Using Solar Thermal to Lower Energy Costs for Small Scale Dairy Processors

Electromagnetic radiation from the sun provides both light and heat on Earth, with much of this radiation being invisible to the human eye. The sun's infrared radiation heats materials, causing their molecular structure to vibrate and resulting in heat. This process is demonstrated by a garden hosepipe left out in bright sun, which can increase in temperature to scalding levels. Solar thermal panels operate in a similar way, with the sun's infrared radiation heating a fluid (usually water or a water glycol mixture) in a solar collector or panel. These panels are typically flat and mounted in an unshaded area on a roof or the ground, with glass covers and multiple interconnected black tubes through which the heat transfer fluid is pumped. Hot fluid is stored in insulated tanks for later use, and in some designs, thermo-siphoning of the fluid to and from the panel obviates the need for a pump. Solar thermal energy can be used in various applications in dairy processing operations, including providing hot water for clean-in-place (CIP) and sanitary use, pre-heating boiler feedwater, and directly heating milk in the pasteurizing process. Each of these applications is discussed briefly below, with their relative economics examined. The costs of electricity and equipment used in these economics are based on actual costs found in Kenya, but the applications would be applicable in any country with suitable adjustment for local economics. Clean-in-place (CIP) and sanitary water heating with solar thermal involves heating water to a temperature of 55 °C (131 °F) for cleaning and sanitizing processing equipment. The energy required to raise water temperature from ambient to 55 °C is 0.045 kWh/litre, which can be provided by a single flat glass panel solar thermal collector of 3.5 m² in a high solar radiation region. This can heat approximately 200 litres/day of water from 16 °C (61 °F) to 55 °C (131 °F), with an installed cost of a single 3.5 m² solar thermal panel estimated at approximately 140,000 – 150,000 KSh (US$1,400 – 1,500). The solar thermal panel pays for itself with a payback period of approximately two years, providing long-term cost savings. Boiler feedwater pre-heating with solar thermal involves heating boiler feedwater used for heating milk prior to and during pasteurization. A heat exchanger is used between the fluid flowing in the solar thermal panel and the boiler feedwater to minimize chemical fouling of the solar thermal panel. Savings, costs, and payback are similar to those for the CIP and sanitation water heating. Pre-heating and heating of milk for pasteurizing involves using solar thermal to partially or completely heat the milk input into the pasteurizing equipment. The actual input temperature required for the milk may vary depending on the pasteurizing process used and the duration the milk is held at the pasteurizing temperature.