EVALUATION OF A GRID-CONNECTED PHOTOVOLTAIC SYSTEM SUPPLYING POWER TO A DAIRY FARM

Transcription

1 CIGR-Ageng212 International Conference of Agricultural Engineering EVALUATION OF A GRID-CONNECTED PHOTOVOLTAIC SYSTEM SUPPLYING POWER TO A DAIRY FARM Lelia Murgia 1 *, Giuseppe Todde 1, Maria Caria 1 1 Dipartimento di Agraria, viale Italia 39, Sassari, 71 Italy) Abstract *Corresponding author. dit_mecc@uniss.it This study aimed to evaluate the operational performances of a photovoltaic power plant installed at a dairy cow farm located in Sardinia, Italy. The electricity production provided by the 48.9 kwp plant had the same seasonal variation as the energy demands for animal farming. During one year the PV array generated 65.7 MWh, with an average of 18 kwh/day, which exceeded by more than 32% the farm daily energy consumption. The peak of monthly production was reached in July (7.9 MWh) whilst the minimum in January (2.4 MWh), corresponding to a mean specific daily yield of 5.4 kwh/kwp and 1.6 kwh/kwp, respectively. Key words: photovoltaic, energy, renewable, dairy, farm. 1. Introduction Photovoltaic (PV) power generation in an agricultural context represents a viable alternative to fossil fuels not only for grid-limited areas (e.g., stand-alone systems) but more extensively for grid-connected installations. Photovoltaic power sources have the potential of reducing peak energy demands, saving electricity expenditures, and producing clean energy. Livestock farming is evolving toward intensive mechanization which requires more energy consumption and result in higher economic and environmental costs (Edens et al., 23; Murgia et al., 28; Dolle and Duyck, 27). In this perspective the increase of photovoltaic energy contribute to improve the environmental sustainability of agricultural activities and generates an additional income through monetary incentives that financially support the electricity produced by renewable sources. The national program Conto Energia started on 24 to promote the PV energy production of grid-connected systems, providing feed-in tariff incentives, based on the power and the integration factor in buildings. This benefits last for 2 years and can be increased by 5% if the electricity consumed results at least 7% of the PV energy generated and if the solar installation replace old asbestos roofs of agricultural buildings (GSE, 21). Furthermore, the quota of self-produced energy exceeding the farm needs and sold to the energy market is considered by the national norms as an economic activity connected to agricultural production. The investment could result interesting especially in Mediterranean area considering the availability of high solar radiation levels and the decreasing costs of the technology. The purpose of this study was to analyze the performance and describe the initial results of a 49 kwp grid-connected PV system installed to supply energy at a cow dairy farm located in Sardinia, Italy 2. Material and Methods The study was conducted on a dairy farm where 15 cows produce 75, kg of milk per year. Farm structures consisted of one free stall barn, a milking parlour, a hay lot and 3

2 silage pits. The photovoltaic array was completely integrated on the roof of the cowshed, covering a surface area of 35 m 2 with a South-West orientation and 11 inclination angle on the horizontal, and at 5 m height from the ground level (Fig. 1). The new covering structure has replaced the previous existent ceiling made by fibrous asbestos cement, and this element adds a bonus on the feed-in tariff paid for the photovoltaic energy produced. The PV system consisted of 272 mono-crystallin modules with 18 Wp nameplate rate (MetaSystem, Type SF125X125-72ML). Each module contains 72 silicon cells encapsulated between two sheets of EVA (ethylene-vinyl-acetate) and glass, and soldered together in series. Cell nominal efficiency is c =.165, while module efficiency is slightly lower ( c =.14). The resulting maximum power of the photovoltaic array is kwp. The connection to the grid is made by three inverters (Danfoss TLX 15K) with three independent string inputs and a three-phase output to the commercial LT (low tension) electrical grid. Table 1 summarizes the main characteristics of the photovoltaic system. Based on local irradiation data, and according to the Italian standards UNI 1349 and UNI 8477/1, the annual available solar energy was expected to be 1693 kwh/m 2, which could provide approximately a total of 68,5 kwh of electricity during in 365 days. FIGURE 1. The photovoltaic array integrated on the cowshed roof Continuous monitoring was carried on to optimize the PV system operation and to record electricity productions and meteorological parameters (solar radiation and temperature). Instant data were collected and managed, both on site and remotely via web, by mean of ComLynx Weblogger (Danfoss), which consent to create numerical and graphical reports on the actual operating conditions. Farm electricity consumptions, output power supplied by PV generator, as well as the daily solar irradiance, were recorded over a period of twelve months (21) to evaluate the technical performances of the system (Marion, 25) and define the solar electricity covering potential in comparison to the energy demands. TABLE 1. Specifications of the photovoltaic system Name Type Open circuit Voltage Short circuit Current Nominal Voltage* Nominal Current Nominal Power Area Weight PV modules MetaSystem SF125X125-72M(L) 44.2 V 5.4 A 35.4 V 5.8 A 18 Wp 1.28 m 2 16 kg Name AC Power (max) AC Voltage Frequency Modules Strings Nominal PV power Inverter System Danfoss- TLX 15K 15, W (3) 43-8 V 5 ± 5 Hz kwp

3 * Standard test conditions: 1 W m-2, 25 C cell temperature, 1.5 AM spectral distribution 3. Results and discussion The annual electricity demand related to animal farming operations accounted for 5.13 MWh, corresponding to 668 kwh/lactating cow and 6.7 kwh/1 kg of milk produced. The most energy demanding operations were milk cooling, ventilation-nebulisation and milking, which required 73% of the total electricity consumed on annual basis. Milk cooling 29% Cow brushing 2% Ventilation- Nebulization 24% Lighting 5% Water pumping 14% Milking 2% Water heating 4% Manure handling 2% FIGURE 2. Main electricity consumptions of livestock. Peak consumptions occurred during summer months (July and August) due to electrical water pumps used for irrigation and to the cowshed cooling systems. These periods of high energy expenditures corresponded to the seasonal trends of greatest productivity of the PV generator. Total PV energy generated over the observation period was 65.7 MWh, exceeding by more than 3% the average annual farm usage recorded before the solar plant installation (29). The peak of monthly production was reached in July (7.9 MWh) whilst the minimum in January (2.4 MWh), corresponding to a mean specific daily yield of 5.4 kwh/kwp and 1.6 kwh/kwp, respectively (Fig 3 and Fig. 4). kwh jan feb mar apr may jun jul aug sep oct nov dec FIGURE 3. Monthly production of PV electricity

4 6, Daily specific production (kwh/kwp) 5, 4, 3, 2, 1, 1,59 2,62 3,51 4,62 4,74 5,35 5,23 5,1 4,31 2,95 2,12 1,95, jan feb mar apr may jun jul aug sep oct nov dec FIGURE 4. Average daily energy produced from 1 kwp. The daily electricity yield resulted on average 18 kwh/day, ranging from 76 kwh/day in January to 262 kwh/day in June. The highest production was recorded June 24 th (Fig. 5). Comparing the amounts of energy produced and consumed on monthly basis, it can be noticed that starting from March, due to the increase of solar radiation, and till November, the energy generation is constantly higher than the demands. Electricity power output (W) Solar radiation Electricity power output Electricity produced KWh /24/ FIGURE 3. Power generated by the PV array, solar irradiation and daily energy yield The annual average efficiency of the system, calculated as the ratio between the energy injected to the grid and the measured solar irradiation, was 12.4%, ranging from 11.7% during summer to 13.3 during winter months. Considering the annual saving in electricity costs because the self-consumption of PV energy (-26.4% respect to 29), the feed-in tariff for the energy produced (.453 /kwh, corresponding about 3, euro in 21) and the energy sold to the National Energy Service (.9 /kwh), a simple payback period of the investment (5,4 /kwp installed) lower than 1 years was estimated. The financial data indicate that PV energy production could represent a profitable activity connected to the main agriculture income Solar radiation (W/m 2 )

5 Moreover, the amount of PV electricity supplied by this plant could reduce the emission of approximately 29 tons of CO 2 eq per year as compared to conventional power from fossil fuels. Conclusions The present study gives some preliminary results on the performances analysis of the PV system. Further investigations and more onsite data measurements are needed in order to evaluate on a larger period the evolution of the electricity consumptions in relation to the PV energy generation. This aspect affects the economic results of the investment. Acknowledgements The authors wish to thank the F.lli Nuvoli Farm, Arborea, and the technical staff ATS Energia for the collaboration in this study References: Edens W.C., Pordesimo L.O., Wilhem L.R., Burns R.T., 23. Energy use analysis of major milking center components at a dairy experiment station. Applied Engineering in agriculture, 19(6): Marion B. et al. 25. Performance parameters for grid-connected PV systems. NREL/CP , feb, 1-6. Dolle J.B., Duyck J.B Les consommations d ènergie en batiments d èelevage bovin. Institut de l Elevage. Chambres de Agriculture de Bretagne et des Pays de la Loire, Compte rendù n GSE, 21. Guida al Conto Energia, Decreto 19 febbraio 27- La richiesta dell incentivazione per gli impianti fotovoltaici, 4a Edizione, marzo 21, Gestore Servizi Elettrici GSE Spa, Roma; GSE, 21. Guida agli incentivi per la produzione di energia elettrica da fonti rinnovabili maggio 21, Gestore Servizi Elettrici GSE Spa, Roma; GSE, 21. Il solare fotovoltaico Dati statistici al 31 dicembre 29, Gestore Servizi Elettrici GSE Spa, Roma; Murgia L., Caria M., Pazzona A., 28. Energy use and management in dairy farms. International Conference Innovation technology empower safety, health and welfare in agriculture and agro-food systems, Ragusa, Italy, sept , 1-7.