Abstract
Water heating accounts for the second largest portion of residential building energy consumption, after space conditioning. Existing HPWH products are a technical success, with demonstrated energy savings of 50% or more compared with standard electric resistance water heaters. However, current HPWHs available on the market cost an average of $1000 or more, which is too expensive for significant market penetration. What is needed is a method to reduce the first cost of HPWHs, so that the payback period will be reduced from 8 years to a period short enough for the market to accept this technology.
A second problem with most existing HPWH products is the reliability issue associated with the pump and water loop needed to circulate cool water from the storage tank to the HPWH condenser. Existing integral HPWHs have the condenser wrapped around the water tank and thus avoid the pump and circulation issues but require a relatively complex and expensive manufacturing process. A more straightforward potentially less costly approach to the integral, single package HPWH design is to insert the condenser directly into the storage tank, or immersed direct heat exchanger (IDX).
Initial development of an IDX HPWH met technical performance goals, achieving measured efficiencies or energy factors (EF) in excess of 1.79. In comparison conventional electric water heaters (EWH) have EFs of about 0.9. However, the initial approach required a 2.5" hole on top of the tank for insertion of the condenser - much larger than the standard openings typically provided. Interactions with water heater manufacturers indicated that the non standard hole size would likely lead to increased manufacturing costs (at least initially) and largely eliminate any cost advantage of the IDX approach. Recently we have been evaluating an approach to allow use of a standard tank hole size for insertion of the IDX condenser. Laboratory tests of a prototype have yielded an EF of 2.02.