Straight-bladed vertical axis wind turbines (SBVAWTs) with a fixed pitch are subject to continuous variation in wind attack angle, which leads to low power generation. Variable pitch techniques are thus required to tackle the problem, but a proper analytical tool for pitch optimization and performance assessment must first be determined. This study thus aims to find a proper analytical tool for high-solidity SBVAWTs of low operational speeds for real-world applications. The applicability of the double-disk multiple stream-tube (DMST) model, a mainstream tool for analyzing moderate-solidity SBVAWTs and optimal pitch angles, is first examined for high-solidity SBVAWTs using measurement data obtained from wind tunnel tests. The complex flow field around a high-solidity SBVAWT is found to make current DMST models' predictions unsatisfactory. A hybrid DMST model, using the attack angle-dynamic aerodynamic force coefficient relationship, is then proposed. The proposed hybrid DMST model predicts the aerodynamic forces acting on the blades with greater accuracy than those from current DMST models. Based on the proposed hybrid DMST model, a method of determining optimal pitch angles is developed and applied to the high-solidity SBVAWT. The results show that the novel method has the potential to significantly enhance power generation in high-solidity SBVAWTs.