Journal of Membrane Science 337 (2009) 113–124 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Modeling of a membrane-based absorption heat pump Jason Woods a,∗ , John Pellegrino a , Eric Kozubal b , Steve Slayzak b , Jay Burch b a Department of Mechanical Engineering, University of Colorado Boulder, 427 UCB, Boulder, CO 80309-0427, USA b National Renewable Energy Laboratory, Golden, CO, USA article info Article history: Received 6 October 2008 Received in revised form 2 March 2009 Accepted 24 March 2009 Available online 1 April 2009 Keywords: Heat pump Energy storage Heat and mass transfer modeling Desiccant Membrane distillation abstract In this paper, a membrane heat pump is proposed and analyzed. Fundamentally, the proposed heat pump consists of an aqueous CaCl 2 solution flow separated from a water flow by a vapor-permeable membrane. The low activity of the solution results in a net flux of water vapor across the membrane, which heats the solution stream and cools the water stream. This mechanism upgrades water-side low-temperature heat to solution-side high-temperature heat, creating a “temperature lift.” The modeling results show that using two membranes and an air gap instead of a single membrane increases the temperature lift by 185%. The model predicts temperature lifts for the air-gap design of 24, 16, and 6 ◦ C for inlet temperatures of 55, 35, and 15 ◦ C, respectively. Membranes with lower thermal conductivities and higher porosities improve the performance of single-membrane designs while thinner membranes improve the performance of air-gap designs. This device can be used with a solar heating system which already uses concentrated salt solutions for liquid-desiccant cooling. © 2009 Elsevier B.V. All rights reserved. 1. Introduction As with other renewable technologies, solar thermal energy for residential water and space heating is plagued by the imbalance between the time the resource is available and the time the resource is needed. This occurs over many timescales, but in the extreme, this can be illustrated by the low-load, high-resource summers and high-load, low-resource winters, as shown in Fig. 1. Thermal energy storage can alleviate this imbalance by storing excess solar heat for later use. The amount of sensible energy stored depends on the heat capacity of the storage medium, the maximum temper- ature limit (e.g., 100 ◦ C for water) and the minimum temperature required to meet the heating load. For the application considered here, the required minimum temperature for space heating and domestic hot water is around 60 ◦ C. Although water has a high spe- cific heat capacity, large storage tanks are still required to meet a large portion of the load. One way to reduce tank size is to use a heat pump during low-resource times to upgrade energy with a temperature too low to be useful to high-temperature energy. Vapor-compression and absorption heat pumps can be used for this purpose. In this paper, we propose a heat pump that uses water and a low-activity salt solution (often referred to as a liquid desiccant). The advantage of this type of heat pump is that the salt solution can be concentrated using solar energy and stored with no (or min- imal) losses. The low-activity of the solution then attracts water ∗ Corresponding author. Tel.: +1 317 435 1207. E-mail addresses: jason.d.woods@colorado.edu, jdwoods21@gmail.com (J. Woods). vapor, and its corresponding latent heat, and provides a “tempera- ture lift” by upgrading low-temperature heat to high-temperature usable heat. In this way, the energy required for heat pumping is also supplied by stored solar energy. Water and a solution of lithium chloride are used in a previously developed absorption heat pump [2]. This device is a closed-system, using evacuated chambers for the heat pumping process. The pro- posed heat pump in our study is an open system using microporous membranes to separate the water and the low-activity salt solution. This eliminates the complexity of a vacuum system and also allows for separate storage of the solution, facilitating optimal design of both the capacity (kWh) of the storage and power (kW) of the heat pump device. In addition, with the open system, the salt solution can be used in the summer for air dehumidification. The purpose of this paper is to determine the feasibility of using a membrane exchanger as a heat pump. In our study we evaluated the performance based on calcium chloride (CaCl 2 ) as the low- activity solution. The paper addresses the concept of a membrane exchanger heat pump, the ideal maximum performance, and the predicted deviation from this ideal case when using a membrane exchanger. The performance is predicted with a numerical model. The theoretical effects of membrane properties are also discussed. 2. A membrane exchanger heat pump 2.1. The heat pump process using membranes The heat pump process proposed here is similar to membrane distillation (MD) and osmotic distillation (OD) processes [3–6]. In these processes, a water vapor pressure difference across a 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.03.039