009. Measured data of the PV systems are compared with the simulated data. The performance measurements of the PV systems were carried out first when the PV systems were in a fixed position and then the PV systems were controlled while tracking the sun in two axis (on azimuth and solar altitude angles) and the necessary measurements were performed. Annual PV electricity yield is calculated as 11.53 MW h with 1459 kW h/kWp energy rating for 28 fixed tilt angle for each system. It is calculated that 30.79% more PV electricity is obtained in the double axis sun-tracking system when compared to the latitude tilt fixed system. The annual PV electricity fed to grid is 15.07 MW h with 1908 kW h/kWp for the double axis sun-tracking PV system between April-2010 and March-2011. The difference between the simulated and measured energy values are less than 5%. The results also allow the comparison of different solutions and the calculation of the electricity output.     

Grape drying by using hybrid photovoltaic-thermal (PV/T) greenhouse dryer: An experimental study

A hybrid photovoltaic-thermal (PV/T) greenhouse dryer of 100 kg capacity has been designed and constructed at Solar Energy Park, Indian Institute of Technology, New Delhi (28°35′N, 77°12′E, 216 m above MSL), India. The developed dryer has been used to dry the Thompson seedless grapes (Mutant: Sonaka) when DC fan was in operation for forced mode convection. The drying of grapes was also performed in open as well as shade for comparison. Experiments were conducted for drying of grapes in the month of April, 2007. Various hourly experimental data namely moisture evaporated, grape surface temperatures, ambient air temperature and humidity, greenhouse air temperature and humidity, etc. were recorded to evaluate heat and mass transfer for the proposed system. It has been found that the value of the convective heat transfer coefficient for grapes (GR-I) lies between 0.26 and 0.31 W/m² K for greenhouse and 0.34–0.40 W/m² K for open conditions, respectively and that for grapes (GR-II) lies between 0.45–1.21 W/m² K for greenhouse and 0.46–0.97 W/m² K for open conditions, respectively.     

Energy and efficiency analysis of environmental heat sources and sinks: In-use performance

A detailed analysis of the heating and cooling performance of environmental heat sources and sinks is presented for 12 low-energy buildings in Germany. In particular, the analysis focuses on the given temperature levels and the efficiency performance of the environmental heat sources and sinks in summer and winter. The investigated buildings employ environmental heat sources and sinks – such as the ground, groundwater, rainwater and the ambient air – in combination with thermo-active building systems (TABS). These concepts are promising approaches for slashing the primary energy use of buildings without violating occupant thermal comfort. A limited primary energy use of about 100 kW h_prim/(m_net² a) as a target for the complete building service technology (HVAC and lighting) was postulated for all buildings presented. With respect to this premise, comprehensive long-term monitoring in fine time-resolution occurred over a period from two to five years. An accompanying commissioning of the building performance took place. Measurements include water supply and return temperatures of the environmental heat sources/sinks, the generated heating and cooling energy, efficiencies of the system, and local climatic site conditions. The comparative evaluation of the systems in all buildings identifies weak points and success factors of the plant. Besides, it characterizes the single component and points out further potential for optimization measures. The annual efficiency performance of the geothermal heat sources and sinks results in a seasonal performance factor of 8–10 kW h_therm/kW h_end, where the end energy use is electricity. The ground, groundwater, rainwater and even the ambient air constitute efficient heat sources/sinks. Energy is needed only for distributing the heat and cold and not for its generation. The choice of suitable plant components, the accurate design of the hydraulic system and the correct dimension of the environmental heat source/sink play a central role in achieving higher efficiencies.     

Modelling and analysis of air-cooled reciprocating chiller and demand energy savings using passive cooling

The study presents modelling and analysis of air-cooled chiller system in an office building at Central Queensland University in Rockhampton, Australia. EnergyPlus, building energy simulation software, has been used to model and to simulate the energy savings. Base case cooling energy has been compared with measured data. The simulated results show a reasonable agreement with the measured data. As a passive cooling means, the effect of economiser usages and pre-cooling have been simulated and analysed to assess annual demand savings for an energy intensive office building at Rockhampton, Australia. It was found that implementation of the pre-cooling and economiser system could save 115 kW/m²/month and 72 kW/m²/month total cooling energy and 26 kW/m²/month and 42 kW/m²/month chiller energy, respectively.     

Hot air balloon engine

This paper describes a solar powered reciprocating engine based on the use of a tethered hot air balloon fuelled by hot air from a glazed collector. The basic theory of the balloon engine is derived and used to predict the performance of engines in the 10 kW to 1 MW range. The engine can operate over several thousand metres altitude with thermal efficiencies higher than 5%. The engine thermal efficiency compares favorably with the efficiency of other engines, such as solar updraft towers, that also utilize the atmospheric temperature gradient but are limited by technical constraints to operate over a much lower altitude range. The increased efficiency allows the use of smaller area glazed collectors. Preliminary cost estimates suggest a lower $/W installation cost than equivalent power output tower engines.     

New lightweight bipolar plate system for polymer electrolyte membrane fuel cells

The use of metal based bipolar plates in polymer electrolyte membrane (PEM) fuel cells, with an active coating on titanium to reduce voltage losses due to the formation of passive layers has been demonstrated. Lifetime data in excess of 8000 h has been achieved and power densities in excess of 1.8 kW dm⁻³ and 1 kW kg⁻¹ are predicted.     

Hybrid electrochemical capacitors based on polyaniline and activated carbon electrodes

The performance of a newly designed, polyaniline–activated carbon, hybrid electrochemical capacitor is evaluated. The capacitor is prepared by using polyaniline as a positive electrode and activated carbon as a negative electrode. From a constant charge–discharge test, a specific capacitance of 380 F g⁻¹ is obtained. The cycling behaviour of the hybrid electrochemical capacitor is examined in a two-electrode cell by means of cyclic voltammetry. The cycle-life is 4000 cycles. Values for the specific energy and specific power of 18 Wh kg⁻¹ and 1.25 kW kg⁻¹, respectively, are demonstrated for a cell voltage between 1 and 1.6 V.     

Heat transfer during air flow in aluminum foams

This paper presents experimental heat transfer coefficients measured during air flow heating in seven different aluminum open-cell foam samples with different number of pores per inch (PPI), porosity and foam core height under a wide range of air mass velocity. Three imposed heat fluxes are considered for each foam sample: 25.0, 32.5 and 40.0 kW m^?2. The collected heat transfer data are analyzed to obtain the global heat transfer coefficient and the normalized mean wall temperature. A model from the open literature has been selected and compared with the experimental database. A new simple heat transfer model, based on the present data, for the global heat transfer coefficient and the foam-finned surface area efficiency estimations has then been developed and compared against the experimental measurements.     

National-scale wave energy resource assessment for Australia

A nationally consistent wave resource assessment is presented for Australian shelf (<300 m) waters. Wave energy and power were derived from significant wave height and period, and wave direction hindcast using the AusWAM model for the period 1 March 1997 to 29 February 2008 inclusive. The spatial distribution of wave energy and power is available on a 0.1° grid covering 110–156° longitude and 7–46° latitude. Total instantaneous wave energy on the entire Australian shelf is on average 3.47 PJ. Wave power is greatest on the 3000 km-long southern Australian shelf (Tasmania/Victoria, southern Western Australia and South Australia), where it widely attains a time-average value of 25–35 kW m^?1 (90th percentile of 60–78 kW m^?1), delivering 800–1100 GJ m^?1 of energy in an average year. New South Wales and southern Queensland shelves, with moderate levels of wave power (time-average: 10–20 kW m^?1; 90th percentile: 20–30 kW m^?1), are also potential sites for electricity generation due to them having a similar reliability in resource delivery to the southern margin. Time-average wave power for most of the northern Australian shelf is <10 kW m^?1. Seasonal variations in wave power are consistent with regional weather patterns, which are characterised by winter SE trade winds/summer monsoon in the north and winter temperate storms/summer sea breezes in the south. The nationally consistent wave resource assessment for Australian shelf waters can be used to inform policy development and site-selection decisions by industry.     

Decentralized/stand-alone hybrid Wind–Diesel power systems to meet residential loads of hot coastal regions

In view of rising costs, pollution and fears of exhaustion of oil and coal, governments around the world are encouraging to seek energy from renewable/sustainable energy sources such as wind. The utilization of energy from wind (since the oil embargo of the 1970s) is being widely disseminated for displacement of fossil fuel produced energy and to reduce atmospheric degradation. A system that consists of a wind turbine and Diesel genset is called a Wind–Diesel power system.The literature indicates that the commercial/residential buildings in Saudi Arabia consume an estimated 10–40% of the total electric energy generated. In the present study, the hourly mean wind-speed data of the period 1986–1997 recorded at the solar radiation and meteorological station, Dhahran (26°32′N, 50°13′E in the Eastern Coastal Region of Saudi Arabia), has been analyzed to investigate the potential of utilizing hybrid (Wind–Diesel) energy conversion systems to meet the load requirements of a hundred typical two bedroom residential buildings (with annual electrical energy demand of 3512 MWh). The long term monthly average wind speeds for Dhahran range from 4.2 to 6.4 m/s. The hybrid systems considered in the present case study consist of different combinations/clusters of 150 kW commercial wind machines supplemented with battery storage and Diesel back-up. The deficit energy generated by the Diesel generator (for different battery capacities) and the number of operational hours of the Diesel system to meet a specific annual electrical energy demand of 3512 MWh have also been presented. The evaluation of the hybrid system shows that with seven 150 kW wind energy conversion system (WECS) and one day of battery storage, the Diesel back-up system has to provide 21.6% of the load demand. Furthermore, with three days of battery storage, the Diesel back-up system has to provide 17.5% of the load demand. However, in the absence of battery storage, about 37% of the load needs to be provided by the Diesel system. The study also places emphasis on the monthly average daily energy generation from different sizes (150 kW, 250 kW, 600 kW) of wind machines to identify the optimum wind machine size from the energy production point of view. It has been noted that for a given 6 MW wind farm size (for 50 m hub height), a cluster of forty 150 kW wind machines yields about 48% more energy as compared to a cluster of ten 600 kW wind machines.     

Energy and exergy analysis of hybrid photovoltaic thermal water collector for constant collection temperature mode

This paper deals with the analysis of hybrid photovoltaic thermal (PVT) water collectors under constant collection temperature mode unlike constant flow rate mode. The analysis has been carried out in terms of thermal energy, electrical energy and exergy gain for two different configurations namely case A (collector partially covered by PV module) and case B (collector fully covered by PV module). The results are compared with the conventional flat plate collector (FPC). The effect of collector area covered by PV module on the performance of hybrid PVT water collector has been studied. The characteristic equations have also been developed for both the cases.It has been observed that case A is more favorable for thermal energy point of view, while case B is suitable for electricity generation. On the basis of the numerical calculations the annual thermal energy gain is found to be 4167.3 and 1023.7 and annual net electrical energy gain is 320.65 and 1377.63 for cases A and B respectively. The annual overall thermal energy gain is decreased by 9.48% and an annual overall exergy gain is increased by 39.16% from case A to case B.     

Improvement of PV module optical properties for PV-thermal hybrid collector application

Photovoltaic-thermal collectors (or PV-T collector) are hybrid collectors where PV modules are integrated as an absorber of a thermal collector in order to convert solar energy into electricity and usable heat at the same time. In most of the cases, the hybrid collectors are made by the superposition of a PV module on the thermal absorber of a solar collector. In this paper, the approach is different and is to analyze thermal and optical properties related to both PV and solar thermal functions in order to identify an optimum combination leading to a maximum overall efficiency. Indeed, although these two functions do not exploit the same range of radiation wavelengths, thermal and PV functions are not so complementary due to photo-conversion thermal dependency. In this context, an alternative PV cell lamination has been developed with increased optical and thermal performance. The improvements were evaluated around 2 mA/cm² in terms of current density in comparison to a standard module encapsulation. Based on this technique, a real size PV-T module has been built and tested at Fraunhofer solar test facilities. The results show a global efficiency of the PV-T collector above 87% (79% thermal efficiency plus 8.7% electrical efficiency, based on the absorber area).     

Potential of geothermal heat exchangers for office building climatisation

Low depth geothermal heat exchangers can be efficiently used as a heat sink for building energy produced during summer. If annual average ambient temperatures are low enough, direct cooling of a building is possible. Alternatively the heat exchangers can replace cooling towers in combination with active cooling systems. In the current work, the performance of vertical and horizontal geothermal heat exchangers implemented in two office building climatisation projects is evaluated.A main result of the performance analysis is that the ground coupled heat exchangers have good coefficients of performance ranging from 13 to 20 as average annual ratios of cold produced to electricity used. Best performance is reached, if the ground cooling system is used to cool down high temperature ambient air. The maximum heat dissipation per meter of ground heat exchanger measured was lower than planned and varied between 8 W m^?1 for the low depth horizontal heat exchangers up to 25 W m^?1 for the vertical heat exchangers.The experimental results were used to validate a numerical simulation model, which was then used to study the influence of soil parameters and inlet temperatures to the ground heat exchangers. The power dissipation varies by ±30% depending on the soil conductivity. The heat conductivity of vertical tube filling material influences performance by another ±30% for different materials. Depending on the inlet temperature level to the ground heat exchanger, the dissipated power increases from 2 W m^?1 for direct cooling applications at 20 °C up to 52 W m^?1 for cooling tower substitutions at 40 °C. This directly influences the cooling costs, which vary between 0.12 and 2.8€ kW h^?1.As a result of the work, planning and operation recommendations for the optimal choice of ground coupled heat exchangers for office building cooling can be given.