Geothermal district heating networks: modelling novel operational strategies incorporating heat storage

The value of integrating a heat storage into a geothermal district heating system has been investigated. The behaviour of the system under a novel operational strategy has been simulated focusing on the energetic, economic and environmental effects of the new strategy of incorporation of the heat storage within the system. A typical geothermal district heating system consists of several production wells, a system of pipelines for the transportation of the hot water to end-users, one or more re-injection wells and peak-up devices (usually fossil-fuel boilers). Traditionally in these systems, the production wells change their production rate throughout the day according to heat demand, and if their maximum capacity is exceeded the peak-up devices are used to meet the balance of the heat demand. In this study, it is proposed to maintain a constant geothermal production and add heat storage into the network. Subsequently, hot water will be stored when heat demand is lower than the production and the stored hot water will be released into the system to cover the peak demands (or part of these). It is not intended to totally phase-out the peak-up devices, but to decrease their use, as these will often be installed anyway for back-up purposes. Both the integration of a heat storage in such a system as well as the novel operational strategy are the main novelties of this thesis. A robust algorithm for the sizing of these systems has been developed. The main inputs are the geothermal production data, the heat demand data throughout one year or more and the topology of the installation. The outputs are the sizing of the whole system, including the necessary number of production wells, the size of the heat storage and the dimensions of the pipelines amongst others. The results provide several useful insights into the initial design considerations for these systems, emphasizing particularly the importance of heat losses. Simulations are carried out for three different cases of sizing of the installation (small, medium and large) to examine the influence of system scale. In the second phase of work, two algorithms are developed which study in detail the operation of the installation throughout a random day and a whole year, respectively. The first algorithm can be a potentially powerful tool for the operators of the installation, who can know a priori how to operate the installation on a random day given the heat demand. The second algorithm is used to obtain the amount of electricity used by the pumps as well as the amount of fuel used by the peak-up boilers over a whole year. These comprise the main operational costs of the installation and are among the main inputs of the third part of the study. In the third part of the study, an integrated energetic, economic and environmental analysis of the studied installation is carried out together with a comparison with the traditional case. The results show that by implementing heat storage under the novel operational strategy, heat is generated more cheaply as all the financial indices improve, more geothermal energy is utilised and less fuel is used in the peak-up boilers, with subsequent environmental benefits, when compared to the traditional case. Furthermore, it is shown that the most attractive case of sizing is the large one, although the addition of the heat storage most greatly impacts the medium case of sizing. In other words, the geothermal component of the installation should be sized as large as possible. This analysis indicates that the proposed solution is beneficial from energetic, economic, and environmental perspectives. Therefore, it can be stated that the aim of this study is achieved in its full potential. Furthermore, the new models for the sizing, operation and economic/energetic/environmental analyses of these kind of systems can be used with few adaptations for real cases, making the practical applicability of this study evident. Having this study as a starting point, further work could include the integration of these systems with end-user demands, further analysis of component parts of the installation (such as the heat exchangers) and the integration of a heat pump to maximise utilisation of geothermal energy.

Regional variation and change in the history of English strong verbs

This thesis investigates how the strong verb system inherited from Old English evolved in the regional dialects of Middle English (ca. 1100-1500). Old English texts preserve a relatively complex system of strong verbs, in which traditionally seven different ablaut classes are distinguished. This system becomes seriously disrupted from the Late Old English and Early Middle English periods onwards. As a result, many strong verbs die out, or have their ablaut patterns affected by sound change and morphological analogy, or transfer to the weak conjugation. In my thesis, I study the beginnings of two of these developments in two strong verb classes to find out what the evidence from Middle English regional dialects can tell us about their origins and diffusion. Chapter 2 concentrates on the strong-to-weak shift in Class III verbs, and investigates to what extent strong, mixed and weak past tense and participle forms vary in Middle English dialects, and whether the variation is more pronounced in the paradigms of specific verbs or sub-classes. Chapter 3 analyses the regional distribution of ablaut levelling in strong Class IV verbs throughout the Middle English period. The Class III and IV data for the Early Middle English period are drawn from A Linguistic Atlas of Early Middle English, and the data for the Late Middle English period from a sub-corpus of files from The Penn-Helsinki Parsed Corpus of Middle English and The Middle English Grammar Corpus. Furthermore, The English Dialect Dictionary and Grammar are consulted as an additional reference point to find out to what extent the Middle English developments are reflected in Late Modern English dialects. Finally, referring to modern insights into language variation and change and linguistic interference, Chapter 4 discusses to what extent intra- and extra-linguistc factors, such as token and type frequency, stem structure and language contact, might correlate with the strong-to-weak shift and ablaut levelling in Class III and IV verbs in the Middle English period. The thesis is accompanied by six appendices that contain further information about my distinction of Middle English dialect areas (Appendix A), historical Class III and IV verbs (B and C) and the text samples and linguistic data from the Middle English text corpora (D, E and F).