地热换热器间歇运行工况分析

Analysis on discontinuous operation of geothermal heat exchangers of the ground-source heat pump systems

地源热泵应用推广的关键和难点是地热换热器的设计和运行模拟 ,大多数工程应用均采用简单而又实用的线热源模型。本文利用线热源解模拟出地热换热器周围土壤的温度响应 ,对于随时间变化的负荷或间歇负荷可以近似用一系列的矩形脉冲热 (或冷 )负荷来代替。因此 ,采用了迭加原理来分析计算随时间变化的间歇负荷引起的温度响应。通过模拟计算发现 ,在计算流体的最大温升值时 ,可以把间歇工作的周期性脉冲热流简化为一个持续作用的平均热负荷和一个脉冲负荷的和。这为地热换热器的设计计算提供了一种简单实用的方法。通过编程模拟还证明 ,对于地热换热器来说 ,冷热负荷平衡的工况是最理想的工况 ,长期运行不会引起地层中热量 (冷量 )积累而使地热换热器性能退化。

A main challenge for GSHP application is the design and performance simulation of the geothermal heat exchangers. Being simple and effective for most engineering applications, the line source model is presented for prediction of the temperature response in the ground around the geothermal heat exchangers. Variable and discontinuous loads can be approximated by a series of rectangular pulses of heating or cooling. Then, superposition is used to deal with such variable and discontinuous loads. Simulations indicate that the maximum temperature rise after a period of cyclic heating of the ground can be approximated by the superposition of a continuous mean load and a single heating pulse. This approach is desirable for engineering applications to deal with long but discontinuous load to geothermal heat exchangers. An ideal situation for the geothermal heat exchanger is also simulated of balanced heating and cooling loads in the years around. On such a condition the heat extracted from and rejected to the ground offsets each other, and there is no net heat accumulation in the ground after long duration of operation. Such a design or operation scheme is favorable for keeping the heat exchanger at high efficiency over its lifetime.