Small Scale Concentrated Solar

Power

Matthew Mobley

Sustainable Energy Engineering | ENGR4973

BackgroundConcentrated solar power is a solar energy capture technique that employs the use of light concentrating devices to focus sunlight on a device capable of extracting useable energy from radiation. The three main types of concentrated solar devices are parabolic trough reflectors, power towers, and dish engines [1]. Parabolic trough collectors utilize either mirrors with a continuous parabolic profile or a spaced and strategically positioned array of flat mirrors, known as a Fresnel arrangement, to direct light along a receiver tube and heat the water inside until it turns into steam and is able to power a turbine [2]. Power towers are similar to Fresnel parabolic systems in that they use an array of flat mirrors to concentrate solar radiation onto a receiver, but different in that they do not attempt to approximate a parabola. Instead, each individual mirror is aligned to reflect the sunlight it receives onto a central receiver tower, which in turn captures the radiant energy as heat and uses the heat to drive a steam turbine [3]. Dish engines are CSP devices that utilize paraboloid reflectors to direct solar energy onto a point for collection. A Stirling engine is typically at the focal point of a dish engine, hence the name [3].

Concentrated solar power is a means of energy capture that has been explored extensively by hobbyists and enthusiasts of renewable energy technologies, and because of this, there are a fair number of resources available that detail how to build various CSP systems inexpensively.

Abstract

The main goal of this effort was to design and build a concentrated solar power device, through experimentation determine the efficiency of the device, then draw conclusions as to the nature of environmental energy extraction. Elements from the dish engine and parabolic trough reflector designs were combined to yield the pursued design. The device was constructed, then testing was conducted. The data collected during testing was analyzed, and an average efficiency of 32.25% was found for the device. It was concluded that concentrated solar power technology is fairly effective in small scale, low cost situations.

Scope

The scope of this effort was defined as follows:

The device must cost less than $150.

Construction of the device must require no more than two weeks.

Experimentation must not require the use of tools more complicated than a digital multimeter.

The efficiency of the device must be non-zero.

The analysis of the design will only encompass the thermodynamics of the system; temperature distributions will be assumed negligible (for better or worse).

Theory

The general energy equation for a heat exchanger follows this form:

𝑄 = 𝑚ℎ𝑓𝑔

Volumetric flow rate (also Q) and inlet and outlet temperature can be measured experimentally, then the relevant values calculated:

𝑚 = 𝜌𝑤𝑎𝑡𝑒𝑟 ∙ 𝑄, and ℎ𝑓𝑔 = ℎ𝑜𝑢𝑡𝑙𝑒𝑡 − ℎ𝑖𝑛𝑙𝑒𝑡, where enthalpy values are acquired via relevant tables.

Multiplying the two calculated values yields the effective rate of energy removal from the system by the water (Q). Comparing this value to the amount of solar energy entering the system will yield the total efficiency of the system.

Specifications Collector area = 0.293 m2

Power use = 12 Watts (pump)

Total cost = $96

System Components

Focal Point Compensation

Solar CollectorWater Pump and Reservoir

Heat Exchanger

Description of Experiment

Experimentation took place on April 18th, 2014. The sky was mostly clear. The device was set up in the vicinity of a power receptacle and pointed westward. Data collection entailed taking temperature measurements at the inlet and outlet of the heat exchanger, then measuring the time required to fill a graduated receptacle to a determined volume with the discharge of the system. This procedure was repeated four times over the course of three hours, and the power output was calculated each time.

Experimentation in Progress

Methods of Data Collection

Outlet temperature measurement

Outlet temperature reading

Volumetric flow rate measurement (mass flow

rate extrapolated)

Available InsolationThe power available in sunlight varies with time of day, time of year, geographic location, and weather conditions, among other things. The power available at any given time can be calculated with the following equations [5] :

𝐼𝑚𝑎𝑥 = 1.1 × 𝐼0 × 0.7 𝐴𝑀 0.678, where 𝐼0 = 1357

𝑊

𝑚2 and 𝐴𝑀 is calculated as follows:

𝐴𝑖𝑟 𝑀𝑎𝑠𝑠 = 𝑟 cos 𝑧 2 + 2𝑟 + 1 − 𝑟 cos 𝑧 , with 𝑧 = zenith of sun, and 𝑟 = 708

Time of Day Zenith (degrees) Calculated AMCalculated Imax

(W/m2)

2:00 PM 30.58° 1.1613 1002.9

4:00 PM 53.43° 1.6763 897.1

4:30 PM 59.66° 1.9756 845.1

5:00 PM 65.94° 2.444 774.1

Calculated Power OutputFollowing the procedure detailed previously, power output was calculated at each time interval.

Time of Day

Time to fill 200mL (s)

Tin (°C) Tout (°C) hfg (J/kg*K) 𝒎 (kg/s)Power Out

(W)

2:00 PM 67.12 28.7 38.2 39704 0.00298 118.2

4:00 PM 49.8 26.5 31.4 20482 0.00400 82.17

4:30 PM 37.97 25.8 31.5 23827 0.00530 126.3

5:00 PM 43.53 26.5 29.5 12541 0.00460 57.69

Calculated EfficiencyEfficiency of the device was calculated for all four trials, and the average efficiency was computed.

𝜀 𝑠𝑦𝑠𝑡𝑒𝑚 =𝑄−𝑃𝑝𝑢𝑚𝑝

𝐼𝑚𝑎𝑥∙𝐴𝑐=

𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑 𝑃𝑜𝑤𝑒𝑟 𝑂𝑢𝑡𝑝𝑢𝑡 −𝑃𝑢𝑚𝑝 𝐷𝑟𝑎𝑤

𝑀𝑎𝑥 𝐴𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝐼𝑛𝑠𝑜𝑙𝑎𝑡𝑖𝑜𝑛 × 𝐶𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 𝐴𝑟𝑒𝑎

Time of DayCalculated

Power Output (Watts)

Max AvailableInsolation

(W/m2)

CalculatedEfficiency

2:00 PM 118.2 1002.9 0.361

4:00 PM 82.17 897.1 0.267

4:30 PM 126.3 845.1 0.461

5:00 PM 57.69 774.1 0.201

Average Efficiency: 32.25%

Conclusions

This device appears to have quite a reasonable efficiency of around 30%, indicating that concentrating solar radiation onto a blackened copper coil can heat water effectively. Additionally, these results appear to indicate that achieving a marginally acceptable efficiency is quite possible given a constricted budget when the renewable technology of choice is CSP.

References

[1] National Renewable Energy Laboratory, "Concentrating Solar Power," [Online]. Available: http://solareis.anl.gov/documents/docs/NREL_CSP_1.pdf. [Accessed 1 4 2014].

[2] National Renewable Energy Laboratory, "Concentrating Solar Power - Parabolic Reflector Technologies," [Online]. Available: http://solareis.anl.gov/documents/docs/NREL_CSP_3.pdf. [Accessed 1 4 2014].

[3] National Renewable Energy Laboratory, "Concentrating Solar Power - Point Focus Reflector Technologies," [Online]. Available: http://solareis.anl.gov/documents/docs/NREL_CSP_2.pdf. [Accessed 1 4 2014].

[4] D. Rojas, "Trash can lid to Parabolic Mirror DIY telescope mirror Signal booster Faster 3G/4G," 28 December 2012. [Online]. Available: http://www.youtube.com/watch?v=_8sd9UgjXLE. [Accessed 1 4 2014].

[5] Wikipedia, "Air mass (solar energy)," [Online]. Available: http://en.wikipedia.org/wiki/Air_mass_(solar_energy). [Accessed 23 4 2014].

[6] Solar Energy Development Programmatic EIS, "Concentrating Solar Power (CSP) Technologies," [Online]. Available: http://solareis.anl.gov/guide/solar/csp/. [Accessed 1 4 2014].