Sun radiates its energy at the rate of about 3.8 × 10²³ kW per second [1]. Solar energy, being the most promising among all the renewable energy sources is transmitted radially as electromagnetic radiation. Solar energy touches the boundary of the atmosphere at about 1.5 kW/m². After traversing the atmosphere, the earth’s surface receives about 1 kW/m² of solar radiation [3]. According to [3], Nigeria receives about 5.08 × 10¹² kWh of energy per day from the sun and if solar energy appliances with just 5% efficiency are used to cover only 1% of the country’s surface area, then 2.54 × 10⁶ Mwah of electrical energy can be obtained from solar energy. This amount of electrical energy is equivalent to 4.66 million barrels of oil per day.

Heat energy from the sun is one of the best renewable energy options for use in adsorption cooling applications, especially in remote and rural locations. Based on the energy requirement of the adsorption cooling system, a flat plate collector may be used because the temperature needed is not very high [4]. In Solar thermal applications, solar energy as electromagnetic waves is first converted into heat energy. The heat energy is then used directly or transferred through a medium (mostly water) to the point of application. Heat is needed in the adsorption refrigeration system to remove the adsorbate (refrigerant) from the adsorbent [7]. Different sources of heat such as industrial heat waste, electrical heating elements, and solar radiation can be used [2].

The adsorption cooling system consists of four major components; adsorber, condenser, expansion device, and evaporator [8]. The arrangement of the major components is represented in Figure 1.

Adsorption refrigeration systems use solid materials like Celica gel, Zeolite, Activated-Carbon, and some specially prepared composite materials as absorbents while the refrigerant fluid (adsorbate) could be Water, Ammonia, Ethanol, or Methanol [9]. The adsorption system does not use mechanical energy but only small heat energy and can either be an intermittent or continuous cycle [8]. According to [10], the important considerations influencing the choice of a suitable adsorbent are:

1) Adsorption of large amounts of adsorbate under low-temperature conditions to yield good COP.

2) Desorption of most of the adsorbate when exposed to thermal energy.

3) Possession of high latent heat of adsorption compared to sensible heat.

4) No deterioration with age or use.

5) Non-toxic and non-corrosive.

6) Low cost and widely available.

Choice of adsorbate depends to a large extent on the following properties [11]:

1) Evaporation temperature below 80˚C.

2) Small molecular size to enable it to be adsorbed into the adsorbent.

3) High latent heat of vaporization and low specific volume.

4) Thermally stable with the adsorbent at the cycle operating temperature ranges.

5) Non-toxic, non-corrosive and non-flammable.

6) Low saturation pressures (slightly above atmospheric) at normal operating temperature.

Performance limitations of adsorption cycles with different adsorbates for solar cooling were investigated by Critoph (2000), concluding that, in general, the activated-carbon/methanol combination was preferable for solar cooling, giving the best COP in a single-stage cycle.

There are other reasons that make activated-carbon/methanol the most favored pair as was reported by [12]. These are briefly outlined below:

1) Activated-carbon is cheaper than zeolite.

2) Activated-carbon can be produced with varying properties by varying the activation time and temperature to suit particular applications.

3) Activated-carbon can be produced from biomass.

The predominant adsorbent/adsorbate pairs are zeolite/water and activated-carbon/methanol. Zeolite/water is usually used for cooling systems while activated-carbon/methanol is used in ice-making systems [13].

In particular, the solid-vapor (adsorbent/adsorbate) equilibrium is obtained from the Clausius-Clapeyron equation [14], this is stated in Equation (1).

d P d T = Δ H ( T ( V v − V l ) ) (1)

where:

P is the vapor pressure.

T is the absolute temperature.

ΔH is the isosteric heat of adsorption per unit mass of adsorbent for the transition, kJ/kg.

V_v and V_l are the specific volumes of the vapor and liquid phases of the refrigerant respectively, m³/kg.

In practice, V v ≫ V l and also it may be assumed that the perfect gas law is obeyed approximately by the vapor, thus (1) becomes:

d P d T = Δ H R T 2 (2)

where:

R is the Universal gas constant.

Hence by integrating (2):

log P = Δ H R T + Constant (3)

This linear relationship between logP and 1 T is determined in practice to a first approximation. From the plot, we can estimate the isosteric heat of adsorption from the gradient of the equilibrium line [14].

The test rig as shown in Figure 2 consists of:

1) 12.5 mm brass pipe and accessories;

2) 0.01 - 2.0 bar pressure gauge;

3) 0˚C - 140˚C temperature gauge;

4) 0.75 Hp electric Vacuum pump;

5) 200 W electric heating element.

A standard equilibrium test was carried out as performed by [15]. The refrigerant (Methanol) cylinder was first weighed. The pressure vessel containing 1 kg of activated-carbon was immersed in oil contained in the constant temperature bath. The adsorbent is heated by heating the oil bath through an electrical heating element. Valve one (V1) was open while valve two (V2) was closed and the pressure vessel was first evacuated at a high temperature using a vacuum pump. The pressure vessel was later allowed to cool. Valve one (V1) was closed and a known mass of adsorbate was administered by opening valve two (V2). The temperature of the vessel was varied between 30˚C to 160˚C and the corresponding pressure was measured at each stage. With the concentration, pressure, and temperature known, the P-T-X data for each of the samples of adsorbent were obtained.

Energy input per unit time for the vapor compression system was obtained from the vapor compression test rig. Also, the coefficient of performance of the system was determined using the data obtained from the vapor compression test rig. For the adsorption cooling system, using (3) as stated in the preceding section log P = Δ H R T + Constant from the plot of the linear relationship between logP and 1 T , isosteric heat of adsorption (ΔH) was estimated from the gradient of the equilibrium line as performed by [12].

Coefficient of performance, COP is given by:

C O P = Q c Q i n (4)

where:

Q_c is the useful cooling in kJ/kg of adsorbent. Q_c given by:

Q c = L ( D X ) − C P R ( T c o n − T e v a ) (5)

Q_in is the heating input per kilogram of adsorbent. Q_in given by:

Q i n = Q s e n + Δ H (6)

Q_sen is the sensible heat needed to heat the adsorbent and its containing vessel to the maximum temperature.

(ΔH) is the isosteric heat of adsorption.

The range of solar flat plate collectors’ thermal energy output (from literature) was analyzed for possible application in the adsorption system based on the adsorption system energy requirement obtained. The COPs obtained for the two systems were also analyzed.

The energy required by the vapor compression test rig and the COP of the system was obtained as 8932.02 kJ/h and 1.2 respectively. These results agreed closely with the results obtained by [11].

The energy required by the adsorbent/adsorbate equilibrium test (adsorption cooling system) and the COP of the system were estimated as 1913.57 kJ/h and 0.39 respectively. This closely agreed with the results obtained by [1], the difference being due to the fact that the mass of the refrigerant slightly differs.

Figure 3 shows a linear plot of log(P) Vs 1/T for the adsorbent/adsorbate (activated carbon/methanol) equilibrium test.

The gradient of the linear plot for activated carbon/Methanol Equilibrium was found to be −61.8. This shows that adsorption is exothermic and that it requires a low amount of thermal energy to induce desorption of the adsorbate adsorbed thus can be derived by low-grade energy. The Clapeyron equation assumes that the vapor phase behaves like perfect (ideal) gas and that the molar volume of the liquid phase is much smaller than the molar volume of the gas phase [12].

From the table of properties, the specific gas constant for Methanol, R_methanol = 8.3145/32.0 × 10⁻³ = 259.83 J/kg·K. Therefore, the Isosteric heat of adsorption for activated carbon/methanol was estimated to be a first approximation to be 16.058 kJ/kg.

From the linear plot, it was observed that at a pressure above atmospheric, the plot is perfectly linear obeying the Clapeyron-Clausius equation. This corresponds to temperatures above the melting point of methanol. However, below this point (1 atm and 64.85˚C) the plots tend to slightly disobey the linearity (Clapeyron-Clausius equation). This is because of the presence of a partial vacuum created initially before raising the temperature of both adsorbate/adsorbent pairs. This is further indicated by the occurrence of negative values of log(P) below atmospheric pressure (partial vacuum). At one atmospheric (1 atm), log(P) becomes zero and gradually positively increases with further addition of heat.

A review of adsorption cooling technology was carried out. Energy requirements and COPs of both the adsorption system and vapor compression system were obtained, evaluated, and compared. Solar flat plate collectors’ thermal energy output (from literature) was analyzed for possible application in the adsorption system based on the adsorption system energy requirement obtained.

Overall, the energy required by the adsorption cooling system for the period (1913.57 kJ) is relatively much lower than that required by the vapor compression system (8932.02 kJ) indicating the huge energy savings potential of the adsorption cooling system.

The COPs of the adsorption cooling system (0.39) are much lower than that of the corresponding vapor compression system (1.2) indicating a relatively higher cooling capacity by the vapor compression system. In adsorption systems, this can be compensated by running the system for a relatively long period of time while yet maintaining the energy-saving potential.

It is also recommended that further work should be carried out towards producing composite adsorbent or adsorbate (like ionic liquids) from two or more different materials with the aim of playing around with some suitable properties of the materials to get adsorbent with better properties.