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The Potential Energy Efficiency of a Hybrid Designed House: A Post-occupancy Case Study on the Heating and Cooling System.


The US DOE Solar Decathlon required net- zero energy performance in 2009. Achieving net- zero requires both decreasing the energy consumption by improving the energy efficiency and increasing the energy supply from sustainable sources. In the investigated case the energy efficiency was improved by integrating passive and active design strategy. The design strategy was confirmed successful during the Solar Decathlon with a relative mild climate in Washington D.C. for three weeks of October of 2009. To examine the potential energy performance of this house for an extreme climate with cold winter and hot summer, the house was monitored with a state-of-art data acquisition system after relocation to the Midwest. Agreement of logging the house operation was also approved by the users. With both sensor measurement and activity log this paper provides a thorough analysis of the post-occupancy energy performance.


The integration of passive and active systems demands a complex interaction of spatial composition, building fabric and thermal and climatic conditions. This complexity is manifest in the open floor plan that uses convection to distribute passive solar heat gained from south-facing windows and natural ventilation for cooling and indoor air quality. The house extended this concept to transient zones in the sun porch where space and envelope interact.

Passive Design Strategy

The spatial composition of the Solar House is seasonal. Thus the Hall and Sun Porch can be reconfigured and opened to the environment. The Sun Porch (sunspace), with added thermal mass in the floor, mediates light and heat and encourages convective loops to heat and cool the house. A louver system spanning the south facade also mediates light and heat and reduces the active cooling load in summer. The house requires active manipulation of its doors, windows and exterior louvers to influence airflow and to maximize heat gain and loss. This reliance on several basic passive solar and ventilation techniques helps reduce the energy loads for the active systems. But the effective meshing of active and passive systems needs an alert and motivated resident.

The building envelope combines a tight, well-insulated wall system with vacuum insulated north doors to facilitate natural ventilation. On the north, east and west, the house's exterior wall construction includes a double cavity with an R value of 48, which is achieved with bio-based foam insulation in a two-by-six framing system twenty-four inches on center with another one-and-one-half-inch additional layer of recycled blue jean insulation. The roof was installed with one layer of R-60 insulation. Glass is minimized on the north, east and west elevation to prevent unwanted heat gains and losses. The fenestration patterns provide adequate day lighting throughout the year (Leysens et al 2013). The east and south windows are multilayered systems and, like all the house's windows, operable to admit fresh air and optimize passive ventilation.

During intermediate seasons the interior spaces are designed to be passively ventilated by maintaining a 250 [ft.sup.2] through-section breezeway in the North-South orientation. The southern side of this breezeway employs two sliding glass walls that allow full opening for multiple seasonal responses, including a solar-collecting porch, which can be opened and closed to suit the season. The glass was specified, so that the outer layer admits maximum solar radiation in winter. The competition team initiated an industry alliance to manufacture R-30 doors with vacuum insulation for the north wall. The northern side of the house incorporates Vacuum Insulated Panels (U value of 0.07 BTU/[degrees]Ff[t.sup.2]) into an operable system of four hinged doors with about 0.95 ratio of opening to frame, that was developed to maintain the thermal resistance of the remainder of the house envelope necessary during peak outdoor winter conditions. Prior to their installation computer-simulated thermal analysis of the assembly and its components was conducted using Therm5 software, which showed both benefits and inefficiencies in the prototype.

Thermal mass was integrated into the building fabric wherever possible, for example in concrete countertops to serve as summer and winter diurnal energy storage.

Active Design Strategy

The heating and cooling system of the house is based on maximizing year-round solar thermal collection. The concept aims to enable heating and cooling powered fully or partially by solar thermal energy. A propylene glycol and water solution functions as the working fluid for thermal energy collection. The glycol solution is heated by a large bank of 60 Apricus evacuated tube collectors. These collectors are capable of heating the fluid to much higher temperatures than those attainable with flat plate collectors. The evacuated tubes are oriented to the south at the same 23[degrees] angle as the roof. The heated glycol solution passes through a heat exchanger that heats thermal storage water, which is stored in a large insulated tank.

For winter heating, the hot water is circulated through a three-loop radiant floor heating system. The use of thermal energy for summer cooling is currently evaluated separately from the house in a test laboratory. In the final completed scenario, the hot water will be used as the heat source to recharge a liquid desiccant dehumidifier. The desiccant dehumidifier removes the latent cooling load. A traditional vapor compression air conditioning system handles the sensible cooling load. Since the dehumidifier handles the latent cooling, the electrically run air conditioner can be smaller than under conventional condition for this size of house. The air conditioning system is widely available commercially and is an efficient version of what homeowners and HVAC installers are already familiar with.

In addition to the radiant heating, desiccant and electric cooling systems, an electrically powered energy recovery ventilator provides fresh air to the interior space. The incoming air is conditioned to the same temperature and humidity as the air in the conditioned space using recovered heat and moisture from the outgoing airstream. Domestic hot water (DHW) is heated by a heat exchanger tied to the main thermal water storage tank. An electric resistance heater is integrated into the DHW tank as a backup heat source.


Energy Performance during Solar Decathlon Competition Event

The EnergyPlus simulation outcome was a fairly accurate prediction of the energy demand and production during the 2009 Solar Decathlon competition. This meant that the house, in spite of extreme weather conditions during the contest week with 3 days of solid rain, was able to produce more energy to perform the necessary tasks conducted during the contests, than needed. In actual numbers, the house produced 170.67 kWh, which was a 3.09 kWh surplus on top of the 167.58 kWh used during the first week of competition. One reason why the house performed so well was the fact that active systems were effectively supported by passive systems and thus used sparsely. The major strategy utilized was natural ventilation as a combination of stack and cross ventilation and a good envelope with high R-value (Lentz 2010).

Energy Performance after Relocation

After the Solar Decathlon competition the house was purchased by the local Department of Natural Resources and utilized as a naturalist's office in a state park resort. The thermal performance of the building envelope is in good condition after relocation. Systematic measurements were taken onsite with smoke test, infrared photos and door blower test.

In the mean time, air temperature sensors in the conditioned space and sunspace, fluid flow meters, fluid temperature sensors and watt nodes in the mechanical room and electrical system are also installed and managed by a data logger for real time data collection and long time monitoring.

With the long time data collection from November 2011, the house energy performance can be analyzed and compared with different operating configurations. From June 2012 to May 2013, the house consumed 13,665kWh electrical energy in total, 29% of this total year round electrical energy comes from the grid, which does not create a net zero energy balance as was predicted.

It's obvious in the Figure 4 that the solar tank heater dominates the energy consumption over the whole year. Energy consumed by the solar water tank heater is 8,757kWh for the whole year, which almost doubles the simulation value of 4,935.9kWh. The AC load occupies the second large portion of the total energy pie at 13%. The cooling consumption by measurement is 1,717kWh year round in total, close to the EnergyPlus simulation 1,525kWh. However, the simulation is based on a residential schedule without natural ventilation and after occupied the users did not utilize natural ventilation as frequently as expected. So there is still some room for cooling energy savings.


The Hot Water System

At the beginning of monitoring the control of the heating and cooling was not in place, which resulted in the radiant floor and AC running simultaneously in June 2012. However, this problem has been solved by manually unplugging the radiant floor pump so in July the heating consumption looks more reasonable.

The current installation problems are 1) one valves between solar collector and solar tank was omitted so when it's cold and cloudy outside, e.g. during night time in winter, the hot water in the tank is losing heat to the cold glycol running in the evacuated tube on the roof; 2) another valve is missing between the solar tank and the radiant floor, which prevents the tank temperature to be set above radiant floor temperature. So the solar hot water tank cannot reach the maximum capacity of heat storage during day time. The consumption of backup electrical heater running at night-time was calculated based on measurements. For an approximate value, the night time is set to be 5pm to 8am in heating season from October 2012 to April 2013.

From the calculation the backup heater consumption during night-time occupies about 75% of the whole day consumption. For the current usage as an office building without occupants at night this amount of energy consumption should be decreased to minimum. The exact detailed control method needs to be investigated further while considering the local climate and system safety.

The Utilization of Natural Ventilation

A passive house needs an active user. However, a residential building being altered to an office building brings along a problem: occupants inside would focus on work, not on their interacting with the outdoor conditions. In the activity log occupants input how the house operations were changed and gave the approximate time and updated the researchers with a monthly report. From the logs collected, the house configuration can be traced with the measurements collected by the data acquisition system. According to the occupant's log, the occupants are not very active in utilizing the natural ventilation when the outside weather is comfortable.

To quantify the natural ventilation savings that the occupants have missed, 75F (23.9C) degrees is chosen as the thresholds to calculate the cooling consumption during the designed natural ventilation period. The conventional air conditioner is set to start cooling at 75F (23.9C). Although an adaptive comfort scenario (ASHRAE 55-2010) would allow the AC to remain shut off below 83F (28.3C), choosing to calculate with 75 F degrees would give us a minimum value of potential savings. From Fig. 6 it can be found that the potential saving percentage is higher in May, June and September when the weather is milder than in July and August. The total potential saving in cooling season would be about 22%.

Another extra cooling load in summer is derived from sunspace solar gain without proper operation. The sliding glass door surrounding the sunspace is difficult to open and close. And the skylight in sunspace was not shaded. Therefore, the sunspace/ sun porch still receives and stores heat in summer. The users opened the inner sliding door to enlarge the activity zone instead of opening the outer one. With opening the inner glass door, the heat collected in the sunspace is mixing with the conditioned air in the main space. But if the outer one would be opened, the heat collected by the sunspace would be considered outside of the building and would not mix with the conditioned air through convection.

The intervention with onsite demonstration how to operate the sunspace took place on June 1, 2013, so the data comparison is chosen from June to September in both 2012 and 2013. It is found that in June and July with sunspace removed the monthly cooling consumption decreased significantly. It's difficult to explain the August data because the occupants' log input were empty, which means the house operation was unclear to the researchers. However, for the increased consumption in September 2013, the occupants' activity log explains a lot. In September 2012, the occupants were very active with natural ventilation because the researchers kept sending email reminders about the outdoor nice weather. So even with the sunspace in place, the total number of days with AC in operation was only 1/3 of the whole month. In September 2013, the researchers only carried out a single day email reminder, and according to the activity log, the AC was always on without enough natural ventilation. Plus, the sunspace was only removed for about half of the month due to driving rain and to prevent rain from leaking into the basement ceiling the south door was kept shut. .

Unfortunately the shading system for the sunspace skylight has not been installed until the end of 2013, although it has already been ordered and has arrived onsite.


The reason of the unexpected energy performance can be summarized as 1) Improper system installation and maintenance; 2) Users' unawareness of the passive design integration and the corresponding behavior; 3) Advanced control system needs to match the complex system.

Users' education is important to develop a good habit but it's hard to receive a quick result as the house is used as a work place. According to the discussion with the users, they would prefer the house telling them what to do to fulfill the passive and active design strategy. Sending email reminders to the occupants about house operation has proved to be effective and acceptable by the users. In the near future, an operation reminder email will be sent out by the new control system instead of the researchers.


This material is based upon work supported by the National Science Foundation under Grant Number EPS-1101284. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Funding was also provided by Iowa's Department of Natural Resources and Iowa State University's Center for Building Energy Research (CBER). Mike Wassmer of Live to Zero Llc provided consultancy for the data acquisition system. The student research team with Jessica Bruck, Clark Colby, Noelie Daviau, Ryan Everly, Mike Garcia, Nick Hulstrom, Suncica Jasarovic Isabelle Leysens, Esdras Murillo, , and Dana Sorensen installed and maintained sensors and wires. Thanks to Timothy Lentz for continuous support and to the whole Iowa State2009 Solar Decathlon faculty, administration and student team as well as to the Iowa Department of Natural Resources in particular Michelle Wilson and Pat Boddy and to Hannah Wiltamuth and Jacob Ahee for their collaborative support.


ASHRAE 55-2010 Standard - Thermal Environmental Conditions for Human Occupancy (ANSI Approved), ASHRAE 2010.

Leysens, Isabelle, Ulrike Passe, Evaluating daylight performance in a passive solar home: a study based on design simulation and post-occupancy monitoring results, In Solar 2013 Conference Proceedings, Proceedings of the annual conference of the American Solar Energy Society (ASES), Baltimore April 2013.

Passe, Ulrike, Evaluating passive and active solar design and performance interaction: First measured results, in Proceedings of The Second International Conference on Building Energy and Environment (COBEE), August 1-4, 2012, Boulder, Colorado, edited by Dr. Zhiqiang Zhai, Dr. Xiangli Li and Haidong Wang, 2012: 265.

Lentz, Tim, Analysis of the passive design and solar collection techniques of the houses in the 2009 U.S. Department of Energy's Solar Decathlon competition, M.S., IOWA STATE UNIVERSITY, 2010, 111 pages; publication number 1479984.

Shan He

Ulrike Passe

Associate Member ASHRAE

Shan He is a research associate in the Department of Architecture, Iowa State University, Ames, Iowa. Ulrike Passe is an associate professor in the Department of Architecture, Iowa State University, Ames, Iowa.
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Author:He, Shan; Passe, Ulrike
Publication:ASHRAE Conference Papers
Article Type:Case study
Date:Jun 22, 2014
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