وابستگی برنامه ریزی تولید به دسترس بودن انرژی حرارتی در گلخانه تجاری - یک مطالعه موردی در آلمان

Commercial greenhouses are often referred to as the optimum heat dump for very low grade waste heat from a variety of sources. However availability, temperature levels and fluctuations in the availability necessitate a very different approach towards production planning as compared to traditional systems where energy supply is adapted to production demand and not the converse. In this study, a commercial ornamental plant nursery that had switched its heat supply from natural gas to utilizing the waste heat of a commercial CHP system in 2007 was analysed. The differences between production planning and temperature regimes before and after switching to waste heat were compared. Furthermore, the degree of utilization of the waste heat available was evaluated. The study undertaken showed three main results: Firstly, greenhouses present a good opportunity for the use of the low grade waste heat. However production needs to be planned very carefully to facilitate the production of high quality plants, for which traditional cultivation planning is unsuitable. Secondly, when planning on utilizing the heat (7.1 GWhth/a) of a CHP system which is sized based on electricity output (6.7 GWhel/a), to the full capacity, additional users need to be found and ideally heat storage integrated. Currently heat utilization amounts to less than 45 % (3.1 GWhth) in total. Especially in summer most of the heat is discarded. Thirdly, several scenarios for utilization optimization were considered. The scenarios described allow for an increase of heat utilization to 62 % or 4.4GWhth using some of the heat to run absorption chillers to provide cooling in summer and storing excess heat in long term heat stores (300 MWhth) to reduce the additional gas demand. The results of the economic evaluation show that the integration of a cold store, supplied by the coolth by using an absorption chiller is financially attractive with a resulting payback period of 2.9 years, whilst consideration of integration of a PCM heat store is far from being viable with a payback period of almost 300 years.

Low grade waste heat often cannot directly be reused within a process without being upgraded. One of the frequently cited solutions to this problem is to use it to warm greenhouses [1]. Intensive horticulture is an important part of the agricultural sector throughout the EU. In Germany 3,700 ha are used for protected horticulture, 91 % thereof for the production of vegetables and ornamental plants [2]. About 40 % of all buildings are older than 25 years and only 20 % being newer than 10 [2]. The typical make up of the buildings in Germany is single glazing (82 %), double PE film with air gaps (8 %), single PE film (3 %), double and single plastic panels (2 % and 1 %). In the UK it accounts for around 12 % of the agricultural output which equates to a total value of over £2bn [3]. Heating usually accounts for 90 % of the energy used in commercial greenhouses [4]. In the UK the primary energy input into protected crops production amounts to 5,207 GJ/a. Compared to residential houses the thermal behaviour of a greenhouse is very different. The greenhouse design itself is a light weight structure, supporting a low inertia transparent envelope that transmits a large amount of solar radiation. The plants within the building capture the bulk of this radiation and convert it mainly into latent heat of vaporization (plant transpiration) and sensible heat. The floor or soil in which the plants grow absorbs the incident radiation and converts some into latent heat [5]. Studies on greenhouse macro climate have been carried out by several researchers. Efforts have been made to predict the greenhouse thermal environment under both steady state and transient conditions [6]. Conventional control techniques are difficult to implement in greenhouses due to the number of variables and their non-linearity. Nonlinear approaches have been developed for temperature and humidity control of greenhouses leading to a more accurate set-point and reduction of disturbance [7]. Fuzzy logic control is used for ventilation of greenhouses [8] as well as temperature and humidity control [9]. Based on the above and other factors, energy management systems have been developed to create the most appropriate microclimate; seeking to maximize plant growth and quality as well as aiming to reduce the final overall cost, eventually bringing about the highest economic profit for the business [10]. According to the Fachagentur für nachwachsende Rohstoffe (FNR) [11] the average energy demand of an ornamental plant production site is 372…453 kWhth/m2a for intensive and 111…131 kWhth/m2a for extensive cultivation. The use of thermal screens can reduce the heat loss from a greenhouse by 20 to 50 %. Costs for installing screens are between 7 and 20 €/m2[12] which results in a payback of less than three years [5]. Electricity is mainly used for hot water circulation, irrigation and fertigation, ventilation and air recirculation and tends to be comparably high for ornamental plants as they have an intensive lighting demand in the winter months. The energy forms used to produce the required process heat in intensive horticulture in Germany are mainly oil and gas with only a few boilers that are newer than 5 years and 42 % being older than 15 years [2]. Due to rising energy costs and environmental considerations in combination with governmental initiatives, renewable energy solutions have become a viable alternative. Schockert [13] gives a comprehensive overview of the most commonly used sources and discusses their suitability. Integration of bespoke CHP systems that utilize bio gas is difficult for the energy demand profiles given in most companies that produce under glass. CHP systems are only viable when run more than 5000 h/a. Significant heat storage facilities are necessary to even out low demand and peak demand periods and backup systems in case of system failure need to be in place etc. Also the heat that is produced at a constant level, needs to be utilized continuously whilst running the system [14]. Many commercial anaerobic digestion (AD) systems with a combined heat and power production usually do not utilize the heat generated. Extensive research has been undertaken to find feasible solutions to overcome these issues but so far only a few satisfactory solutions have been found [15]. In Germany one of the governmental incentives to make the utilization of the heat more attractive is the so called KWK-Bonus, which grants 0.02 €/kWhth of heat utilized but not for emergency cooling [14]. Therefore many owners of greenhouses use the waste heat of commercial bio-gas driven CHP systems for free, while the owner of the system receive the KWK-Bonus [13]. In most cases, the heat demands in summer are significantly lower than in winter or are not present at all. Therefore long term heat storage or heat driven refrigeration systems have to be considered to increase the waste heat utilization and thermal efficiency of the system. In horticulture, long term thermal storage can be realized using different systems such as water stores, rock bed stores and PCM [16]. For storing heat generated by a CHP PCM with a melting point >70 °C are considered suitable. Sethi and Sharma [16] as well as Sharma et al. [6] give comprehensive overviews of PCM with potential for greenhouse application and research undertaken. However, at present several obstacles still have to be overcome before big scale application is possible. On the one hand the issue of evenly distributed heat exchange in large scale applications has not been solved. On the other hand PCM are still very expensive and, therefore, economically not viable at present. The PCM selected for this study, Na2SiO3*5H2O, with a melting point of 72.2 °C currently is traded at around 740 €/t at a capacity of 79.2 kWhth/t. Today’s markets require plants with correct morphological characteristics, predictable flowering and high aesthetic value [17]. Plant growth is determined by several factors such as irrigation, substrate, fertilization, photo period, irradiance [18] and surrounding temperature [19]. Historically, chemical growth retardants were used to control plant morphology in many greenhouse crops. In recent years thermal growth control has gained increased importance. Cultivation temperature has a direct influence on growth and bud development [17]. According to Myster & Moe [20] the so called DIF method which stands for different temperatures during the day and the night is directly related to internode length, plant height, leaf orientation, shoot orientation, chlorophyll content, lateral branching and petiole and flower stalk elongation. Depending on the strategy, positive, zero or negative growth and flowering can be controlled very precisely. Many growers use artificially induced quasi dormancy at 6 °C in cold stores to prevent plants that have already reached the specified characteristics from further growth until the plants are sold. If such a cooling demand is present the use of waste heat to produce cooling using absorption refrigeration is a possibility. This topic is well-researched and there is a multitude of existing applications. When waste heat from a CHP is utilized the term trigeneration is used. Wu & Wang [21] give a comprehensive overview of the state of the art in combined heating, cooling and power generation. As the heat consequently is utilized, the KWK-Bonus will still be in place. The work presented investigates the impact of switching from classical production and energy management in an ornamental plant nursery in Germany to a situation where a constant amount of heat is available from a renewable energy source, namely a bio gas driven CHP system. The degree of utilization of the available heat was evaluated and potentials for the increase of utilization investigated. The nursery investigated has been utilising this waste heat for heating their greenhouses since 2007. Currently there are no other users involved. An energy audit for 2010 was carried out to determine the degree of utilization of the energy available for every month of the year. Furthermore, the impact of changing the availability of thermal energy on production planning was analysed.

The utilization of low grade waste heat in greenhouses presents a good opportunity for the use of waste heat related to power generation and industrial applications as well as for the improvement of production conditions in the horticultural sector. In the case presented 3.1 MWhth (of 7.1 MWhth available) of heat, which otherwise would be wasted are already utillised. With heat available in abundance, higher cultivation temperatures can be used, having a direct positive impact on the use of fungicides, pesticides and other chemicals. Availability of cheap heat in winter allows the nursery in question to produce energy intensive plants in periods of the year when they are rare and therefore sought after, which increases profitability significantly. At the same time the case investigated showed a reduction of direct energy costs for heating by 93 %. However, although bio gas by definition is CO2 neutral, the fact that energy crops are used as feed stock for AD is increasingly controversially discussed, particularly in cases where the heat generated is not or only insufficiently utilised. Therefore it is important for the nursery to increase the degree of i significantly. Cold stores, which are exclusively needed in summer to control the growth of some plants, could potentially be integrated in the system, increasing the degree of heat utilsation from currently 45 % to 62 % and give the company additional opportunities to increase their production efficiency and therefore profitability. At the same time a payback time of 2.9 years for a refrigeration system is well within acceptable limits for investment in the horticultural sector. Through a planned extension of production space in the next two years, the degree of heat utilization will increase to at least 65-70 %, which in combination with the integration of a cold store would result in a total utilization of more than 80 %. Through the integration of long term heat stores utilising PCM the degree of utilization could be increased by another 4 % and therefore the dependency on fossil fuels could be further decreased if not completely eliminated which in turn could reduce the net CO2 to zero. However, with material costs that are more than 260 times higher than the cost for the equivalent amount of natural gas and the very low potential for increase of system efficiency combined with the need of further technological development, this currently is not an option. However, long term heat stores based on revisable chemical reactions, which are based on cheaper materials could be an economically more viable solution even in large scale applicatons in the near future.