As homes move toward zero energy performance, some designers are drawn toward the solar combisystem due to its ability to increase the energy savings as compared to solar water heater (SWH) systems. However, it is not trivial as to the extent of incremental savings these systems will yield as compared to SWH systems, since the savings are highly dependent on system size and the domestic hot water (DHW) and space heating loads of the residential building. In this paper, the performance of a small combisystem and SWH, as a function of location, size, and load, is investigated using annual simulations. For benchmark thermal loads, the percent increased savings from a combisystem relative to a SWH can be as high as 8% for a 6 m(2) system and 27% for a 9 m(2) system in locations with a relatively high solar availability during the heating load season. These incremental savings increase significantly in scenarios with higher space heating loads and low DHW loads.

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.

Residential heat pump water heaters (HPWHs) have recently reemerged on the U. S. market. These units have the potential to provide homeowners significant cost and energy savings. However, actual in-use performance of an HPWH will vary significantly with climate, installation location, HVAC equipment, and hot water use. In conditioned space, the cooling provided by the HPWH can be either a net benefit or a penalty depending on climate; in unconditioned space the ambient air temperature has a significant impact on its performance. To determine the in-use energy consumption of an HPWH in different regions, annual simulations of 50 and 80 gallon HPWHs as well as a standard electric resistance water heater installed in conditioned and unconditioned spaces were performed for more than 900 locations across the U. S. The simulations included a benchmark home to account for interactions between the space conditioning equipment and the HPWH and a realistic hot water draw profile that varied between 45 and 60 gallons per day with local mains water temperature. Results showed that the HPWH will always save some source energy compared to a standard electric water heater, although savings vary widely with location. In addition to source energy savings, the breakeven cost (the net installed cost an HPWH would have to have to be a cost neutral replacement for a standard water heater) was also examined. The highest breakeven costs were seen in cases with high energy savings, such as the Southeast, or high energy rates, such as New England and California. Although the breakeven cost is higher for the 80 gallon HPWH than for the 50 gallon HPWH, the 80 gallon unit's higher net installed costs makes it likely that the 50 gallon HPWH will be more cost effective.

Heat pump water heaters (HPWHs) have recently re-emerged in the U.S. residential water heating market and have the potential to provide homeowners with significant energy savings. However, there are questions as to the actual performance and energy savings potential of these units, in particular in regards to the heat pump's performance in unconditioned space and the impact of the heat pump on space heating and cooling loads when it is located in conditioned space. To help answer these questions, NREL performed simulations of a HPWH in both conditioned and unconditioned space at over 900 locations across the continental United States and Hawaii. Simulations included a Building America benchmark home so that any interaction between the HPWH and the home's HVAC equipment could be captured. Comparisons were performed to typical gas and electric water heaters to determine the energy savings potential and cost effectiveness of a HPWH relative to these technologies. HPWHs were found to have a significant source energy savings potential when replacing typical electric water heaters, but only saved source energy relative to gas water heater in the most favorable installation locations in the southern United States. When replacing an electric water heater, the HPWH is likely to break even in California, the southern United States, and parts of the northeast in most situations. However, the HPWH will only break even when replacing a gas water heater in a few southern states.

Long-term, compact thermal energy storage (TES) is essential to the development of cost-effective solar and passive building-integrated space heating systems and may enhance the annual technical and economic performance of solar domestic hot water (DHW) systems. Systems should provide high energy storage density, charge and discharge temperatures that match the heat source used for charging and the intended load, adequate charge/discharge power, and employ storage materials that are stable over many cycles, non-toxic, environmentally safe. Moreover, these systems must compete effectively in the marketplace and therefore should provide an economic benefit compared to conventional heating and DHW systems, which have traditionally relied on sensible water storage. The IEA joint Task/Annex 42/24 brings together experts in materials development and systems integration to develop advanced materials and systems for the compact storage of thermal energy. Working Group WB2 addresses applications of compact thermal energy storage for space heating and hot water (20 to 100 oC). The activities in this working group are aimed at guiding materials development, within the limitations of these two applications, and at development of systems that use compact storage medium with the goals of improved technical and economic performance compared to traditional approaches. Because solar assisted heating of buildings is the main focus of the SHC IEA, this topic is addressed by many of the contributors to this report. However, because there are many relevant applications for TES, the working group activities encompass other applications including systems for passive building integrated climate control. The primary activities of the working group include: election of candidate materials and system configurations; assessment of the technical performance of systems through laboratory tests of components and systems, case studies, modeling, and/or field studies; assessment of the economic performance through modeling and evaluation of market potential. This report summarizes the contributions of the working group over the period from February 2009 through December 2012. The focus is on the development of system concepts (item 1 from the list above) and technical evaluation of proposed systems and the storage components of these systems through predictive modeling and laboratory testing.

The zero energy community considered here consists of tens to tens-of-thousands of residences coupled to a central solar plant that produces all the community's electrical and thermal needs. A distribution network carries fluids to meet the heating and cooling loads. Large central solar systems can significantly reduce cost of energy vs. single- family systems, and they enable economical seasonal heat storage. However, the thermal distribution system is costly. Conventional district heating/cooling systems use a water/glycol solution to deliver sensible energy. Piping is sized to meet the peak instantaneous load. A new district system introduced here differs in two key ways: i) it continuously distributes a hot liquid desiccant (LD) solution to LD-based heating and cooling equipment in each home; and ii) it uses central and local storage of both LD and heat to reduce flow rates to meet average loads. Results for piping sizes in conventional and LD thermal communities show that the LD zero energy community reduces distribution piping diameters meeting heating loads by ∼5X and meeting cooling loads by ∼8X for cooling, depending on climate.

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.

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.

The market environment for solar water heating technology has changed substantially with the successful introduction of heat pump water heaters (HPWHs). The addition of this energy-efficient technology to the market increases direct competition with solar water heaters (SWHs) for available energy savings. It is therefore essential to understand which segment of the market is best suited for HPWHsand focus the development of innovative, low-cost SWHs in the market segment where the largest opportunities exist. To evaluate cost and performance tradeoffs between high performance hot water heating systems, annual energy simulations were run using the program, TRNSYS, and analysis was performed to compare the energy savings associated with HPWH and SWH technologies to conventional methods ofwater heating.

A membrane absorption heat pump uses absorbent and refrigerant (solvent) flows separated by a membrane to create temperature gradients (aka temperature lifts) used for heating or cooling. Compared to vacuum absorption heat pumps, an atmospheric-pressure membrane heat pump provides more compact designs, potentially enabling applications such as energy-efficient cooling for electronics. In addition, storing concentrated absorbent offers unique options for energy storage for solar heating and cooling of buildings. A new membrane heat pump module was built using two sets of rows of hollow fibers with stagnant air between the fibers to reduce conductive heat transfer. Transport coefficients for the complex air-gap geometry were estimated with a three-dimensional finite-volume heat transfer analysis of the air gap region with results fitted to a modified conduction shape factor. A two-dimensional finite-difference model of the entire process shows good agreement with experiments performed over different air-gap widths, flow rates, inlet temperatures, and absorbent concentrations. Temperature lifts up to 9°C were achieved with 39% (mass) LiCl (aq) feed solution and 35°C inlet temperatures. Extensions of the modeling to higher-porosity, larger-pore-size membranes suggest that temperature lifts of 14°C at ambient inlet temperatures are achievable in our module geometry.

Solar hot water (SHW) systems are being installed by the thousands. Tax credits and utility rebate programs are spurring this burgeoning market. However, the reliability of these systems is virtually unknown. Recent work by Sandia National Laboratories (SNL) has shown that few data exist to quantify the mean time to failure of these systems. However, there is keen interest in developing new techniques to measure SHW reliability, particularly among utilities that use ratepayer money to pay the rebates. This document reports on an effort to develop and test new, simplified techniques to directly measure the state of health of fielded SHW systems. One approach was developed by the National Renewable Energy Laboratory (NREL) and is based on the idea that the performance of the solar storage tank can reliably indicate the operational status of the SHW systems. Another approach, developed by the University of New Mexico (UNM), uses adaptive resonance theory, a type of neural network, to detect and predict failures. This method uses the same sensors that are normally used to control the SHW system. The NREL method uses two additional temperature sensors on the solar tank. The theories, development, application, and testing of both methods are described in the report. Testing was performed on the SHW Reliability Testbed at UNM, a highly instrumented SHW system developed jointly by SNL and UNM. The two methods were tested against a number of simulated failures. The results show that both methods show promise for inclusion in conventional SHW controllers, giving them advanced capability in detecting and predicting component failures.

NREL has developed the novel concept of a desiccant enhanced evaporative air conditioner (DEVap) with the objective of combining the benefits of liquid desiccant and evaporative cooling technologies into an innovative 'cooling core.' Liquid desiccant technologies have extraordinary dehumidification potential, but require an efficient cooling sink. DEVap's thermodynamic potential overcomes manyshortcomings of standard refrigeration-based direct expansion cooling. DEVap decouples cooling and dehumidification performance, which results in independent temperature and humidity control. The energy input is largely switched away from electricity to low-grade thermal energy that can be sourced from fuels such as natural gas, waste heat, solar, or biofuels.

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.

This paper studies a collector design that utilizes unglazed photovoltaic/thermal (PV/T) collectors preheating air for glazed air heating modules. The performance modeling of these collectors is examined both individually and in series. For each collector type, a dynamic, finite difference, first-law model has been created using literature correlations for friction. The models were compared to performance data, calibrating the models by scaling of friction terms for best fit. The calibrated models generally agree well with the experimental data; even during sudden changes to ambient conditions. The root mean square error between the unglazed PV/T model and experiment results for the useful thermal energy gain and the outlet air temperature are 7.12 W/m2 and 1.07°C, respectively. The annual source energy performance of the building-integrated PV/T (BIPV/T) array is then simulated for residential applications in seven climate zones of the United States of America. The performance of the BIPV/T array is characterized by the amount of net electrical energy and useful thermal energy produced. The useful thermal energy is defined as the amount of energy offset by the BIPV/T system for water heating and space conditioning. A BIPV/T system composed 87.5% of PV modules, and 12.5% of glazed air heating modules, offsets the same amount of source energy as a roof-mounted PV system of the same area. This array composition increases the thermal energy gain by 47% over a BIPV/T array composed solely of PV modules.

The installed energy savings for advanced residential hot water systems can depend greatly on detailed occupant use patterns. Quantifying these patterns is essential for analyzing measures such as tankless water heaters, solar hot water systems with demand-side heat exchangers, distribution system improvements, and recirculation loops. This paper describes the development of an advanced spreadsheet tool that can generate a series of year-long hot water event schedules consistent with realistic probability distributions of start time, duration and flow rate variability, clustering, fixture assignment, vacation periods, and seasonality. This paper also presents the application of the hot water event schedules in the context of an integral-collector-storage solar water heating system in a moderate climate.

Does pore-size distribution need to be considered to accurately model vapor transport in membrane distillation (MD)? This paper addresses that question from a theoretical perspective. Although some previous work has discussed pore-size distribution in MD, there has yet to be a comprehensive, general analysis of its effects on MD and its various configurations. In this work, a numerical model is used to calculate the flux through all pore sizes to estimate the effect of pore-size distribution on MD flux. The modeling shows that the error in the calculated flux incurred by neglecting pore-size distribution is largest for a microfiltration process, where viscous flow dominates, somewhat smaller for vacuum MD, where Knudsen flow dominates, smaller still for direct-contact MD, where molecular diffusion usually dominates, and smallest for air-gap MD, where the air gap dominates the overall mass transfer resistance. Considering a membrane with a mean pore size of at least 100nm and a geometric standard deviation of the pore size of 1.2, the error is: 9% for vacuum MD, 3.5% for direct-contact MD, and less than 1% for air-gap MD.