2
General Considerations

Stephen White, Daniel Mugnier, Daniel Neyer, and Jacqueline Neyer

2.1 Solar Thermal Air-Conditioning General Flowsheet

Numerous technology options are possible in the general class of solar thermal air-conditioning. The key components are the solar collector (source of heat) and the thermal cooling device (producer of cold). A wide variety of alternatives are available for each of these two key components, as shown in Table 2.1.

Table 2.1 Technology options for solar thermal air-conditioning

• - Air collectors
• - Flat plate collectors
• - Evacuated tube collectors

• - Rotary solid desiccant cooling
• - Liquid desiccant cooling
• - Adsorption chillers
• - Single effect absorption chillers

• - Compound parabolic collectors
• - Single axis tracking concentrating collectors

• - Double/triple effect absorption chillers
• - Low-temperature refrigeration

Each of these component technologies is mature and more or less commercially available. Characteristics of these components are discussed extensively elsewhere (e.g., the Handbook). The key issue addressed in this Guide is how the two key components can be combined into a robust and cost-effective solar thermal air-conditioning (SAC) system.

The generic flowsheet for combining the two components is illustrated in Figure 2.1.

Fig. 2.1 Generic flowsheet of a solar air-conditioning system [1]

The generic system contains two flow loops separated by a hot thermal storage (buffer) tank:

• - A solar heat collection flow loop: In this loop, the heat transfer fluid (generally water) is taken from the bottom of the thermal storage tank, heated in the solar collector and returned to the top of the storage tank. Thermal stratification ensures that the coolest fluid is sent to the solar collector, thereby maximizing solar collection efficiency. An additional intermediary heat exchanger and secondary heat transfer fluid is sometimes used (not shown) to either protect against frost or manage high temperatures when there is no demand for solar heat.
• - A process heat flow loop: In this loop, the heat transfer fluid is taken from the top of the storage tank and fed to the desired processes (thermal cooling, space heating and/or hot water production). Numerous heat requirements can be satisfied simultaneously by the one solar thermal system in a well-integrated system. Thermal stratification ensures that the hottest fluid is sent to the process, thereby maximizing chiller capacity and efficiency.

The thermal storage tank (thermal buffer) is a vital part of the system, acting to decouple the intermittent availability of solar heat with the variable demand for cooling by the building. It also provides residence time buffering to prevent the chiller cycling on and off. It should be noted, however, that the thermal storage tank will be subject to heat losses if not adequately insulated.

An auxiliary heat source can be used to provide heat when the sun is not available. However, care should be taken to avoid backup fossil fuel heat completely filling the thermal storage tank and thereby (a) reducing the amount of storage available for storing solar heat and (b) reducing the efficiency of the solar collectors. In Figure 2.1, the flow through the backup auxiliary gas heater is in parallel with the storage tank, so that process fluid flows either from the charged (solar only) thermal storage tank or from the auxiliary gas heater, but not from both. This "one or the other" strategy prevents heat from the auxiliary gas heater ending up in the thermal storage tank. It is a good approach, but not the only suitable approach.

In the specific case of an absorption chiller, additional flow loops are required for both rejecting heat and for circulating produced chilled water to the building (Figure 2.2). Additional cold storage may also be included (not shown).

Fig. 2.2 Generic flowsheet of absorption chiller and auxiliaries [1]

It is evident from this general description that a solar air-conditioning system has a number of additional auxiliary component parts, in the form of pumps and fans, which consume parasitic electrical power. Optimal operation of all these components requires a good logic control strategy that satisfies both the need for robust operation under a wide range of transient conditions and delivers an overall energy saving system.

2.2 Key Design Principles

After a number of years of active research, there are sufficient demonstrations and enough research and monitoring reports to begin to identify:

1. key features that are the hallmarks of good solar air-conditioning design, and,
2. undesirable features that generally lead to poor outcomes, and should be avoided.

This section aims to articulate these features as a set of high-level principles that the engineer should pay particular attention to when designing a new solar air-conditioning installation. This chapter considers these principles at a qualitative level. The subsequent chapters (3, 4 and 5) focus on how these principles have been addressed in the respective case study designs, and provide more quantitative guidance relevant to the specific designs.

Principle 0: Reduce energy demand before applying renewables

Before designing a solar heating and cooling system for a building's heating, ventilation and air-conditioning (HVAC) needs, apply appropriate building design and energy-efficiency measures to reduce both (a) cooling and heating peak loads and (b) overall energy demand. If the demands are low and the building is highly efficient, fluid supply temperatures can be set as low (for heating) and as high (for cooling) as possible. This generally makes the solar heating/cooling system and the overall HVAC system more efficient and reduces capital cost.

Principle 1: Choose applications where high annual solar utilization can be achieved

Given that solar cooling installations require significant capital investment in the solar collector field, it is important to achieve a rapid payback on this capital investment by maximizing the amount of solar heat collected, and gainfully used, over the entire year.

This is complicated by the fact that air-conditioning is a seasonally varying energy demand. In temperate climates, the air-conditioning requirement shifts from heating in winter to cooling in summer, and there are shoulder seasons where air-conditioning demand is relatively low. It is important to maintain maximum solar energy utilization during these shoulder seasons. Even in hot climates, demand for cooling can vary significantly by season.

Two options for maximizing solar collector yield and capacity utilization include:

• - Choose the size of the solar cooling system such that it satisfies only a fraction of the building air-conditioning peak demand (base load covering). In this case, collected solar energy is used as the first preference energy source, and the backup heating and cooling energy source covers the remaining more variable part of the overall building thermal load.
• - Design the solar collector field to first satisfy an alternative, more year-round energy demand (rather than designing for space heating and cooling alone). The most common year-round energy requirement is domestic hot water production (e.g., hotels and residential). In this application the solar collector field would be designed for winter or shoulder season hot water demand, and the resulting excess heat in summer (when more solar radiation is available) is used to assist the peak summer air-conditioning demand.

Summarizing, at the current level of technology and commercial maturity, solar thermal systems for "cooling only" applications may not be economically attractive in temperate climates. Consequently, the designer should be considering opportunities for integrated (multi-application) building thermal energy opportunities. With increasing maturity, the technology will likely become more competitive for air-conditioning only applications.

Principle 2: Avoid using fossil fuels as a backup for single-effect ab-/adsorption chillers

It may seem attractive, from a first-cost perspective, to use fossil fuel resources as a backup energy source for the chiller when solar heat is not available (particularly when a gas boiler is already being installed for domestic hot water or space heating). This would enable the absorption chiller to be sized to deliver all of the required cooling and building air-conditioning demand, obviating the need for a backup vapor compression chiller. However, the efficiency of converting fossil primary energy to cooling, in single-effect absorption and adsorption chillers, is low compared with that of a vapor compression chiller driven by electricity derived from fossil primary energy sources.

A number of studies have shown that primary energy savings can still be...