Shower Heat Recovery
The shower heat recovery system is the central component of what will be a rather expansive energy recycling system for the Smart House. The intent of this project was to design and test a device that utilizes the heat energy dispensed down the drain during a hot shower, or more generally during any process that continually uses and dispels hot water. The draining gray water can be used to preheat incoming cold water, thus reducing the amount of water taken from the water heater. In order to be attractive to the mass market as well as the Smart House, the device should not only reduce the energy used by the water heater, but should also be cost effective to the user. The savings incurred through its use should equally counteract the cost of the packaged product and its implementation. Through a number of design development techniques that will be discussed further in the report, two heat exchangers were devised to meet the function required.
Two slightly different models were examined. Both designs are based on the concept of concentric counter-flow heat exchange, with the draining hot-water flowing on the outside and the incoming cold water flowing in the opposite direction on the inside. A special modification to this classic heat exchanger is the alteration of the inner pipe geometry from simple tube to a coil. This modification effectively lengthens the duration of heat exchange for the cold water without altering the length of the device.
Two prototypes were constructed and tested, one with the inner coil system mated against the inside of a two inch diameter PVC pipe, the other with the inner coil system having space between it and the inner wall of a three inch PVC pipe, allowing hot draining water to fall both inside and outside of the coil. Both of these models were tested in the lab, and the results were tabulated and analyzed, with specific concentration on the calculated values of heat transfer coefficient and effectiveness. Multiple simulations were run to determine the nature of the device in terms of mass-production and varying use.
Advantage: Amount of water taken from the water can be reduced. It is also cost effective to the user.
The effectiveness of the exchangers was calculated to average around 85%, a value that is higher than any competing product on the market today. Furthermore, simulations reveal that the device will be cost effective and functional in any environment it is placed in. Because no significant difference in response was found between the two different designs of the heat exchanger, the mated version is the best option due to its easier manufacturability and the ease with which it can be cleaned.