###### Abstract  
The EU energy and climate policy has set quantitative goals for decarbonization based on the energy efficiency and the evolution of energy systems. The utilization of demand side flexibility can help towards this direction and achieve the target of higher levels of penetration in regard to intermittent renewable energy production and carbon emission reduction. This paper presents a simulation-based assessment of thermal flexibility in a typical office building in Greece, which is a representative Mediterranean country. The use of variable speed heat pumps coupled with hydronic terminal units was evaluated. The research focused mainly on the evaluation of energy flexibility offered by energy stored in the form of thermal energy by utilizing the building's thermal mass. The demand response potential under hourly CO\({}_{\text{2 eq}}\) intensity and energy prices was investigated. The flexibility potential was evaluated under different demand response strategies, and the effect of demand response on energy consumption, operational costs, CO\({}_{\text{2 eq}}\) emissions and thermal comfort was analyzed and discussed. The results showed that both control strategies based on both the CO\({}_{\text{2 eq}}\)intensity signal and spot price signal have, in some cases, the potential for cost and emission savings, and in other cases, the potential to depreciate in terms of emissions and cost the increase of energy consumption due to load shifting. energy efficiency; CO\({}_{\text{2 eq}}\) intensity; energy prices; energy flexibility; predictive rule-based control; office building; energy simulation; TRNSYS +
Footnote †: journal: Applied Sciences  
residential buildings [6]. Commercial HVAC, lighting and other electrical equipment in office buildings can adjust their operational profile to support the energy grid. Several researchers have come to appreciate the importance of buildings as a source of energy flexibility to the energy grid and tried to quantify the demand side flexibility of the building sector by implementing DR in buildings and their energy systems. In their research [7], they tried to quantify the flexibility that a building with CHP and energy storage can offer, by considering the magnitude of a CHP operation's delay or forced operation time. [PERSON] et al., 2019 [8] introduced a multicriteria indicator and tried to quantify the energy flexibility of a residential building with AWHP and TES. [PERSON] and [PERSON] in 2016 [9] introduced cost curves as a metric for measuring flexibility that is applicable to a wide variety of building typologies and energy systems. In [10], power shifting indexes were employed to evaluate an office building's flexibility under model predictive control. [PERSON] et al., 2015 [11], developed a generic flexibility assessment methodology that was applied to various building typologies equipped with heat pumps. In [12; 13], indicators that cover time, size and cost were used to quantify the energy storage in building thermal mass as a mean of flexibility in different building typologies. [PERSON] et al., 2019 [14], implemented different DR scenarios to assess the flexibility of a building equipped with a hybrid heating system with thermal energy storage located in Italy. Several temperature regulation strategies were implemented in detailed resistance-capacitance models of various buildings in order to quantify their demand response potential in [15]. Several researches studied the impact of DR control on equivalent carbon dioxide emissions. The authors of [16; 17; 18] tested the implementation of MPC in residential buildings. In every case, a reduction in dioxide emissions was observed. Other researchers studied the implementation of two different CO\({}_{2}\) MPCs to a large commercial building with two different HVAC settings and found decreases in emissions, especially in the case of exploiting renewable energy [19].