Capstone II · Team Project

Investigating Heat Transfer Enhancement in a Solar Trough

Global energy demand is rising, yet renewable technologies like concentrated solar power (CSP) remain limited by low operating temperatures. Our team built a rotating particle-flow receiver to test whether integrating solar particles into a parabolic trough system meaningfully increases working fluid temperature.

Team 19 standing with the solar trough prototype on the rooftop

How It Works

Outside air enters the inner tube at a set volumetric flow rate
The rotating system transfers the particles' energy to the working fluid via convection and conduction
Particles absorb radiation from the concentrated sunlight at the focal length
Heated working fluid is measured at the outlet
Labeled CAD diagram of the solar trough system

System CAD Design

  • Borosilicate glass tube
  • Helical copper coil
  • Rotational subassembly
  • Motor subassembly
  • Bevel gears
  • Structural frame
  • Reflective sheet

Design & Testing Process

From first sketches to a full outdoor test rig on a Boston rooftop.

1

Sketch Development

Hand sketches of the rotating airflow system and coil/bevel gear components
Sketches of rotating system
2

Fixture Trough

CAD model of scaled down trough fixture
CAD model of scaled-down trough
3

Fixture Testing

Fixture test setup with heat lamp
Fixture test set up
4

Trough Indoor Testing

Indoor solar trough test setup
Solar trough indoor test set up
5

Trough Outdoor Testing

Outdoor solar trough test setup with city skyline
Solar trough outdoor test set up

Key Design Decisions

Coil Geometry

GeometryLength (in)Air residence (s)Surface area (in²)
Straight220.226
Coil620.573
CAD diagram of the coil showing 22 in overall length, 1.5 in pitch, and 1 in coil diameter

~3× increase in air residence time and surface area with the coil geometry. Performed fluids and heat transfer analysis using MATLAB and hand calculations to validate a coiled copper tube geometry, applying a Nusselt number correlation for coiled-tube internal flow that accounts for the tube-to-coil diameter ratio alongside the Reynolds and Prandtl numbers of the internal airflow:

Nusselt number correlation derivation for coiled-tube internal flow

Rotational Cap

  • High-temp O-ring for seal strength
  • High-temp silicone sealant
  • Single bearing for axial rotation
Rotational cap CAD model with high-temp O-ring Rotational cap end view with silicone sealant Threaded fitting for the rotational cap

Motor & Rotation

  • Bevel gears drive the rotational subassembly
  • DC motor with dedicated motor mount
  • Coupler links motor to the end flange (bearing press fit)
CAD render of the bevel gear and motor mount setup Close-up CAD render of the rotating bevel gear assembly

Copper Tube Fabrication

The capstone machine shop couldn't fabricate the helical coil, and an external vendor quoted ~$700–800 for a single unit. We worked with the machinist and online resources to develop an in-house method through trial and error.

Sand packing the copper tubing Heating and bending the copper tube Early copper coil attempt
Sand packing → heating & bending → early coil
Progression of handmade copper coils
Progression of handmade copper coils

Particle Selection

Two rounds of heat-lamp testing narrowed candidate particles down by absorptivity, weight, thermal conductivity, and cooling rate.

Phase I particle testing setup
Phase I — silicon carbide, graphite & sand vs. control
Phase II particle testing setup
Phase II — silicon carbide vs. graphite, 50–300g

Silicon carbide was selected for its lower cooling rate and lack of glass-tube shading. The 200g test quantity was scaled proportionally to the full 22″ focal tube, landing at 340g.

340g of silicon carbide inside the full-scale focal tube

Final Testing

We reserved the Columbus Garage rooftop and waited for a sunny day, then tested with and without particles at three flow rates: 5, 10, and 20 L/min.

Full test rig setup with instrumentation labeled
Arduino & TCs
Sun Dial
Thermometer
Close-up of Arduino and thermocouple wiring
Arduino & Thermocouples
Close-up of the sun dial mounted on the frame
Sun Dial
Close-up of the handheld thermocouple thermometer readout
Thermocouple Thermometer
Focal tube assembly rotating during testing
Thermal camera image of the prototype during testing, reading 100.9°C
Thermal camera reading (100.9°C)
Compressor attached to the system
Compressor setup
Testing results, with vs. without particles
MetricWith ParticlesWithout Particles
Average ambient temp8°C19°C
Max outlet temp161°C174°C
Max glass tube temp173°C144°C

Final Conclusions

Graph of copper outlet and glass tube temperature over time during testing with particles on April 8th, 2026
Copper outlet and glass tube temperature during testing with particles, tracked across flow-rate and rotation changes
Graph comparing inlet to outlet temperature difference with particles and without particles in the focal tube assembly
Inlet-to-outlet temperature difference with vs. without particles — average ambient temperature was 7.8°C (with particles) vs. 19.2°C (without)

Future Improvements

Kai Yoshida and Daniel Riddick presenting the Solar Particle Receiver at the final presentation
  • Improved assembly process
  • Copper tubing fabrication
  • Testing across varied weather conditions
  • Particle fluidization
  • Energy storage