# New Developments and Progress in Absorption Chillers for Solar Cooling Applications
## 2 Solar-Powered Absorption Cooling Systems
### Primary Energy Use  
Another main issue for the supply of cost-competitive cold using absorption chillers is the effective use of the primary energy of the entire solar absorption cooling system (Figure 1) [17]. Hence, apart from increasing thermal efficiency (i.e., high thermal COP, Equation (1)), energy density (coolingcapacity per chiller volume), and reducing capital investment, the consumption of parasitic electricity is a crucial factor in solar space cooling systems based on absorption chillers. The thermal COP, \(COP_{th}\), is defined as the ratio of the cooling capacity, \(\dot{Q}_{cold}\), to the driving heat input (\(\dot{Q}_{dhs}\)):  
\[COP_{th}=\frac{\dot{Q}_{cold}}{\dot{Q}_{dhs}} \tag{1}\]  
The parasitic electricity consumption is the electricity consumed by the circulation pumps to drive heat, reject heat, and carry cold, and other systems such as fans and the control system. The electrical efficiency of the system is determined in terms of electrical COP, \(COP_{el}\), defined as the ratio of the cooling capacity to the total parasitic electricity consumption (\(\dot{W}_{total,el}\)):  
\[COP_{el}=\frac{\dot{Q}_{cold}}{\dot{W}_{total,el}} \tag{2}\]  
Furthermore, the contribution of solar energy (\(\dot{Q}_{sol}\)) to cold production is often expressed in terms of solar fraction (\(SF\)) as:  
\[SF=\frac{\dot{Q}_{sol}}{\dot{Q}_{dhs}} \tag{3}\]  
In solar-assisted absorption cooling systems, the driving heat is the sum of solar heat and auxiliary heat from the backup system (e.g., gas-fired heater (or boiler)): \(\dot{Q}_{dhs}~{}=~{}\dot{Q}_{sol}+\dot{Q}_{b}\).  
The primary energy, \(PE_{abs}\), demand of the solar absorption cooling system includes: (i) the part associated with the parasitic electricity demand (i.e., \(\dot{W}_{total,el}/\eta_{el}\)) and (ii) the demand of the backup heater (\(\dot{Q}_{dhs}(1-SF)/\eta_{b}\)). \(\eta_{el}\) and \(\eta_{b}\) are the electric power plant and backup heater efficiencies, respectively. Then, the primary energy rate (\(PER_{abs}\)) of the system becomes [18]:  
\[PER_{abs}=\frac{\dot{Q}_{cold}}{PE_{abs}}=\frac{\dot{Q}_{cold}}{\left(\dot{Q}_{ dhs}(1-SF)/\eta_{b}\right)+\left(\dot{W}_{total,el}/\eta_{el}\right)} \tag{4}\]  
The solar-assisted absorption cooling system is more competitive than the conventional cooling system when its primary energy rate, \(PER_{abs}\), is higher than the electricity-driven vapor compression chiller (\(PER_{com}\)) which is the benchmark for space cooling technology. This means that the primary energy rate ratio (\(\pi\)), defined in Equation (5), must be higher than one [18].  
\[\pi=\frac{\eta_{b}COP_{th,abs}}{\eta_{el}\cdot COP_{com}}\left(\frac{1+p_{a} \cdot(1+COP_{com})}{1+p_{a}\cdot\left(1+COP_{th,abs}\right)\cdot(\eta_{b}/ \eta_{el})-SF}\right) \tag{5}\]  
where \(COP_{th,abs}\) is the absorption chiller thermal \(COP\), \(COP_{com}\) the compression chiller \(COP\), and \(p_{a}\) the parasitic electrical power demand ratio. The value of \(p_{a}\) is equivalent to the ratio of the parasitic electrical power demand to the heat rejected by the absorption chiller, i.e., approximately \(\dot{W}_{total,el}/\left(\dot{Q}_{dhs}+\dot{Q}_{cold}\right)\)[17].