Introduction

In Mexico, coffee is one of the crops of greater extension and economic importance, since it is among the five most important export products. The production areas are concentrated in 12 states, mainly in small production units in mountain areas, where the states of Oaxaca, Chiapas, Veracruz and Puebla, contributing more than 80% of national production.

The industrial process for the transformation of coffee cherry to parchment, it is called profit and comprises the steps of: sorting, washing, pulped, mucilage removal and drying. Of you are stages, drying is critical because it is intended to reduce the moisture content of washed coffee (48-56% wet basis) to a range ranging between 10% and 12% humidity.

The traditional process of drying coffee beans in the coffee regions of the country, is outdoors on sheets of concrete (asoliaderos), where the moist grains are directly exposed to sunlight for nine to about twelve days rem returning The grain periodically until reaching a moisture content of 12% on average. The drying system is simple, but there are limitations that reduce the quality of the same; such as sudden rains, dust, garbage and animals; you should also store or cover the coffee at night to avoid re-absorbing moisture and the inability to dry high volume of coffee.

One of the solution alternatives is to perform the drying process of the coffee bean inside a solar dryer, whereby; greater opportunity to market coffee with fewer defects (stain), more safety and better sensory quality, approaching the concept of organic coffee, where specific requirements are met in the drying step such is obtained maximum use of: the energy of the sun, avoid contact of the grain with the soil (^{CERTIMEX, 2009}). In addition, only plastic made of polyethylene or polypropylene protection purposes always allowed and when the coffee does not come into direct contact with plastic (^{Paswan and Mohit, 2010}). In addition, the use of solar dryers for drying parchment coffee responds to a trend in the development of sustainable technologies, designed to protect the grain from adverse weather conditions and a dry grain quality.

The operation of solar driers is based on the greenhouse principle, where solar energy is trapped by manifolds which raise the temperature of the fluid (air), which performs the drying process, so that the efficiency of a drying system usually expressed by different parameters such as efficiency of the dryer, the solar collector efficiency, extraction rate specific humidity, evaporative capacity, among others.

The thermal performance evaluation in drying applications sun is considered a means to evaluate as it operates a dryer under solar certain conditions and is defined as the ratio of energy for drying and amount of water evaporated during the process. Therefore, the total thermal efficiency is the ratio of the gain useful heat of solar energy available inside the solar dryer (^{Almuhanna, 2012}). Thus, an efficiency from a solar dryer it is a measure of the effectiveness with which used the radiation solar system to dry the product and evaluate your performance (^{Keke et al., 2014} and ^{Mustapha et al., 2014}). That is, the energy needed to evaporate water grain (kJ h^-1) divided by power input to the dryer and is expressed in percentage (^{Parra-Coronado et al., 2008}; ^{Chavan et al., 2011}). Typical values of this parameter are in the range 10-50% depending on the operating mode and the type of dryer (^{Tiwaria et al., 2013}, ^{Zakaria, 2013}).

In relation to the evaluation of the profitability of a solar dryer with respect to a dryer that uses fossil energy, the comparisons are required to be carried out under conditions of commercial use. Such a comparison provides the unit cost of drying and of course, the difference in cost of each method. Thus, the initial investment in a solar dryer is lower or at most equal to a conventional dryer of the same volume, so the differentiation is in the costs of energy to perform the drying process, with lower times of amortization (^{Sreekumar, 2013}). According to ^{Ferro et al. (2000)}, the following cost elements can be used for economic comparison calculations between solar dryers.

Direct costs. Raw materials, operating labor and supervision and office labor.

Costs of services. Electricity, maintenance and repairs (2-10% of fixed capital), operating supplies (10-20% of operating labor) and laboratory charges.

Indirect costs. General (payroll and plant), packing, storage, local taxes (1-2% of capital), insurance (0.4-1% of capital).

Furthermore, ^{Boonyasri et al. (2011)} recommend as the basis for calculations of the total cost to the sum of the cost of the dryer, maintenance costs, labor, the cost of the product to be dried and depreciation. Another way to get the economic feasibility of a solar dryer is by calculating the savings obtained by using a solar installation against an alternative conventional drying. But is requires assessing the feasibility of a solar dryer drying only option, we suggest using the present value or discounted cash flow, which determines whether the present value of the expected cash flows justifies the initial investment.

The economic attractiveness of a potential investment in solar energy for industrial processes can be determined by comparing the price of solar energy with the price of the fossil fuel used, both expressed in unit terms or equivalent energy. This requires the calculation of the internal rate of return of the solar investment or, in some cases, the growth rate of return.

Therefore, the objective of this study is to evaluate the thermal and financial efficiency of the drying process of grain coffee washing in an active solar dryer greenhouse.

Materials and methods

The design of the dryer is under study greenhouse, whose measurements are 2.4 m wide and 5.6 m long, with a useful drying chamber to 5.8 m³. Collector consists of a 15 m2 part of the drying chamber, built with galvanized sheets of the lowest caliber, painted black “mate” on the sunny side, with an orientation to the south and an inclination equal to the latitude of the place part. The construction is based on PTR frames and covered with transparent polyethylene, caliber 720 with treatment for UV rays, commonly used in the construction of greenhouses. To move the air into the dryer at a speed of 1.5 m s-1, an electric fan motor is used 0.186425 kW. In addition, it has manual closing vents for extraction of humid air. The grain was placed in trays made with wooden frame and 80% shade cloth with dimensions of 0.3*0.85 m, which served as drying beds, bed trays being formed in august, forming five lines inside the dryer bed. The proportion of grain was 19.5 kg m^-2 (^{Menya and Komakech, 2013}).

The thermal efficiency of the solar dryer is determined using the following equation (^{Monrudee et al., 2011}; ^{Tiwaria et al., 2013}; ^{Zakaria, 2013}):

Where: ƞs= efficiency of the solar dryer; Wo= weight the starting material (kg); Wt= weight the the material at time t (kg); Lv= latent heat of vaporization of water in the parchment coffee (kJ kg^-1); Ht= time radiation incident on the inclined surface of the collector; (kJ m^-2); A= collector area (m²); Pv= consumption energy by fans (kJ).

The latent heat of vaporization of water in the parchment coffee (Lv) heat is obtained using the following equation:

Where: Lv = value latent vaporization of parchment coffee, kJ kg^-1; T= temperature coffee, °C; M= contents moisture coffee, decimal, bs.

For the economic evaluation, the following data were considered: a volume of 200 kg (19.5 kg m²) coffee bean dried load on the solar dryer with an average time drying of 5 days (moisture loss from 55% to 12%) and given that the harvest period lasts coffee 25 weeks or so, so it is assumed to be performed on average 25 drying loads (one per week) per year.

In the evaluation of the present value (PV) or discounted cash flow (FED), the following equation was used to determine if the present value of the expected flows justifies the investment (^{Purohit et al., 2006}; ^{Purohit and Purohit, 2010}):

Where: Vp= value present of future total flows; Ij= ingress in the j-th load; Gj= expenses incurred in the solar dryer at the j-th; n= planning horizon; i= discount rate

The payback period of the investment was calculated with the following formula (^{Passamai and Passamai, 2006}):

And the profitability of the inversion of the solar dryer, was determined with the following formula:

Results and discussion

The traditional process of drying the coffee bean is about 9-12 days while using a solar dryer was 44 hours of accumulated sunshine (5 days on average). About 7 h sun first day were recorded as the dryer was prepared in the morning, 10 days 2-4 and 7 day 5, since the dryer was emptied afternoon. The average maximum temperature reached within the dryer was 46 °C, with a relative humidity of 60% on average. A moisture percentage of the parchment coffee of 11.8% was obtained. These results show that by using the solar dryer it is reduced up to 40-60% drying time compared to traditional drying (Figure 1). These results are consistent with those reported by ^{Henry et al. (2013)}, who counted 11 hours of daily sunlight and a time between 5 to 7 days, to dry pepper by solar dryers.

Figure 1 Drying curve for coffee beans solar dryer.  

Compares the existing temperature conditions inside the dryer and the environment, are always higher values of temperature inside the dryer by the greenhouse effect (Figure 2), 20 °C the greatest difference and presents approximately at 16 h, this enabled a faster drying environmentally. The difference between the internal temperature and the environment is greater than the difference of 13.6 °C reported by ^{Menya and Komakech (2013)} in a greenhouse dryer, located in Uganda, and also greater than the temperature difference of 12.8 °C reported by ^{Monrudee et al. (2001)} and 14.1 °C reported by ^{Almuhanna (2012)}, both in greenhouse dryers, and less than the maximum difference of 27 °C reported by ^{Ferreira et al. (2008)} using a solar chimney or the difference 29.9 °C of temperature inside a solar drier over the ambient temperature, reported by ^{Sreekumar (2013)}.

Figure 2 Dryer interior temperature- external environment, in a normal day to start the fall (September).  

However, the temperature profile is not constant during the hours of one day, nor during the days of the drying process. This is because the temperature of the dryer depends on solar radiation, which is not constant. Also the relative humidity of the drying air is a critical factor in controlling the drying rate of the product. The lower the relative humidity, the greater the drying air absorption capacity. In order to determine the overall efficiency of the dryer, the average insolation data was taken in the area where the dryer was evaluated, which was determined using equation 1, resulting in an estimated 12%.

Furthermore, moisture loss was faster in the initials stages of drying, since the grain has higher moisture content and this water is easier to remove, becoming increasingly difficult to remove moisture at the end of the drying process. These results are consistent with the assessments made by ^{Rajeshwari and Ramalingam (2012)}, who reported an efficiency of 15% to 18%, in all conditions test design solar dryer box type, but are low values regarding the efficiency of 29% by dry coffee with an indirect solar dryer with forced convection, reported by ^{Restrepo and Burbano (2005)} and the value of 52.55% average efficiency for a day, reported by ^{Sreekumar (2013)}. ^{Bergues-Ricardo and Díaz-López (2014)} when evaluating a sample of existing solar dryers in Latinoamerica, found that its efficiency varies between 2 and 50% on average.

As the drying process greatly affects to quality coffee in terms of appearance and taste. when the drying process coffee bean done properly within the recommended conditions, a quality higher product is obtained (^{Menya and Komakech, 2013}). Thus, depending on the location of the dryer and the conditions of insolation, at certain times of the day temperatures up to 50 °C within the dryer, which are suitable for drying coffee (^{Restrepo and Burbano, 2005}). This is because when you exceed 50 °C temperature in the drying process of coffee beans for a period of 4 to 10 h, there are losses in the taste of coffee. Overheating during drying produces citrus or “cooked” flavors in the coffee beverage or a minimum layer thickness of the grains to dry, generating a hardening of the outer surfaces of the coffee beans that compromise the body and flavor thereof (^{Menya and Komakech, 2013}).

An important aspect to keep in mind is what happens during the night, when there is a cooling of the drying chamber, resulting in condensation on the walls of the dryer. One way to handle this aspect is to keep the vents open and turn on fan to move the relative humidity of the inside the chamber.

Regarding the economic evaluation, applying the methodology used by ^{Purohit et al. (2006)} to determine the cost of drying, which is a function of time and inputs (electrical energy, equipment depreciation, volume of parchment coffee, salaries per load, land cost and administration), the result was $4.00 per kilogram of parchment coffee (Table 1). The expenses to carry out the drying process of a load of washed coffee made a total of $3 200.00 (total cost of drying plus the cost of coffee without dry) and cost of installing a solar dryer of $25 000.00. The market price of parchment coffee is $32.50 kg. Marketing the dried grain in the solar dryer under this scheme generates an income of $6 500.00, which implies a profit of $3 300.00 for drying load. Under the assumption of intensive use (carrying 25 loads per year), during the first year there is a net flow of $57 500.00 (Table 2).

Table 1 Costs per kilogram and total cost of solar drying of coffee beans  

Table 2 Net flow ($) of solar drying of coffee beans in the first year of service.  

By applying the present value pre equation (VP) or discounted cash flow (FED) with data Table 2 and assuming a planning horizon of 5 years (average useful vine solar dryer), with a spending annual maintenance of $5 000.00 (replacement cover transparent polyethylene) and an annual discount rate of 8%. Yearly income is $74 500.00 and a value present at the end of the planning horizon equal to $335 004.00 (Table 3), all under consideration washing and drying coffee with full time use of solar dryer (25 loads per year).

Table 3 Present value of project solar drying of coffee bean. 

With net income for the first year of using the dryer and applying the equation of payback period of investment, it has a value of 0.435, which implies a recovery in less than a year and to know which load is, the operation is performed following:

This means that the recovery is performed at the end of the eleventh drying load in the solar dryer. That if used full time, this would be approximately three months after the investment was made. These results are in line with the recovery period of approximately six months when solar dryers are used for solar dehydrated apple, and lower the recovery period reported by ^{Monrudee et al. (2011)} of 1.15 years, considered quite short in relation to the useful life of a solar dryer.

The return on investment of the solar dryer had a final value of 2.3. This is that having an annual flow of $57 500.00 by performing drying 25 loads, have a 230% rent reliability, being very attractive investment. This despite the low volume dried load, but ^{Mohod et al. (2011)} report a cost benefit ratio of 1.21 and a recovery period of 2.84 years.

This recovery of the investment and its respective profitability is possible under the assumption that the dryer will be used at its maximum capacity and all the time, otherwise, just as conventional dryers will be underutilized and generating economic losses.

Conclusions

The time for drying coffee using a active solar dryer type greenhouse in the coffee in growing area of northeastern mountains of Puebla, it is 5 days on average. This shows that the drying time is reduced by 40-50% approximately, with an increase of the inner temperature of 20 °C over the ambient temperature, which generates an overall efficiency of solar dryer estimated at about 12%, which makes it suitable to obtain dry parchment coffee at the level of small producers.

The present value of the flows is positive from the first three months of use, therefore, it’s recommended to use solar dryers in the drying of coffee beans. The period of recovery of the investment in solar drying is achieved by performing the eleventh drying load, so that the recovery time depends on the intensity of use of the solar dryer.