Jay Burch

Development of Standardized Domestic Hot Water Event Schedules for Residential Buildings

The Building America Research Benchmark is a standard house definition created as a point of reference for tracking progress toward multi-year energy savings targets. As part of its development, the National Renewable Energy Laboratory has established a set of domestic hot water events to be used in conjunction with sub-hourly analysis of advanced hot water systems.

Advances in Solar Buildings

In the autumn of 2002, 14 universities built solar houses on the National Mall in Washington, DC, in a student competition-the Solar Decathlon-demonstrating that homes can derive all the energy they need from the sun and celebrating advances in solar buildings. This paper describes recent progress in solar building technology that expands the designers palette and holds the potential to radically improve building energy performance. The discussion includes market conditions and solar resource data; design integration and modeling; window technology, daylighting, passive solar heating; solar water heating; solar ventilation air preheating; building-integrated photovoltaics; and solar cooling. The Solar Decathlon competition highlighted ways in which these strategies are integrated in successful solar buildings.

Thermal Penalty of an Immersed Heat Exchanger in Integral Collector Storage Systems

The use of a heat exchanger in solar water heating systems reduces the overall system efficiency, In this paper, we derive a simple penalty factor that gives the fractional reduction in thermal efficiency imposed by a heat exchanger immersed in an isothermal integral collector storage (ICS) system. The penalty factor depends only on the effectiveness of the heat exchanger, the capacitance of the hot water delivered, and the product of the loss coefficient and aperture area of the ICS. This factor allows the designer to select an appropriate heat exchanger without extensive simulation or experiments. Application of the penalty factor for a load-side (or on-demand) heat exchanger, which operates intermittently at varying flow rates, is discussed and illustrated through example.

Geographic constraints on passive solar domestic hot water systems due to pipe freezing

The supply and return piping of passive solar domestic hot water systems (SDHWS) is typically exposed to ambient weather conditions, and damaging pipe freeze is a major concern. This paper presents a pipe-freeze model that accounts for hot water draws and uses 30 years of actual weather data. The simulation results are cast in terms of pipe-freeze probabilities. Using contour maps of the continental United States, the pipe-freeze probabilities for residential, commercial, and seasonal passive SDHWS are presented for various pipe and insulation geometries.

Potential for Energy Savings Through Residential Hot Water Distribution System Improvements

Hot water distribution systems have received a great deal of attention recently, as residential energy efficiency programs target measures that can drive whole-house energy savings beyond the 50% level. Unfortunately, evaluating distribution losses and the resulting thermal interactions with space conditioning loads is very complicated, and must be performed using advanced simulation tools and realistic hot water event schedules, including appropriate event volumes, draw sequencing, and time between draw events. The authors developed a simplified methodology for the analysis of distribution losses based on detailed modeling of alternative system designs using HWSIM, a model specifically designed for this application. Curve fits were then applied to the results so they could be extrapolated to many different house designs and climate regions. The authors also performed preliminary analysis of whole house energy effects of alternate hot water distribution systems in two climates.

Geographical Limitations on Integral-Collector-Storage Collectors due to Collector Freeze

A major challenge for solar water heaters is to provide heat at a cost comparable to or lower than conventional fuels. Since the price of a passive integral-collector-storage (ICS) solar water heater has historically been less than that for active systems with freeze protection, they can potentially heat water at a lower cost. However, ICS panels are subject to freeze damage, as the collector generally has metal tubes carrying pressurized water that can freeze and burst. In order to delineate the geographical areas where ICS panels can be deployed safely, it is necessary to experimentally characterize the conditions causing freeze damage, to develop a model relating the freeze behavior to climatic conditions, to validate that model with experimental data, and to run the model against long-term weather data across the U.S. Two variations of an ICS panel and/or their bare tubes were tested in a walk in freezer and subjected to freezing conditions until freeze damage occurred. The units tested include both a single and double glazed tubular ICS panel. Key data includes the volume expansion of the tube(s) at burst and the collector loss coefficient near 0 degrees C. Under freezing conditions the insulated supply/return lines would freeze solid initiating a pressure-buildup and eventual burst in the collector tubes due to further internal freezing. An additional test on the single glazed unit was also conducted in which heat tape was installed on the inlet and outlet pipes to prevent them from freezing, which increases the freeze tolerance of the panel by forcing small internal interconnection pipes to freeze solid before damage occurs. Existing models for ICS thermal performance were modified to incorporate the freezing process, and have been validated with the experimental data. The validated models were used to predict regions of the country that are safe for installing the ICS panels. Simulations were run using 30 years of weather data available for all TMY2 sites, and maps were created to illustrate regions of safe installation throughout the US for both the with and without heat tape scenarios for the two ICS models. A correlation using record minimum temperature was developed to generalize the maps to any location for which the record minimum is known. The maps show quantitatively the expected conclusions: 1) that double glazing and higher insulation will extend the safe region; and 2) that the use of heat tape on the inlet and outlet pipes significantly increases the region in which ICS panels can be safely installed in the US.Copyright

Behavior and Testing Performance of a Gas Tankless Water Heater

Tankless water heaters present an opportunity to dramatically reduce water heating energy use. These impacts are possible because of their dramatic reduction of environmental losses through lower heat transfer areas and not keeping the heat exchanger at operating temperature between draws. The potential for energy savings has caused a lot of interest in the scientific community. However, the scientific community has not yet gained an understanding of these devices and several questions regarding their behavior remain. The areas of uncertainty include the following: 1) how these heaters behave around the minimum flow rate, 2) how well they adapt to changes in water flow rate, 3) how they behave in situations with preheated water (i.e. when used with solar water heaters) and 4) whether or not draw characteristics impact the steady state efficiency. Tests have been performed on a Rinnai R75Lsi to determine the answers to these questions for a specific heater. Tests were performed with 1) gradually changing flow rate to identify the minimum flow rate, 2) rapidly adjusting the flow rate to observe how the heater responded to suddenly changing draws, 3) temperature-flow combinations such that the minimum heat rate exceeded the required heat rate, and 4) draws under steady state conditions with varying flow rates and set temperatures. Minimum flow rate results indicate that the heater will not fire unless the flow rate surpasses 2.8 L/min and will cease firing if the flow rate decreases below 2.15 L/min while the owners manual states that the minimum flow rate is 2.3 L/min. Rapidly changing flow rate results indicate that there can be temperature fluctuations up to 9 °C and unsteady operation for up to 1 minute depending on the magnitude of the flow rate change. Tests with preheated inlet water showed that the heater uses feedback controls to avoid unstable operation at low heat rates. Steady state efficiency tests did not identify any variables which impact efficiency. Future work should include testing additional units to determine how other heaters, particularly those not manufactured by Rinnai, behave in similar situations.© 2011 ASME

Liquid Calcium Chloride Solar Storage: Concept and Analysis

Aqueous calcium chloride has a number of potential advantages as a compact and long-term solar storage medium compared to sensibly heated water. The combination of sensible and chemical binding energy of the liquid desiccant provides higher energy densities and lower thermal losses, as well as a temperature lift during discharge via an absorption heat pump. Calcium chloride is an excellent choice among desiccant materials because it is relatively inexpensive, non-toxic, and environmentally safe. This paper provides an overview of its application for solar storage and presents a novel concept for storing the liquid desiccant in a single storage vessel. The storage system uses an internal heat exchanger to add and discharge thermal energy and to help manage the mass, momentum, and energy transfer in the tank. The feasibility of the proposed concept is demonstrated via a computational fluid dynamic study of heat and mass transfer in the system over a range of Rayleigh, Lewis, Prandtl, and buoyancy ratio numbers expected in practice.Copyright

A Model of Radiation-Induced Thermal Stratification in an Integral-Collector-Storage Tank

A finite difference model was developed that predicts the transient temperature profile of the water in an integral collector-storage unit during charge from solar radiation incident on the absorber. The model uses a forward difference technique. A Nu(Gr*) correlation for a flat plate in an infinite medium under prescribed uniform heat flux was used on the inside surface of the absorber to relate imposed flux to boundary layer mass flow and temperature. Nodal mass-energy balances were imposed to derive the overall mass flows and temperatures. Model results were compared to experimental and numerical data with good agreement.Copyright

Geographical Limitations on Integral Collector Storage Collectors Due to Freezing

Passive integral collector storage (ICS) solar water heaters can potentially heat water at lower costs then active systems with freeze protection. However, ICS panels can freeze in cold weather. This study developed a model relating the freeze behavior to climate conditions, validated the model experimentally and then ran the model with long term U.S. weather data to delineate regions safe for the passive solar heaters. Both, a single- and a double-glazed tubular ICS panels were modeled and tested. It was found that freezing begins when the water in the supply/return lines freezes and initiates a pressure build up in the collector which can eventually burst the large collector tubes when the water inside freezes and expands. It was found that freezing can be delayed by installing heat tape over the supply/return lines. Using a model of the collector and TMY2 weather data, correlation maps were developed to show in which regions of the U.S. ICS panels with and without heat tapes can be installed safely.