Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Day-to-night heat storage in greenhouses: 4. Changing the environmental bounds
    Seginer, Ido ; Straten, Gerrit van; Beveren, Peter J.M. van - \ 2020
    Biosystems Engineering 192 (2020). - ISSN 1537-5110 - p. 90 - 107.
    Control bounds - Greenhouse control - Greenhouse model - Heat buffer

    Controlling the greenhouse environment usually involves bounds (restrictions) on the indoor conditions. In model-based control, these bounds are meant to keep the plant environment away from high risk zones, the effects of which are not sufficiently well described by the model. The objective is to estimate the potential energy saving and gain in profit resulting from relaxing the bounds. The calculations employed a previously developed simulation-optimization program in conjunction with a new, solar-driven evapotranspiration model. Spanning a whole year, the simulations were carried out for a typical Dutch tomato-greenhouse configuration, utilising a gas-fired boiler for both heat and CO2 production, and a water tank for day-to-night heat storage. The main findings are as follows: Provided that the crop is not damaged by the change, the expected gain from increasing the permissible humidity is about 0.74 € m−2y−1 per one percent relative humidity, and from reducing the minimum temperature − about 0.87 € m−2y−1 per degree. Roughly 2% of the energy is saved by a 1K reduction of temperature or a 1% increase of the relative humidity. Adding a heat buffer has no noticeable effect on the total amount of gas used. It does, however, increase the effectiveness of CO2 enrichment, thus increasing the yield and the economic gain (by 3.4 €m−2y−1). Replacing the profit goal by energy-use-minimisation goal, results in a substantial loss (−11.5 € m−2y−1).

    Optimal utilization of a boiler, combined heat and power installation, and heat buffers in horticultural greenhouses
    Beveren, P.J.M. van; Bontsema, J. ; Straten, G. van; Henten, E.J. van - \ 2019
    Computers and Electronics in Agriculture 162 (2019). - ISSN 0168-1699 - p. 1035 - 1048.
    Dynamic optimization - Energy cost saving - Equipment deployment - Greenhouse - Greenhouse operational management - Zero-or-range constraint

    In the daily operation of a greenhouse, decisions must be made about the best deployment of equipment for generating heat and electricity. The purpose of this paper is twofold: (1)To demonstrate the feasibility and flexibility of an optimal control framework for allocating heat and electricity demand to available equipment, by application to two different configurations used in practice. (2)To show that for a given energy and electricity demand benefit can be obtained by minimizing costs during resource allocation. The allocation problem is formulated as an optimal control problem, with a pre-defined heat and electricity demand pattern as constraints. Two simplified, yet realistic, configurations are presented, one with a boiler and heat buffer, and a second one with an additional combined heat and power generator (CHP)and a second heat buffer. A direct comparison with the grower is possible on those days where the other equipment that was at the grower's disposal was not used (63 days in the available 2012 data set). On those days overall costs savings of 20% were obtained. This shows that a given heat demand does not come with a fixed price to pay. Rather, benefits can be obtained by determining the utilization of the equipment by dynamic optimization. It also appears that prior knowledge of gas and electricity prices in combination with dynamic optimization has a high potential for cost savings in horticultural practice. To determine the factors influencing the outcome, different sensitivities to the optimization result were analyzed.

    Drying of yam with solar adsorption system
    Amankwah, Emmanuel Yaw Adu - \ 2019
    Wageningen University. Promotor(en): K.J. Keesman, co-promotor(en): A.J.B. van Boxtel; K.A. Dzisi; G. van Straten. - Wageningen : Wageningen University - ISBN 9789463434393 - 138

    Yam is an important food crop in Ghana. It is a delicacy that is eaten after cooking or frying the fresh sample when cut into slices. The powders of the dried yam can be incorporated into soups, baby food and bread as composite flours. However, the high moisture content of yam when harvested (about 70%wb) makes it susceptible to spoilage leading to reduced shelf life and onward rejection by the consumer. This adds to the waste basket. Drying is a worldwide postharvest method for extending the shelf life of agricultural food products such as yam. An energy source that is cheap and clean has always been from the sun. The open sun method has been utilized severally and for many years, but this is subject to various contaminants and thus reducing its quality. A confined solar drying method is a better option but the energy source is limited to the day’s solar energy. Solar adsorption drying is an opportunity to dry day and night in an efficient and product conserving manner with a reduced drying cycle. Prior to this thesis work, several aspects were insufficiently known to really judge the potential. The thesis tackles some of the bottlenecks and problems: lack of knowledge about the drying properties of yam; how to design appropriate solar collectors for drying and regeneration and to share experiences with the actual implementation of the system to see its possibilities an opportunities. In this thesis the sorption properties of yam which gives an indication of the equilibrium moisture content in relation to water activity at constant temperature, but has been given less attention in literature, is studied. The analytical Crank solution to the Fick’s second law has been utilized in the prediction of drying curves of many food products. However the Crank solution puts limitations on the extent to which the equation could be utilized. The limitation is that the particle of food sample to be dried must be infinite, shrinkage should not exist, while moisture diffusion coefficient must be constant. In practice, these limitations are not realistic since most biological food products go through shrinkage together with variable moisture diffusion.

    Meanwhile the source of energy for drying has largely come from the sun and has led the use of the so-called open sun drying (OSD) method especially in tropical countries, for so many years. This method exposes the food to weather conditions and contaminants reducing the quality of the dried product. Recent research has shown that a confined drying system is most appropriate in terms of reduced drying time and quality of the final product. The shortcoming of the solar drying (SD) method is that, drying is limited to a time interval of 6-9 hours of the day after which the sun’s energy can no more be explored. Solar adsorption drying (SAD), as an alternative drying to solar drying, is an integrated drying method that helps in drying day and night in a continuous manner. Research in solar adsorption drying systems is limited in literature, therefore its performance is yet to be explored. Solar collectors are an integral part of solar and solar adsorption dryers. However, most researchers construct collectors to see what happens or use heuristic rules, whereas others do not link to dryers. The output air temperature has an effect on the quality of the dried food product, therefore much attention must be given to the output temperature of the air that enters the dryer. On the other hand after adsorption drying, the adsorbent needs to be regenerated. The easiest way to regenerate the adsorbent (silica gel), which requires temperatures over 70°C is by conventional ovens, which is costly.

    In this thesis, the condition and requirements for solar adsorption drying of yam (Dioscorea rotundata) is investigated. The approach is first to investigate the sorption properties of yam, then the drying characteristics of yam by modifying the Crank solution of the Fick’s diffusion equation, subsequently the model based design and construction of solar collectors for drying and regeneration purposes, and then finally, the proof of principle of solar - adsorption for both drying and regeneration.

    The first part of this thesis concerns investigating the equilibrium moisture isotherms for yam at 25°C and 50°C, both for sorption and desorption, experimentally using the gravimetric method. The strength of this work is that a Dynamic Vapour Sorption (DVS) analyzer, which is very sensitive to micro changes in weight loss during dehydration or rehydration, was used. A graphical presentation of equilibrium moisture content, as a function of water activity from the outcome of nonlinear regression analyses of desorption and adsorption isotherms respectively, is shown. The findings contradict most of the findings in literature. The parameters and their uncertainty range for each of the models (GAB, Henderson, Halsey, Oswin, Smith, BET and Peleg) are estimated. The strength of the statistical analysis is that an objective criteria is used to select the model that best describes the experimental desorption and adsorption isotherms over the relevant range of moisture contents. This has been presented in a table form where statiscally, the standard error (SE), the percent average relative deviation (PRD), Akaike Information Criterion (AIC), coefficient of performance (R2), and most importantly, the 2-σ bound confidence interval of the parameters, in particular those that bear the same unit as the equilibrium moisture content (Xe), have been taken into consideration. The GAB model was selected to describe both the desorption and adsorption isotherms, because it allows a physical interpretation.

    Next, Crank’s analytical approximation to Fick’s diffusion equation is used to investigate the effect of moisture dependent sample thickness and diffusivity on the drying behavior of yam (Dioscoreaceae rotundata) cubicles. Separate experiments of drying and shrinkage at constant temperatures of 30, 40 and 50oC were conducted in a cabinet dryer. The comparative study shows an interdependence between diffusivity and shrinkage due to water loss during drying. The analytical expression for the diffusion in a slab results in non-Fickian behavior for smaller cubicles and consequently, results in a higher effective diffusion coefficient. The drying rate trajectory shows two stages as a function of moisture ratio. The drying behavior of yam is better described by observing both drying and drying rate curves concurrently. This gives good agreement between observed and model data by a combination of fractal moisture dependent shrinkage and moisture dependent diffusion. The advantage of this modified Crank’s approximation is that the moisture trajectory of finite food cubicles can be predicted appropriately.

    Coupled partial differential equations are developed to investigate which collector lengths are appropriate for drying and for adsorbent regeneration. The output air temperature of the model was validated with an experiment. The spatially distributed model is a powerful tool to aid in multi-purpose collector design leading to solar dryer and regeneration system construction. The advantage of this model is that, the spatial mean temperature of the absorber plate, a requirement for the determination of the overall heat loss coefficient, which hitherto is absent in the approach of Duffie and Beckmann, is no more a problem. The result shows a spatially distribution of temperature in air, absorber plate, and glass cover as functions of collector length and time (day) at constant air velocity and presented in a graphical from. While the temperature of absorber plate, glass cover and air are functions of time (day), the air temperature additionally varies directly with collector length. The study shows the operational overall heat loss coefficient, radiative and convective heat gain and efficiency as functions of the operational air velocity. There was a good agreement between the observed and the model air output temperature.

    The proof of principle of solar adsorption drying (SAD) and its benefits, compared to solar drying (SD) and open sun drying (OSD) and the effects of these drying methods on the quality of different yam cultivars and regeneration of silica gel is considered. The drying methods SAD, SD and OSD are compared in terms of drying cycles. The SAD had, the shortest drying cycle while maintaining the whitish colour of yam, followed by SD and OSD. However, due to the night drying, the SAD dried product is longer exposed to the drying medium (air), leading to slightly lower vitamin C content of dried sample compared to SD dried samples. OSD had the worst effect on vitamin C. The study shows that the different cultivars of yam dried at different rates. Drying has no effect on composition. Regeneration was possible with solar energy but more work must be done to obtain the information needed for improved designs and operation strategies for the desorption.

    Finally the perspective, impact and the potential application of the study has also been discussed.

    An improved methodology to evaluate crop salt tolerance from field trials
    Straten, G. van; Vos, A.C. de; Rozema, J. ; Bruning, B. ; Bodegom, P.M. van - \ 2019
    Agricultural Water Management 213 (2019). - ISSN 0378-3774 - p. 375 - 387.
    Crop salinity tolerance - Parameter estimation - Salinization

    The salt tolerance of crops is commonly expressed in descriptive parameters such as threshold or 50%-yield soil salinity and shape parameters describing the yield curve. Estimation by visual or simplified ordinary least squares (OLS) regression methods has multiple issues: parameter bias due to uncertainty in soil salinity, lack of independent estimates of the reference yield, questionable robustness of the threshold parameter and missing information about uncertainty and correlation of the parameter estimates. Here, we present a comprehensive OLS method together with an analysis of its statistical properties to alleviate and overcome such issues, on the basis of a numerical experiment that mimics observed yield responses to saline groundwater across a range of salinities in the experimental test facility Salt Farm Texel. The results indicate under which experimental conditions bias is not a major problem. The method allows estimation of the zero-observed-effect yield from the data, which is relevant to agricultural practice. Estimates for zero-observed-effect yield and threshold ECe are negatively correlated, underlining the difficulty of obtaining reliable threshold values. The estimated confidence regions are reliable and robust against soil salinity uncertainty, but large observation error jeopardizes the confidence intervals, especially for the slope parameter. Data uncertainty alone can be responsible for substantial differences from experiment to experiment, providing a partial explanation for the wide variety in reported parameters in the literature, and stressing the need for long-term repetitions. Given the lack of robustness of the threshold parameter, we propose to adopt the 90%-yield EC (ECe90) as tolerance parameter. Its confidence bounds can be obtained from a simple reformulation of the original models. We also present uncertainty ellipses as a suitable tool to unite multiple-year estimates. The method is offered as a solid and generic basis for reliable assessment of the cultivation potential of varieties and crops on salt-affected soils.

    Moisture Dependent Diffusion and Shrinkage in Yam during Drying
    Amankwah, Emmanuel A. ; Dzisi, Komla Agbeko ; Straten, Gerrit van; Boxtel, Anton J.B. van - \ 2018
    International Journal of Food Engineering 14 (2018)7-8. - ISSN 2194-5764 - 15 p.
    drying curves - effective diffusion - water transport - yam (dioscoraeceae rotundata)

    Crank's analytical approximations for Fick's diffusion equation were used to investigate the effect of moisture dependent sample thickness and diffusivity on the behavior of yam (Dioscoreaceae rotundata) cubicles during drying processes. Drying and shrinkage experiments were separately conducted at temperatures of 30, 40 and 50 °C in a cabinet drier. The comparative study of moisture dependent shrinkage and moisture dependent diffusivity justifies the interdependence of diffusivity and shrinkage due to water loss during drying. The behavior for yam is best explained by a combination of fractal moisture dependent shrinkage and moisture dependent diffusion, describing both the drying and rate curves better with good prediction of the high moisture regions. This assertion was reached as a result of low mean square error, standard error, percentage relative deviation, Akaike's Information Criterion and high coefficient of determination. The results may indicate a varying mobility of water in food matrix of different moisture content in the multilayer and monolayer regimes.

    Modeling the equilibrium moisture content of desorption and adsorption of yam (Dente)
    Amankwah, E.A. ; Dsizi, K.A. ; Straten, G. van; Boxtel, A.J.B. van - \ 2018
    Agricultural Engineering International 20 (2018)1. - ISSN 1682-1130 - p. 184 - 192.
    Equilibrium moisture content - GAB - Sorption isotherm - Yam

    The experimental equilibrium moisture content of yam (Dioscorea rotundata; cultivar: Dente) at temperatures of 25°C and 50°C were determined at relative humidity from 0% to 95% employing the dynamic vapor sorption analyzer. Wet yam samples with about 68% initial moisture content were used, first for desorption and subsequently for sorption. Water activity decreased with increased temperature at constant equilibrium moisture content. The desorption and adsorption isotherms were fitted by the Guggenheim-Anderson-de Boer (GAB), Henderson, Halsey, Oswin, Smith, Brunauer-Emmett-Teller (BET) and Peleg models. On the basis of the fit the Peleg, GAB and Oswin models were most suitable for describing the observed data. When the focus is on drying, the GAB (3 parameters) and empirical Peleg model (4 parameters) performed best. Of these, GAB is preferable because it has fewer parameters, which, moreover, have a physical meaning.

    Day-to-night heat storage in greenhouses : 3 Co-generation of heat and electricity (CHP)
    Seginer, Ido ; Beveren, Peter J.M. van; Straten, Gerrit van - \ 2018
    Biosystems Engineering 172 (2018). - ISSN 1537-5110 - p. 1 - 18.
    Co-generation of heat and electricity - CO enrichment - Greenhouse environmental control - Heat storage
    Day-to-night heat storage in water tanks (buffers) is common practice for cold-climate greenhouses, where gas is burned during the day for carbon-dioxide enrichment. In Seginer, I., van Straten, G., van Beveren, P. (2017). Biosystems Engineering, 161, 188–199, an optimal environmental control approach was outlined for conventional greenhouses, the idea being that a virtual value of the stored heat (its ‘co-state’) could be used to guide instantaneous control decisions. The value of the co-state was heuristically adjusted to minimise the time the buffer was ineffective (being empty or full). Here the same approach is applied to greenhouses with co-generation of heat and electricity (CHP). The parameters-set and weather are characteristic of tomato greenhouses in The Netherlands. The main results are: (1) The heuristic control method is easily adapted to systems with CHP; (2) Buffers are more useful to CO2 enrichment in the summer than to heating in the winter; (3) There is strong synergy between the two production systems – tomatoes and electricity. The tomato crop benefits from the by-products of electricity generation, namely CO2 and heat, sharing this benefit to support low electricity prices; (4) The combined system produces less CO2 pollution than the two production systems operating independently; (5) The contribution of the CHP and buffer to the economic performance of the system is very significant, while that of the thermal screen and boiler is marginal; (6) Flexibility in the system is important. A buffer and/or continuously controlled boiler and CHP are essential to achieving high profitability.
    Zouttolerantie aardappelen SWAP-WOFOST toepassing op Zilt Proefbedrijf Texel
    Mulder, H.M. ; Bakel, P.J.T. van; Vos, A. de; Straten, G. van; Heinen, M. ; Kroes, J.G. - \ 2018
    Amersfoort : Stowa (Stowa rapport 2018-01) - ISBN 9789057737695 - 28
    Op het Zilt Proefbedrijf Texel ‘SaltFarm Texel’ (www.SaltFarmTexel.com) worden proeven uitgevoerd om voor verschillende landbouwgewassen de zouttolerantie te onderzoeken. Gedurende de periode 2012 tot en met 2015 zijn onder gecontroleerde veldomstandigheden de zouttolerantie van de aardappelvariëteiten Miss Mignonne en Achilles vastgesteld (De Vos et al., 2016). In dit rapport wordt beschreven in hoeverre het model SWAP-WOFOST (Kroes et al., 2009) de meetgegevens van het Zilt Proefbedrijf Texel kan simuleren. In het laatste hoofdstuk wordt beknopt beschreven hoe met deze kennis metarelaties kunnen worden afgeleid voor het bepalen van de langjarig gemiddelde opbrengstderving als gevolg van zout schade.
    Day-To-night heat storage in greenhouses : A simulation study
    Seginer, I. ; Straten, G. Van; Beveren, P.J.M. Van - \ 2017
    In: 5th International Symposium on Models for Plant Growth, Environment Control and Farming Management in Protected Cultivation, HortiModel 2016 International Society for Horticultural Science (Acta Horticulturae ) - ISBN 9789462611788 - p. 119 - 127.
    Co-state-based policy - Co2 enrichment - Day-To-night heat storage - Greenhouses
    In cold-climate locations, where natural gas is burned during the day to enrich greenhouses with carbon dioxide, water tanks (buffers) are often used to store the surplus daytime heat for nighttime heating. A practical control strategy for filling (charging) and emptying (discharging) of the buffer, based on a virtual value (costate) of the stored heat (the state), is suggested and illustrated by simulation. Heating and ventilation decisions are obtained by maximizing, at each time step, the virtual increase in value of the greenhouse system (including stored heat). As long as the heat buffer is neither empty nor full, the virtual value (co-state) of the stored heat remains constant. When the buffer is full (towards the end of a day), this value is gradually decreased, until the buffer starts to discharge. When the buffer empties (towards the end of a night), the virtual value is gradually increased, until recharging of the buffer starts again. This heuristic strategy is meant to minimize the time that the buffer is empty or full, because in these states the buffer is inactive (ineffective). Simulations with an annual weather sequence show the following: (1) the winter-Time virtual value of stored heat is about equal to the actual cost of heat, while in summer it is close to zero; (2) the utilization of the buffer, judged by the time on the storage bounds (full or empty), is roughly uniform along the year; (3) the performance of the system improves asymptotically with an increase of the installed capacity of the buffer; (4) expensive energy (heat) results in reduced intensity of cultivation (less heat and less yield).
    Day-to-night heat storage in greenhouses : 2 Sub-optimal solution for realistic weather
    Seginer, Ido ; Straten, Gerrit van; Beveren, Peter J.M. van - \ 2017
    Biosystems Engineering 161 (2017). - ISSN 1537-5110 - p. 188 - 199.
    CO enrichment - Greenhouse - Heat buffer - Optimal control - Self-adjusting co-state
    Day-to-night heat storage in water tanks (buffers) is common practice in cold-climate greenhouses, where gas is burned during the day for carbon dioxide enrichment. In Part 1 of this study, an optimal control approach was outlined for such a system, the basic idea being that the virtual value (shadow price) of the stored heat (its 'co-state') could be used to guide the instantaneous control decisions. The results for daily-periodic weather showed: (1) The optimal co-state is constant in time. (2) The optimal solution is associated with minimum time on the storage bounds (buffer empty or full). With these conclusions as guidelines, a semi-heuristic procedure of optimisation for realistic (i.e. not strictly periodic) weather is developed. The co-state remains constant while the storage trajectory is between the heat storage bounds. It is gradually increased while the buffer is empty, and decreased when the buffer is full, attempting to push the trajectory away from the bounds, thus minimising the time that the buffer is idle. The main outcomes are: (1) No information about the future is required. (2) The algorithm changes the co-state automatically, producing the correct annual variation (high in winter and low in summer). (3) The predictions of yield and heat requirement compare favourably with practice. (4) The gain in performance achievable with the suggested method is probably 75% or more of the true optimum. (5) The procedure can be used in the design stage to determine the optimal buffer size and the usefulness of other modifications of the system.
    Day-to-night heat storage in greenhouses : 1 Optimisation for periodic weather
    Seginer, Ido ; Straten, Gerrit van; Beveren, Peter J.M. van - \ 2017
    Biosystems Engineering 161 (2017). - ISSN 1537-5110 - p. 174 - 187.
    CO enrichment - Constant co-state - Greenhouse - Heat buffer - Optimal control - Periodic weather
    Day-to-night heat storage using water tanks (buffers) is common practice in cold-climate greenhouses, where gas is burned during the day for carbon dioxide enrichment. In this study an optimal control approach is outlined for such a system, based on the idea that the virtual value (shadow price) of the stored heat, its 'co-state', could be used to guide the instantaneous control decisions. If this value is high, the system has an incentive to fill the heat storage (buffer), and vice versa if the co-state is low. The optimal co-state trajectory maximises the net income (performance criterion). To illustrate the method, a system description and a parameter-set roughly representative of tomato greenhouses in The Netherlands is used. The results, for daily-periodic weather, show: (1) The optimal co-state is constant (same value night and day), in contrast to the varying set-points and control fluxes. (2) The optimal solution is associated with minimum time on the storage bounds (minimum time of full or empty buffer). (3) The optimal virtual value (co-state) of stored heat is about the same as the actual cost of boiler heat during winter and about zero in summer. (4) The gain from installing a buffer is highest during spring and minimal in winter. (5) The intensive utilisation of the heat buffer in summer and its low utilisation in winter indicate that the justification of the heat storage practice, under the assumed conditions, is more the need for CO2 enrichment in summer than the need for heating in winter.
    Distributed mathematical model supporting design and construction of solar collectors for drying
    Amankwah, E.A.Y. ; Dzisi, K.A. ; Straten, G. van; Willigenburg, L.G. van; Boxtel, A.J.B. van - \ 2017
    Drying Technology 35 (2017)14. - ISSN 0737-3937 - p. 1675 - 1687.
    Low temperature drying - SADS - solar collector - yam

    Coupled partial differential equations were developed to investigate which collector lengths are appropriate for drying and adsorbent regeneration under prevailing Ghanaian weather. Unlike approaches based on empirical data, the numerical model is more flexible. Effects of operational and design variables on outlet temperature and performance were systematically studied. Collector length and air speed affect performance indicators. Operational overall heat loss coefficient, an important characteristic of the collector, is not constant but varies during the day. With plausible physical parameters, the model describes the experimental data well. Collector lengths of 1.5 and 4.5 m suited drying and regeneration, respectively.

    Microsatellite diversity of the Nordic type of goats in relation to breed conservation : How relevant is pure ancestry?
    Lenstra, J.A. ; Tigchelaar, J. ; Biebach, I. ; Hallsson, J.H. ; Kantanen, J. ; Nielsen, V.H. ; Pompanon, F. ; Naderi, S. ; Rezaei, H.R. ; Sæther, N. ; Ertugrul, O. ; Grossen, C. ; Camenisch, G. ; Vos-Loohuis, M. ; Straten, M. van; Poel, E.A. de; Windig, J. ; Oldenbroek, K. - \ 2017
    Journal of Animal Breeding and Genetics 134 (2017)1. - ISSN 0931-2668 - p. 78 - 84.
    Conservation - Diversity - Goats - Microsatellite

    In the last decades, several endangered breeds of livestock species have been re-established effectively. However, the successful revival of the Dutch and Danish Landrace goats involved crossing with exotic breeds and the ancestry of the current populations is therefore not clear. We have generated genotypes for 27 FAO-recommended microsatellites of these landraces and three phenotypically similar Nordic-type landraces and compared these breeds with central European, Mediterranean and south-west Asian goats. We found decreasing levels of genetic diversity with increasing distance from the south-west Asian domestication site with a south-east-to-north-west cline that is clearly steeper than the Mediterranean east-to-west cline. In terms of genetic diversity, the Dutch Landrace comes next to the isolated Icelandic breed, which has an extremely low diversity. The Norwegian coastal goat and the Finnish and Icelandic landraces are clearly related. It appears that by a combination of mixed origin and a population bottleneck, the Dutch and Danish Land-races are separated from the other breeds. However, the current Dutch and Danish populations with the multicoloured and long-horned appearance effectively substitute for the original breed, illustrating that for conservation of cultural heritage, the phenotype of a breed is more relevant than pure ancestry and the genetic diversity of the original breed. More in general, we propose that for conservation, the retention of genetic diversity of an original breed and of the visual phenotype by which the breed is recognized and defined needs to be considered separately.

    Crop salt tolerance under controlled field conditions in The Netherlands, based on trials conducted at Salt Farm Texel
    Vos, Arjen de; Bruning, Bas ; Straten, Gerrit van; Oosterbaan, Roland ; Rozema, Jelte ; Bodegom, Peter van - \ 2016
    Den Burg : Salt Farm Texel - 39
    Optimal Day-to-Night Greenhouse Heat Storage : Square-Wave Weather
    Seginer, Ido ; Straten, Gerrit van; Beveren, Peter van - \ 2016
    IFAC-PapersOnLine 49 (2016)16. - ISSN 2405-8963 - p. 375 - 380.
    Co-state - Constrained optimum - Greenhouse - Heat storage - Optimal control - Time on bounds

    Day-to-night heat storage is often practiced in cold-climate greenhouses. It is suggested to manage the heat storage by considering the co-state (virtual value) of the stored heat in the on-line optimization of the greenhouse environment. Examples worked out for a periodic square-wave weather show that a properly selected constant co-state can produce an optimal solution to the control problem. The optimal co-state is shown to change with time over the year. Maximizing the performance criterion can also be achieved by minimizing the time that the heat buffer is either completely empty or completely full.

    Field tests of dielectric sensors in a facility for studying salt tolerance of crops
    Straten, Gerrit Van; Vos, Arjen De; Vlaming, Rik ; Oosterbaan, Roland - \ 2016
    International Agricultural Engineering Journal 25 (2016)2. - ISSN 0858-2114 - p. 102 - 113.
    Calibration - Electrical conductivity - Irrigation - Salinity - Salt tolerance - Soil sensor - Volumetric water content

    Seven salinity levels ranging from 1.7 to 35 dS/m are applied to groups of eight fields each in a field facility for testing the salt tolerance of crops. Each of the 56 test fields is equipped with one or two dielectric sensors for soil volumetric water content (VWC) and bulk electric conductivity (ECb). Several models for calibrating the sensors in the laboratory were tested and parameterized. Overall, the root mean square error was in the range of 0.57-0.59 dS/m in terms of soil bulk EC. The models differed in their robustness against inversion to obtain pore water EC from measured bulk EC. The laboratory calibration formula overestimates the pore water EC at low EC (5 dS/m), and underestimates it at high EC (25 dS/m). In practice, calculated sensor pore water EC's in fields with the same salinity treatment differ among each other, showing the limitations of laboratory calibrations. However, in fields where pore water samples are available, a direct proportionality between pore water EC and sensor bulk EC suffices without correction for VWC in this well irrigated case. Moreover there is a good correlation between the low frequent EC time series of suction cup samples and the high frequent sensor readings. When used with care, sensors can give valuable information about the dynamics of soil conditions during crop salinity tolerance tests.

    Assessing micro-algae productivities with scenario studies
    Slegers, P.M. ; Straten, G. van; Wijffels, R.H. ; Boxtel, A.J.B. van - \ 2016
    - 1 p.
    Logistic aspects of algae cultivation?
    Slegers, P.M. ; Leduc, S. ; Wijffels, R.H. ; Straten, G. van; Boxtel, A.J.B. van - \ 2016
    Quantitative modeling and analytic assessment of the transcription dynamics of the XlnR regulon in Aspergillus niger
    Omony, Jimmy ; Mach-Aigner, A.R. ; Straten, Gerrit van; Boxtel, A.J.B. van - \ 2016
    BMC Systems Biology 10 (2016). - ISSN 1752-0509 - 10 p.
    Aspergillus niger - CreA - D-xylose - Dynamic modeling - Parameter estimation - XlnR regulon

    Background: Transcription of genes coding for xylanolytic and cellulolytic enzymes in Aspergillus niger is controlled by the transactivator XlnR. In this work we analyse and model the transcription dynamics in the XlnR regulon from time-course data of the messenger RNA levels for some XlnR target genes, obtained by reverse transcription quantitative PCR (RT-qPCR). Induction of transcription was achieved using low (1 mM) and high (50 mM) concentrations of D-xylose (Xyl). We investigated the wild type strain (Wt) and a mutant strain with partial loss-of-function of the carbon catabolite repressor CreA (Mt). Results: An improved kinetic differential equation model based on two antagonistic Hill functions was proposed, and fitted to the time-course RT-qPCR data from the Wt and the Mt by numerical optimization of the parameters. We show that perturbing the XlnR regulon with Xyl in low and high concentrations results in different expression levels and transcription dynamics of the target genes. At least four distinct transcription profiles were observed, particularly for the usage of 50 mM Xyl. Higher transcript levels were observed for some genes after induction with 1 mM rather than 50 mM Xyl, especially in the Mt. Grouping the expression profiles of the investigated genes has improved our understanding of induction by Xyl and the according regulatory role of CreA. Conclusions: The model explains for the higher expression levels at 1 mM versus 50 mM in both Wt and Mt. It does not yet fully encapsulate the effect of partial loss-of-function of CreA in the Mt. The model describes the dynamics in most of the data and elucidates the time-dynamics of the two major regulatory mechanisms: i) the activation by XlnR, and ii) the carbon catabolite repression by CreA.

    Optimal control of greenhouse climate using minimal energy and grower defined bounds
    Beveren, P.J.M. van; Bontsema, J. ; Straten, G. van; Henten, E.J. van - \ 2015
    Applied Energy 159 (2015). - ISSN 0306-2619 - p. 509 - 519.
    Cooling - Energy - Greenhouse climate - Heating - Optimal control

    Saving energy in greenhouses is an important issue for growers. Here, we present a method to minimize the total energy that is required to heat and cool a greenhouse. Using this method, the grower can define bounds for temperature, humidity, CO2 concentration, and the maximum amount of CO2 available. Given these settings, optimal control techniques can be used to minimize energy input. To do this, an existing greenhouse climate model for temperature and humidity was expanded to include a CO2 balance. Heating, cooling, the amount of natural ventilation, and the injection of industrial CO2 were used as control variables.Standard optimization settings were defined in order to compare the grower's strategy with the optimal solution. This optimization resulted in a theoretical 47% reduction in heating, 15% reduction in cooling, and 10% reduction in CO2 injection for the year 2012. The optimal control does not need to maintain a minimum pipe temperature, in contrast to current practice. When the minimum pipe temperature strategy of the grower was implemented, heating and CO2 were reduced by 28% and 10% respectively.We also analyzed the effect of different bounds on optimal energy input. We found that as more freedom is given to the climate variables, the higher the potential energy savings. However, in practice the grower is in charge of defining the bounds. Thus, the potential energy savings critically depend on the choice of these bounds. This effect was analyzed by varying the bounds. However, because the effect can be demonstrated to the grower, the outcome has value to the grower with respect to decision making, an option that is not currently available in practice today.

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