Abstract:
Lightweight buildings have low thermal mass. This means their day and night temperatures fluctuate a great deal. Comfort is reduced and heating and air conditioning (HVAC) has to work harder. If a building’s thermal mass could be increased, it would improve comfort and reduce the HVAC energy requirement. Phase Change Material (PCM) is an innovative solution to this problem. It uses a material’s latent heat to absorb heat during day and release it at night, effectively acting like thermal mass but at one tenth the volume of sensible heat materials. This research investigates the economics of PCM. The HVAC energy savings are estimated and compared against the cost of PCM. One limitation of PCM is that its performance is climate dependent, it works better in some climates than others. This research also investigates PCM performance in different cities to identify the cities where it performs best and thus has the greatest chance of commercial success. Computer simulation was used to conduct this research. EnergyPlus is a building simulation software which is powerful, flexible, and has an input specifically for PCM. A comprehensive validation study was conduct to assess its accuracy. This validation study included a literature review of EnergyPlus PCM modelling as well as simulating a building with PCM called the Tamaki Offices. The Tamaki Offices contain PCM in the walls as well as instruments to measure temperature and humidity. For validation, the measured indoor temperature was compared against the simulated indoor temperature to assess the amount of agreement. It was found EnergyPlus modelled PCM reasonably accurately. Its output matched the measured data with an average difference of 1.8°C. To find the HVAC energy savings from PCM, a real house in Auckland was modelled. The house construction was reproduced from the building plan and the material properties were from reliable online sources. The house has a total of five bedrooms, a kitchen, living room, four bathrooms, and garage. PCM was put in the walls and ceiling of every room. An HVAC system was placed in the three most used rooms, the living room, kitchen, and master bedroom. It was found that a 24 hr HVAC over one year used 4.6 gigajoules (GJ) less energy in the house with PCM, a 17% saving. In another simulation, PCM was only placed in the HVAC rooms and 3.0 GJ less energy was used. The 24 hr HVAC was changed to a more realistic HVAC schedule which only turned on when people were present, this HVAC saved only 0.6 GJ mainly due to infrequent usage. The economic analysis found the cost of PCM to be approximately $12,000 NZD if placed in all rooms or $4,000 if placed only in the HVAC rooms. The 4.6 GJ, 3.0 GJ, and 0.6 GJ energy saving equates to $336, $219, and $47 saved per year respectively assuming 2011 average New Zealand electricity prices. This gives a straight payback period of 36 years, 18 years, and 255 years. Assuming some hidden costs, only the 18 years scenario can give a positive return on investment for an estimated 50 years building lifespan. The 255 years scenario shows that PCM likely cannot be economical if the HVAC is used infrequently. The same Auckland house model was simulated in the climate of 18 other cities around the world. The model was unchanged to allow easy comparison between cities. It was found that equatorial cities were too hot and PCM was ineffective. Polar cities were too seasonal and PCM was only effective for a few months of the year. Overall this shows that PCM operates best in a narrow temperature range and its effectiveness is greatly reduced outside of this range. Cities with moderate temperature such as San Francisco and Brisbane were best for PCM use. Normally an equatorial city would be too hot for PCM, however high altitude equatorial cities are much cooler and have been found to be the perfect cities for PCM. They are even better than San Francisco and Brisbane because they don’t have seasons, the temperature is consistent all year. One of these cities is Bogota, Columbia. A simulation conducted with PCM only in the HVAC rooms and operating 24 hrs showed a 7.7 GJ saving. Assuming New Zealand electricity prices, this gives an annual saving of $562 which is a payback period of 7 years. This shows strong promise for commercial success, it can return several times its investment during a buildings lifespan. In future work more detailed simulations can be conducted for Bogota and similar cities which show strong promise for commercial success. Also the accuracy of EnergyPlus validation simulations can be improved by measuring material properties and on site weather measurements such as direct and diffuse radiation.