1、 E Elshik is a PhD student in the Department of Mechanical Engineering Science, University of Johannesburg, SA, CR Bester is a Senior Lecturer in the Department of Mechanical Engineering Science, University of Johannesburg, SA, AL Nel is a Professor in the Department of Mechanical Engineering Scienc
2、e, University of Johannesburg, SA Appropriate Solar Spectrum Usage: The Novel Design of a Photovoltaic Thermal System Ebrahim Elshik Rudolf Bester Andre Nel ABSTRACT The path towards zero energy buildings is fraught with many challenges, the onsite renewable energy production to drive consumer appli
3、ances that are not low or zero energy is an important challenge. Therefore, developing the energy production such that the production mode is matched to the usage mode is the simplest manner to improve efficiency. As such, energy consumption for lighting could be significantly reduced by optimizing
4、the buildings design to maximize direct daylight usage, similarly cooking using solar stoves, or water heating using solar geysers eliminates the need for PV cells to generate electricity. The most important energy consumption in most buildings is HVAC (accounting for approximately 40% of a building
5、s energy consumption) which can be addressed with the use of a solar power absorption chiller. This article introduces the design of a novel solar concentrated photovoltaic thermal (CPVT) system that produces electricity and thermal energy simultaneously from the same surface area. The goal of the p
6、roposed system is to provide sufficient heat for an absorption cooling system, water heating as well as to produce electricity in a cost effective way. The CPVT system is designed to operate over a wide spectrum (400nm upward contains around 90% of the incident solar radiation spectrum). In the prop
7、osed system, solar irradiation is highly concentrated (to the equivalent intensity of approximately 100 suns) onto a single point, using a dual axis sun tracking concentrator with a Fresnel lens. A filter then separates the infrared (IR) from the visible light (VL) components using an imaging lens (
8、viz. a hot mirror which has approximately a 98% filter efficiency). The IR is then utilized for heating while the VL components power the PV cell. The efficiency of the electricity generation in the PV cell improves when the IR component is removed from the incident solar irradiance. High-temperatur
9、e high pressure water, at approximately 95-120oC (203248oF), is generated by the IR and serves as a heat source for the absorption cooling system (lithium bromide water / ammonia-water). The proposed system is expected to deliver electricity at the rate of 0.08 W/cm2 (0.2032 W/in2) of PV cell area,
10、and around 0.04W/cm2 (0.1.016 W/in2) collector area. Given that the ratio of collector area to PV cell area is 9:1 this allows us to design the relative size to suit the building requirements. INTRODUCTION “Net Zero Energy Building” (NZEB) is an ambitious title that is unlikely to be obtained withou
11、t scrupulous attention to every aspect of the design of a building. A typical buildings path towards being a NZEB starts with achieving significant energy load reductions by means of building design optimization and the usage of low or zero energy consumption systems, plus meeting the remaining load
12、s with on-site clean energy generation. Traditionally the onsite renewable energy has been produced in one of two forms: thermal energy using solar thermal collectors or electricity generation using PV cells which are used for different applications. Transforming the energy from the produced form (t
13、hermal/electricity) into the form required often results in a significant loss. The best and most efficient technique for renewable energy production in buildings is the one that is developed to directly drive the consumer appliances. For example, energy consumption for lighting could be significant
14、ly reduced by optimizing the buildings design to maximize direct daylight usage, similarly cooking using direct collection solar stoves, or water heating using solar geysers, all these eliminate the need for PV cells. The most important energy consumption in many buildings is HVAC systems which can
15、be addressed by means of a solar power driven absorption chiller. Since HVAC and water heating contribute significantly to the energy consumption in both commercial and residential buildings (around 27-40%) (C2ES 2014), treating these while still supplying electricity for other uses can be seen as a
16、 key to achieving the NZEB. Thermally driven absorption chillers are considered as one of the greenest HVAC systems primarily because they can be powered directly by solar thermal energy. Most of the absorption chiller types provide an opportunity to produce almost free space heating and cooling if
17、a suitable solar thermal source is available. For example, a 17.585 kW (5 TR) absorption chiller provides chilled water at inlet of 12C (53.6F) and outlet of 7C (44.6F) when the hot medium ranged from 70-95C (158-203F) supplied at the generator. From a production perspective, PV cells present a clea
18、n renewable energy that should be used widely. One of the main limiting factors which has slowed the acceptance and more extensive use of PV cells in applications is the relatively low conversion efficiency of PV cells (Saad and Masud 2009). The main reason behind low PV cell efficiency is: the heat
19、 buildup under the PV cells as well as the PV cells solar spectrum operating range. Commercial single layer PV cells are predominantly designed to operate in the VL range of 400-700 nm as illustrated in figure 1. Both the above mentioned reasons are the result of the solar spectrum IR component. Fig
20、ure 1 VPC-AFM value with light wavelength of poly-Si solar cell devices (Heo 2013) The new development of the multi-layer PV cell which is designed to operate over a wider solar spectrum has overcome the low conversion efficiency of the traditional PV cells. These newer PV cells achieve efficiencies
21、 approaching 40% in the laboratory. This new trend still faces many challenges, one of which is the high cost of the fabricated material compare to the retail price of the single layer commercial PV cell (McGehee 2012). Another challenge is related to the undesired heat build up under the PV cell su
22、rface. One of the leading manufacturers of a sophisticated multi-layer PV cell states that as much as 39% of the solar spectrum which strikes their PV cell surface is transformed into heat (Kribus et al. 2006). Although, the PVT and CPVT concept of removing the undesired heat of the incident radiati
23、on from the PV cells and turning it into a useful energy has gained more attention, harvesting this energy is still quite limited. An increase of only 1C in the PV cell temperature results in a 1% decrease of the PV efficiency (Hollick 2011). Moreover, the PV structure is only capable of tolerating
24、temperatures up to 100C (212F) (in the case of the most recent multi-layer PV cells (Mittelman et al. 2007). In response to this dilemma, this project seeks to develop a novel PVT design that uses an inline filtering element. PRINCIPLES Solar Spectrum Utilization The solar irradiance striking the ea
25、rths surface is divided based on wavelength and energy contents into three ranges: ultraviolet (UV) 5-10% energy content (below 400 nm), visible light (VL) around 40% energy content (400 to 700 nm) and infrared (IR) 50% of the energy content (above 700 nm). The target of the proposed system is to ge
26、t the most out of the sun light energy, mainly by using various components of the solar spectrum in a more appropriate way. Mostly the proposed system focuses on the IR and the VL which counts for more than 90% of the total spectrum. The IR range is considered as the only source of heat in the solar
27、 spectrum and it is most useful for the thermal energy collection. While, VL range shows the highest electricity production from the commercial single layer PV cells. To this end, the appropriate utilization of solar irradiance should be governed by the ability to collect the energy across the total
28、 spectrum by means of utilizing the VL for electricity production and the IR for thermal energy collection as illustrated in figure2: Figure 2 (a) Concentrated solar collector concept 2 (b) PV cell electricity production concepts 2 (c) Proposed concentrated PVT concept Figure 2 explanations: Figure
29、2 (a): Solar thermal collector concentrates sun light in order to increase thermal density results in the production of thermal energy mainly using IR component Figure 2 (b): PV cell operates mostly on the VL range with a reduction in performance due to the incident IR component. Figure 2 (c): The p
30、roposed system aims to concentrate the sun light while separating the IR and the VL. The IR component is used for heating and the VL light for electricity production. Reducing the incident IR component from the surface of PV cell may even increase the overall PV cell efficiency. Water Sun Light Abso
31、rption Water properties have been extensively studied in the past century; several studies have been performed to investigate the absorption of solar irradiance by water in different fields (space, ocean, swimming pool, etc.). Most studies agreed that water predominantly absorbs the IR spectral comp
32、onents and very little of the VL components (Chaplin 2015; Prahl 1998; Geek 2007). Figure 3 demonstrates this phenomenon as it shows that the waters lowest absorption coefficient occurs in the VL range; in other words it is very transparent toward the VL light, while still heated up by the IR portio
33、n. Figure 3 Water absorption coefficient across sun spectrum (Chaplin 2015) IR/VL Light Filtration Imaging theory has confirmed the possibility to easily separate solar spectrum components (UV, VL and IR). Whereby, several mirrors and lenses are highly sensitive to specific range of light wavelength
34、 has been commercially produced. For instance, “hot mirror” and the “cold mirror” shows a very high efficiency up to 98% to reflect the IR and VL lights respectively as illustrated in figure 4. Figure 4 (a) hot mirror trasmission and reflection and incident angle 4 (b) cold mirror trasmission and re
35、flection and incident angle DESIGN AND LAYOUT Figure 5 illustrates the proposed system layout which consists of 4 major elements: Figure 5 system layout (a) Fresnel lens specification (b) water container (c) PV cell (d) solar tracker holder 1. Concentrator (detailed specification and layout illustra
36、ted in figure 5 (a) ) To maximize the harvested thermal energy it is mandatory to increase the total heat density by concentrating sun light. There are different types of concentrated solar collector (CSC) mostly differ based on several parameters but most importantly driven by the required output t
37、emperature. The selection of this concentrator was based on two parameters: a) Single point collectors (dishes, Fresnel lens collectors and helio towers) have the highest output temperature ever achieved (Stine et al. 2001). Even though the Fresnel lens is considered among the highest CSC temperatur
38、e achievers, it is also one of the cheapest, simplest and lightest concentrators. b) The feature of the buildings, namely the buildings foot print, occasionally limits the option of which concentrator can be used. The fact that it is a PVT system leads to a preference for the Fresnel lens over the d
39、ish concentrator. 2. Water Container Water was selected as the thermal energy collection medium due to its: high thermal conductivity; high absorption of the IR spectrum; high transmission of the VL spectrum. Water light absorption and transmission are best described by Beers law using the following
40、 equations (Chaplin 2015): A = - log10(I/Io) = l. (1) I/Io = 10-L (2) 3. Sun Light Filtration As illustrated in figure 5 (b) the water container has a top glass and a bottom glass: Top glass: a very clear and highly light transmitted glass cover is used. It allows most of the concentrated light to p
41、enetrate to the water. The selected cover glass has a transmission efficiency of 90% across the solar spectrum from 300 to 2000nm. Bottom glass: a hot mirror was selected to reflect the unabsorbed IR component back into the water. This process will allow as much of the IR component to be absorbed as
42、 possible while still using thin layer of water, which in turn minimize the amount of the VL absorbed by the water. 4. Electricity Generation PV Since a standard PV cell reaches its highest efficiency with incident radiation in the VL range and shows a negative response to IR (responsible for the un
43、desired heat buildup in the PV cell) the removal of the IR component is essential for optimal performance. Thus removing the IR component and its associated heat from the surface of the PV cell allows the usage of the single layer commercial PV illustrated in figure 5 (c). The aim of developing the
44、reduced area concentrator is that this suits many buildings footprints and allows us to use a limited lens size of only 1000 mm (39.37 in) diameter. To retain the correct incident VL at the PV cell surface the concentration ratio of the Fresnel lens (based on incident area) to the PV cell area must
45、be 9:1 area ratio, therefore the cell area is 300 x 300 mm (11.81 x 11.81 in). Electricity output could be calculated using below equation: PEL= Io x lens x top Glass x 10-L(water hight) x hot mirror x pv (3) 5. Solar Tracker In order to achieve the highest possible concentration (+100 sun) it is im
46、portant to use a dual axis solar tracker. The selected tracker consists of 3 major parts as illustrated in figure 6: 1. sun sensors on the four directions, 2. control which receives the sensor signal and transfers it to the actuators 3. two actuators 1500N (337.21 lbf) each, one for north- south and
47、 one for East- West Figure 6 Solar tracker kit SYSTEM OPERATION The proposed system was designed to operate in the following manner: 1. Light strikes the lens surface at a 0 incident angle (due to the usage of a dual axis solar tracker). 2. The lens concentrates 90% of the sun light at a distance of
48、 1300 mm (51.18 in) away from the lens surface into a focal point of 70 mm (2.76 in) diameter. 3. 90% of the concentrated light penetrates through the cover glass into the water. 4. Water with a thickness of 100 mm (3.94 in) absorbs most of the IR component and a small portion (38%) of the VL. 5. Th
49、e remaining light strikes the hot mirror, 98% of the IR component is reflected back into the water and the VL portion continues to the PV cell. 6. The water is circulated from the cold water tank into the heating container and back to the hot water tank. By monitoring the container inlet and outlet temperatures by means of thermocouples the circulation tempo of the pump is managed. For this design the stored water temperature was set at 95C (203F) to suit the operation requirement of the common 17.585 kW (5 TR) absorbtion c
