Qiyuan Li

Experimental investigation of a nanofluid absorber employed in a low-profile, concentrated solar thermal collector

Recent studies [1-3] have demonstrated that nanotechnology, in the form of nanoparticles suspended in water and organic liquids, can be employed to enhance solar collection via direct volumetric absorbers. However, current nanofluid solar collector experimental studies are either relevant to low-temperature flat plate solar collectors (<100 °C) [4] or higher temperature (>100 °C) indoor laboratory-scale concentrating solar collectors [1, 5]. Moreover, many of these studies involve in thermal properties of nanofluid (such as thermal conductivity) enhancement in solar collectors by using conventional selective coated steel/copper tube receivers [6], and no full-scale concentrating collector has been tested at outdoor condition by employing nanofluid absorber [2, 6]. Thus, there is a need of experimental researches to evaluate the exact performance of full-scale concentrating solar collector by employing nanofluids absorber at outdoor condition. As reported previously [7-9], a low profile (<10 cm height) solar thermal concentrating collector was designed and analysed which can potentially supply thermal energy in the 100-250 °C range (an application currently met by gas and electricity). The present study focuses on the design and experimental investigation of a nanofluid absorber employed in this newly designed collector. The nanofluid absorber consists of glass tubes used to contain chemically functionalized multi-walled carbon nanotubes (MWCNTs) dispersed in DI water. MWCNTs (average diameter of 6-13 nm and average length of 2.5-20 μm) were functionalized by potassium persulfate as an oxidant. The nanofluids were prepared with a MCWNT concentration of 50 ± 0.1 mg/L to form a balance between solar absorption depth and viscosity (e.g. pumping power). Moreover, experimentally comparison of the thermal efficiency between two receivers (a black chrome-coated copper tube versus a MWCNT nanofluid contained within a glass tubetube) is investigated. Thermal experimentation reveals that while the collector efficiency reduced from 73% to 54% when operating temperature increased from ambient to 80 °C by employing a MWCNT nanofluid receiver, the efficiency decreased from 85% to 68% with same operating temperature range by employing black chrome-coated copper tube receiver. This difference can mainly be explained by the reflection optical loss off and higher thermal emission heat loss the front surface of the glass tube, yielding a 90% of transmittance to the MWCNT fluid and a 0.9 emissivity of glass pipe. Overall, an experimental investigation of the performance of a low profile solar collector with a direct volumetric absorber and conventional surface absorber is presented. In order to bring nanotechnology into industrial and commercial heating applications,

A new optical concentrator design and analysis for rooftop solar applications

In this paper, a new type of linear focus, linear-tracking, catadioptric concentrator system is proposed and analysed for roof-integrated solar thermal applications. The optical concentrator designs have a focal distance of less than 10cm and are analysed using optical simulation software (Zemax). The results show that a relatively high concentration ratio (4.5 ~ 5.9 times) can be obtained and that the concentrators are capable of achieving an average optical efficiency around 66 - 69% during the middle 6 hours of a sunny day (i.e. a day with ~1000W/m2 global irradiance). Optical efficiency is analysed for perfect and non-ideal optical components to predict the collector performance under different ‘practical’ circumstances. Overall, we intend for this paper to catalyse the development of rooftop solar thermal concentrators with compact form factors, similar to PV panels.

Design and indoor testing of a compact optical concentrator

Abstract. We propose and analyze designs for stationary and compact optical concentrators. The designs are based on a catadioptric assembly with a linear focus line. They have a focal distance of around 10 to 15 cm with a concentration ratio (4.5 to 5.9 times). The concentrator employs an internal linear-tracking mechanism, making it suitable for rooftop solar applications. The optical performance of the collector has been simulated with ray tracing software (Zemax), and laser-based indoor experiments were carried out to validate this model. The results show that the system is capable of achieving an average optical efficiency of around 66% to 69% during the middle 6 (sunniest) h of the day. The design process and principles described in this work will help enable a new class of rooftop solar thermal concentrators.

Development of a mobile groundwater desalination system for communities in rural India

The consumption of saline groundwater has contributed to a growing incidence of renal diseases, particularly in coastal communities of India. Although reverse osmosis (RO) is routinely used to remove salt from groundwater, conventional RO systems (i.e. centralized systems using spiral wound RO elements) have limited utility in these communities due to high capital and maintenances costs, and lack of infrastructure to distribute the water. Consequently, there is a need to develop an appropriate solution for groundwater treatment based on small-scale, mobile and community-led systems. In this work, we designed a mobile desalination system to provide a simple platform for water treatment and delivery of goods to rural communities. The system employs tubular RO membranes packed in a single, low-profile vessel which fits below the cargo space. The low-profile enables minimal intrusion on the space available for the transportation of goods. Pressure is delivered by a belt driven clutch pump, powered by the engine. Water is treated locally by connecting the intake to the village well while the vehicle idles. A combined numerical and experimental approach was used to optimise the module/system design, resulting in ∼20% permeate flux enhancement. Experimental results revealed that the system can produce 16 L per square meter of membrane area per hour (LMH) at a salinity level of 80 ppm from a ∼2000 ppm groundwater when it is feed at 1 m3/h at 8 bars. This indicates that a vehicle equipped with 12 m2 of tubular RO membranes can deliver 1 m3 of drinkable water by using ∼0.9 L of diesel. Assuming eight such systems could be implemented in a community to fulfil the water demands for a village with 2000 residents, a social business study revealed that a payback time of 2.5 years is achievable, even if the sale price of the water is relatively low, USD 0.18 (Rs 12, which is half of the lowest market price) per 20 L, including providing a goods transportation service at price of USD 5.25 (Rs 350) per 100 km.

Bright ideas for the future of industrial heating

The sun represents an abundant, sustainable—yet critically underutilized—energy resource. At present, we use less than 0.0002% of Earth’s solar resources to power our economy,1 and the majority of what is used goes toward the relatively low-tech goal of raising the temperature of water by 30–50C. According to an International Energy Agency report, solar thermal water heaters had a global installed capacity of 400GWth in 2014 (180GWth in China alone), more than double the 180GWe of photovoltaics installed in the same year.2 Photovoltaics—which convert solar energy into an electrical current—are an economically viable way to produce electricity. However, they harvest only about one-third the energy per square meter as compared with solar thermal collectors. The demand for heat in factories and mines, and in chemical, metal, and agricultural industrial operations, is in excess of 11 million GWth hours.2 In total, these activities consume 40% of our energy resources and generate much of our society’s wealth. As such, it is worth investing in and deploying sustainable energy solutions for this market. Since heat is hard to transport over large distances, solar technologies represent a clean, distributed means for industries to exploit their unused real estate (large, flat rooftops, for example). Rooftop heat production also represents an inimitable opportunity for industries to convert volatile energy expenses into stable, internal capital investments. The use of parabolic troughs (where solar thermal collectors lined with mirrors heat a tube of fluid) has made small inroads into this market for the supply of 100–400C heat.2 However, trough systems are difficult to mount on rooftops. Thus, there is a clear and present demand for new technologies that can easily integrate with rooftops to offset fossil fuel consumption for industrial heating needs. The Australian Renewable Energy Agency has funded a

A hybrid PV/T collector using spectrally selective absorbing nanofluids

3.2 million (AUS

Experimental and numerical investigation of volumetric versus surface solar absorbers for a concentrated solar thermal collector

4.5 million) project known as Micro-Urban Solar Figure 1. Prototype of the solar concentrator, which comprises concentration and volumetric absorbers, for rooftop-mounted heat production.3