The method of surrogate data has been extensively applied to hypothesis testing of system linearity, when only one realization of the system, a time series, is known. Normally, surrogate data should preserve the linear stochastic structure and the amplitude distribution of the original series. Classical surrogate data methods (such as random permutation, amplitude adjusted Fourier transform, or iterative amplitude adjusted Fourier transform) are successful at preserving one or both of these features in stationary cases. However, they always produce stationary surrogates, hence existing nonstationarity could be interpreted as dynamic nonlinearity. Certain modifications have been proposed that additionally preserve some nonstationarity, at the expense of reproducing a great deal of nonlinearity. However, even those methods generally fail to preserve the trend (i.e., global nonstationarity in the mean) of the original series. This is the case of time series with unit roots in their autoregressive structure. Additionally, those methods, based on Fourier transform, either need first and last values in the original series to match, or they need to select a piece of the original series with matching ends. These conditions are often inapplicable and the resulting surrogates are adversely affected by the well-known artefact problem. In this study, we propose a simple technique that, applied within existing Fourier-transform-based methods, generates surrogate data that jointly preserve the aforementioned characteristics of the original series, including (even strong) trends. Moreover, our technique avoids the negative effects of end mismatch. Several artificial and real, stationary and nonstationary, linear and nonlinear time series are examined, in order to demonstrate the advantages of the methods. Corresponding surrogate data are produced with the classical and with the proposed methods, and the results are compared.

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A new class of hybrid (heating and cooling) solar collector has been designed, built and tested. The collector combines an adsorption generator and a heat removal tubing. The tubing speeds up the evaporation stage of the cooling cycle and can also be used to deliver hot water. A small prototype (0.97 m2collection area) has been incorporated to an installation, which was tested in Burgos (northern Spain, 42°20′ N, 3°40′ W, 840 m above sea level). Two operation modes (summer and winter) are allowed. In summer mode, the installation delivers and stores domestic hot water and cold water; solar COP averaged 0.13 in our experimental campaign, and domestic hot water production was 3.1 MJ per day. When domestic hot water production is disabled, the average solar COP is reduced by 30%. In winter mode, domestic hot water and hot water for space heating are produced; combined production reached 55% of average daily solar radiation.