Hybrid Hot Water System

     Natural gas is a great way to heat water....right?  Well for most of us it certainly is, but why?  Probably because natural gas is abundant is many places.  This is not the cas in Hawaii where natural gas is very expensive.  So the people of Prince Jonah Kuhio Kalanianaole (PJKK) Federal Building decided to find a new way to heat their water.

     The building uses about 2,800 gallons of hot water every day.  Not bad, until you consider that in Hawaii, natural gas cost about three times as much as in most places.  With some help from the Sandia National Laboratory, a system was implemented which took advantage of the solar resources and waste heat from the building's chiller.

     According to an energy analysis, this system has reduced natural gas consumption by 83%.  The solar collector consists of 768 square feet of flat panels, a 1300 gallon storage tank and a plate and frame heat exchanger to recover waste heat from the chiller.  Typically, the water is heated to 21 ⁰C (70 ⁰F) by the waste heat from the chiller and then is brought to its usable temperature of 60 ⁰C (140 ⁰F) by the solar panels.

     Performance monitoring shows that on average 28% of the total energy needed to heat the water annually comes from the chiller heat exchanger, 55% of the load is supplied by the solar panels with the remaining 17% being supplied by the boiler.  The estimated cost of the project is $58, 400 with a life cycle cost of $84,000.  This corresponds to a payback period of about 9 years.