Solar Water Heater: Thermal Energy from Sunlight
Build a functional solar water heating system using recycled black plastic bottles, cardboard, and transparent plastic sheet. Measure temperature increase over 5 days and calculate thermal energy gain. Zero-cost materials, complete SBA rubric included.
Scientific principles behind this project
The solar water heater demonstrates the greenhouse effect and thermal energy transfer. Sunlight passes through the transparent plastic cover and is absorbed by the black-painted bottles. Black surfaces absorb most visible light and convert it into heat (infrared radiation). The plastic cover traps this heat by preventing convection (air circulation) from carrying the warmth away. This is exactly how commercial solar water heaters work, as well as greenhouses used for crop production.
- Absorption: Dark colours absorb more electromagnetic radiation than light colours.
- Greenhouse effect: Short-wave solar radiation passes through transparent material, but long-wave thermal radiation is trapped.
- Conduction: Heat transfers from the warm bottle surface to the water inside.
- Convection prevention: The plastic cover stops air movement that would otherwise cool the bottles.
Complete materials list
Core structural materials (collect these first):
- Cardboard box - medium size (approximately 50cm x 40cm x 30cm). A shoe box or small carton works. The box serves as the insulated container.
- Aluminium foil or shiny chip packet (crisp packet) - enough to line the inside of the box. This reflects infrared heat back toward the bottles.
- Transparent plastic sheet - clear polythene (from a new shopping bag) or transparent廢棄 report cover. Must be clean and without holes.
- Black plastic bottles - 2 to 3 empty soda bottles (1L or 1.5L size). Dark soft drink bottles (Coca-Cola or similar) are already black. If unavailable, paint clear bottles with black tempera paint or charcoal paste.
Insulation and assembly materials:
- Newspaper or dry grass - for filling gaps and insulating the space between bottles.
- Black paint or charcoal powder - if your bottles are not black. Mix charcoal powder with a little water to make paint.
- Scissors or craft knife - for cutting cardboard and plastic (adult assistance recommended).
- Transparent adhesive tape - for sealing the plastic cover.
- Ruler or measuring tape - for dimensions and placement.
Measurement and recording tools:
- Thermometer (0-100°C range) - a simple kitchen or laboratory thermometer. If unavailable, use a digital probe thermometer or ask your science teacher to borrow one.
- Clock or watch - to record time of each measurement.
- Notebook and pen - for recording 5 days of temperature data.
- Measuring cup - to ensure same volume of water (500ml or 1 litre per bottle).
Total cost: 0 KES (all items are household waste, recycled materials, or borrowed from school).
Step-by-step construction guide
Follow these 9 steps carefully. The construction takes approximately 60 minutes. After building, you will conduct 5 days of temperature measurements.
Prepare the cardboard box
Select a cardboard box without large holes. Cut off the top flaps completely so the box is open at the top. The box depth should be approximately 10cm to 15cm. If the box is deeper, cut down the sides to reduce depth. This creates a shallow tray that will be covered with plastic.
Line the box with aluminium foil (reflector)
Cut aluminium foil (or open a crisp packet) to fit the inside bottom and sides of the box. Use tape to hold the foil in place, shiny side facing upward and inward. The foil reflects infrared radiation back toward the bottles, increasing efficiency by up to 30 percent. Smooth out wrinkles as much as possible.
Prepare the water bottles
Remove labels from the plastic bottles. If bottles are not black, paint them with black tempera paint or a mixture of charcoal powder and water. Apply two coats for best absorption. Allow paint to dry completely (approximately 2 hours). Black bottles absorb maximum solar radiation. Fill each bottle with the same measured volume of water (500ml or 1 litre) - consistency is essential for scientific comparison.
Arrange bottles inside the box
Place the filled bottles in the foil-lined box. Leave at least 2cm space between bottles for air circulation. If using multiple bottles, arrange them in a single layer, not stacked. Fill empty spaces between bottles with crumpled newspaper or dry grass - this insulation reduces heat loss through the sides.
Create the transparent cover
Cut a piece of transparent plastic sheet slightly larger than the top of the box (allow 5cm overhang on each side). The plastic must be clean and without holes. Stretch the plastic tightly over the top of the box. Tape it down securely on all four sides to create an airtight seal. The cover should be slightly domed (not touching the bottles) so that condensed water drips down the sides rather than onto the bottles.
Position the heater for maximum sunlight
Place the solar water heater in a location that receives direct sunlight from 10:00 AM to 3:00 PM (the period of strongest solar radiation). Tilt the box slightly toward the sun - propping the back edge up by 10cm to 15cm using a brick or stone. This tilt angle increases the amount of sunlight captured. Face the box toward the North if you are in Kenya (since the sun is always in the northern part of the sky).
Baseline temperature measurement (Day 1, 9:00 AM)
Before exposing the heater to sunlight, measure the initial temperature of water in each bottle. Record this as the "starting temperature". Place the thermometer into the water through the bottle opening (if cover is on, lift one corner temporarily). Replace the cover immediately after measurement.
Daily data collection protocol (Days 1-5)
For five consecutive sunny days, measure and record water temperature at the same times each day: 9:00 AM (before heating starts), 12:00 PM (noon), and 3:00 PM (end of heating period). Also record ambient air temperature (shade temperature) for comparison. If a day is cloudy, repeat the same measurement schedule but note the weather condition in your log.
Calculate temperature rise and thermal energy
At the end of Day 5, calculate the average temperature rise: (temperature at 3:00 PM) - (temperature at 9:00 AM). Then calculate thermal energy gained using the formula:
Energy (joules) = mass of water (kg) × specific heat capacity of water (4184 J/kg°C) × temperature rise (°C)
For example, if you used 1 litre of water (1 kg) and the temperature rose by 12°C, the energy gained is 1 kg × 4184 × 12 = 50,208 joules (approximately 50 kilojoules).
Five-Day Temperature Log Sheet
Copy this table into your notebook. Record temperatures with one decimal place precision (e.g., 28.5°C). Calculate daily rise and 5-day average rise.
| Day | Date | Weather | 9:00 AM (Start) Water Temp (°C) | 12:00 PM Water Temp (°C) | 3:00 PM (End) Water Temp (°C) | Temperature Rise (°C) | Ambient Air Temp (°C) |
|---|---|---|---|---|---|---|---|
| 1 | _____ | _____ | _____ | _____ | _____ | _____ | _____ |
| 2 | _____ | _____ | _____ | _____ | _____ | _____ | _____ |
| 3 | _____ | _____ | _____ | _____ | _____ | _____ | _____ |
| 4 | _____ | _____ | _____ | _____ | _____ | _____ | _____ |
| 5 | _____ | _____ | _____ | _____ | _____ | _____ | _____ |
| Average Rise (Days 1-5) | _____°C | _____ | |||||
Required analysis (write in your report): Create a line graph with Day on the x-axis and Temperature on the y-axis. Plot both 9:00 AM and 3:00 PM temperatures on the same graph to show the daily heating pattern. Explain why Day 3 might have different results if weather changed.
KNEC SBA Rubric – Solar Water Heater Project
| Criteria | Exceeds (5) | Meets (4) | Approaching (3) | Below (2-1) |
|---|---|---|---|---|
| Design and construction quality | Box properly lined with reflector, airtight plastic cover, black bottles, good insulation. Heater functions without structural failure. | Most components present, minor air leaks or insufficient insulation. | Missing one major component (e.g., no reflector, or no black paint). | Poor construction, heater falls apart or does not function. |
| Data collection (5 days) | Complete 5-day log with all 15 temperature readings. Weather recorded each day. Ambient temperature noted. | 4 days of data, some missing times. | 3 days of data or significant gaps. | Fewer than 3 days of data or no log sheet. |
| Data analysis and graph | Line graph correctly plotted with labeled axes. Average temperature rise calculated. Energy gain calculated correctly using formula. | Graph present, minor labeling errors. Average calculated but energy calculation missing. | Graph incomplete or no energy calculation. | No graph or analysis. |
| Scientific conclusion | Explains absorption, greenhouse effect, heat transfer. Identifies sources of error (e.g., cloudy day, wind). Suggests improvement (e.g., use glass cover instead of plastic). | Basic explanation of why water heated, but missing technical terms. | Very brief conclusion, no scientific principles named. | No conclusion written. |
- Build a second solar heater without the transparent cover (or without black paint) as a control experiment. Compare the temperature rise of both devices over 2 days.
- Calculate the efficiency of your heater: (energy gained by water) divided by (solar energy incident on the box area). Estimate solar intensity as 1000 W/m² during midday.
- Photograph the heater with a visible clock showing the time for each measurement day to prove consistency.
- Have a parent or science teacher sign each page of your data log.
Research extension (bonus marks)
Investigate how commercial solar water heaters work in Kenya. Research the following and write a one-page report comparing your device to commercial systems:
- Materials used in commercial solar collectors (copper pipes, selective coating, tempered glass).
- Typical temperature rise achieved by commercial systems (40°C to 60°C).
- Cost of a household solar water heater in Kenya (approximately 30,000 to 80,000 KES).
- How evacuated tube collectors differ from flat plate collectors.
Include a diagram comparing your device to a commercial one. This demonstrates real-world application and can earn up to 3 bonus points.
Project Gallery – add your own images
Print or save as PDF
Take this guide outdoors. Print the temperature log sheet and rubric to fill by hand.